Hallo all,
attached to this mail you will find a couple of patches fixing one bug I
have with kernel 3.10 (all subreleases).
These patches have been developed originally by Jan Kara (jack@xxxxxxx,
I guess you
know him better than I do) for kernel 3.13 and can be found here:
https://lkml.org/lkml/2014/3/25/343
I just have backported them to kernel 3.10.
Also I have created (at least it was a try by myself) a bugentry on
kernel.org:
https://bugzilla.kernel.org/show_bug.cgi?id=71231
(I have to update it yet)
Summarize:
Creating lots of output on console that is going to be printed on serial
port too (via kernel command line "console=ttyS0") will hang several
systems (mostly Dell servers with multiple CPUs).
I can easily reproduce this behaviour by creating about 50 virtual LUNs
via fibrechannel at once.
This patch from Jan introduces a buffering and better locking in printk
(as far as I understood him or his patches right).
Without this patches I get teh following stacktraces and the system is
not responding after a couple of seconds:
119.504736] Code: 00 48 89 f0 48 c7 06 00 00 00 00 48 89 e5 48 87 07
48 85 c0 75 09 c7 46 08 01 00 00 00 5d c3 48 89 30 8b 46 08 85 c0 75 f4
f3 90 <8b> 46 08 85 c0 74 f7 5d c3 0f 1f 44 00 00 48 8b 16 55 48 89 e5 [
119.504740] NMI backtrace for cpu 44
[ 119.504746] CPU: 44 PID: 21877 Comm: lsscsi Not tainted 3.10.32-64bit #1
[ 119.504748] Hardware name: Dell Inc. PowerEdge R910/0P658H, BIOS
1.0.1 02/19/2010
[ 119.504751] task: ffff881063510d60 ti: ffff88106351c000 task.ti:
ffff88106351c000
[ 119.504760] RIP: 0010:[<ffffffff8105e505>] [<ffffffff8105e505>]
mspin_lock+0x35/0x40
[ 119.504763] RSP: 0018:ffff88106351db38 EFLAGS: 00000246
[ 119.504765] RAX: 0000000000000000 RBX: ffffffff81aa9220 RCX:
0000000000000028
[ 119.504768] RDX: ffff88106351dfd8 RSI: ffff88106351db60 RDI:
ffffffff81aa9240
[ 119.504770] RBP: ffff88106351db38 R08: 70722f3374736f68 R09:
fefefefefefefeff
[ 119.504772] R10: ffff88086ddc5bc0 R11: 8f8dd0cc8b8c9097 R12:
ffff88106351db60
[ 119.504774] R13: ffffffff81aa9240 R14: ffff881063510d60 R15:
ffff88106351dca8
[ 119.504777] FS: 0000000000000000(0000) GS:ffff88046fd60000(0063)
knlGS:00000000f762f8d0
[ 119.504780] CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
[ 119.504782] CR2: 00000000f777c000 CR3: 00000010684ea000 CR4:
00000000000007e0
[ 119.504785] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 119.504788] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[ 119.504789] Stack:
[ 119.504794] ffff88106351dba8 ffffffff81766d5d ffff88106351db58
ffff88106351dfd8
[ 119.504798] ffff88106351dba8 ffff881066ebfb90 ffff881000000000
ffffffff00000003
[ 119.504803] ffff88106351db98 ffffffff81aa9220 ffff88106351dc98
ffff88046c3a94d0
[ 119.504804] Call Trace:
[ 119.504812] [<ffffffff81766d5d>] __mutex_lock_slowpath+0x5d/0x1d0
[ 119.504817] [<ffffffff817667dd>] mutex_lock+0x1d/0x40
[ 119.504823] [<ffffffff81156ba5>] sysfs_dentry_revalidate+0x35/0x100
[ 119.504828] [<ffffffff810ff389>] lookup_fast+0x299/0x2e0
[ 119.504833] [<ffffffff810ff586>] ? __inode_permission+0x46/0x70
[ 119.504838] [<ffffffff810ff79a>] link_path_walk+0x19a/0x810
[ 119.504843] [<ffffffff810ff994>] link_path_walk+0x394/0x810
[ 119.504848] [<ffffffff810b9b8f>] ? release_pages+0x18f/0x1e0
[ 119.504853] [<ffffffff81103094>] path_openat.isra.72+0x94/0x460
[ 119.504859] [<ffffffff810cbf84>] ? tlb_flush_mmu+0x54/0x90
[ 119.504864] [<ffffffff810d2c79>] ? unmap_region+0xd9/0x120
[ 119.504869] [<ffffffff8110368c>] do_filp_open+0x3c/0x90
[ 119.504875] [<ffffffff8110ee52>] ? __alloc_fd+0x42/0x100
[ 119.504882] [<ffffffff810f3a3f>] do_sys_open+0xef/0x1d0
[ 119.504888] [<ffffffff8113a036>] compat_SyS_open+0x16/0x20
[ 119.504893] [<ffffffff8176b598>] sysenter_dispatch+0x7/0x25
[ 119.504938] Code: f0 48 c7 06 00 00 00 00 48 89 e5 48 87 07 48 85 c0
75 09 c7 46 08 01 00 00 00 5d c3 48 89 30 8b 46 08 85 c0 75 f4 f3 90 8b
46 08 <85> c0 74 f7 5d c3 0f 1f 44 00 00 48 8b 16 55 48 89 e5 48 85 d2 [
119.504942] NMI backtrace for cpu 45
[ 119.504946] CPU: 45 PID: 0 Comm: swapper/45 Not tainted 3.10.32-64bit #1
[ 119.504949] Hardware name: Dell Inc. PowerEdge R910/0P658H, BIOS
1.0.1 02/19/2010
[ 119.504952] task: ffff88046dcd6b00 ti: ffff88046dcf6000 task.ti:
ffff88046dcf6000
[ 119.504960] RIP: 0010:[<ffffffff8102318a>] [<ffffffff8102318a>]
mwait_idle_with_hints+0x5a/0x70
[ 119.504962] RSP: 0018:ffff88046dcf7dd8 EFLAGS: 00000046
[ 119.504965] RAX: 0000000000000000 RBX: ffff88106d07d46c RCX:
0000000000000001
[ 119.504967] RDX: 0000000000000000 RSI: 0000000000000001 RDI:
0000000000000000
[ 119.504970] RBP: ffff88046dcf7dd8 R08: ffff88046dcf7fd8 R09:
000000000000001c
[ 119.504972] R10: 0000000000000166 R11: 00000000016f1e50 R12:
0000000000000001
[ 119.504975] R13: ffff88106d07d400 R14: 0000000000000001 R15:
ffffffffa0004830
[ 119.504978] FS: 0000000000000000(0000) GS:ffff88086fd60000(0000)
knlGS:0000000000000000
[ 119.504981] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 119.504984] CR2: 00000000f7746000 CR3: 000000046cb57000 CR4:
00000000000007e0
[ 119.504986] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 119.504989] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[ 119.504989] Stack:
[ 119.504994] ffff88046dcf7de8 ffffffff810231cd ffff88046dcf7df8
ffffffffa0001515
[ 119.504999] ffff88046dcf7e28 ffffffffa000164a 00000000000000f5
ffff88106c669000
[ 119.505003] ffffffffa00047c0 0000001aaf4aa38f ffff88046dcf7e88
ffffffff8164e83a
[ 119.505004] Call Trace:
[ 119.505010] [<ffffffff810231cd>]
acpi_processor_ffh_cstate_enter+0x2d/0x30
[ 119.505021] [<ffffffffa0001515>] acpi_idle_do_entry+0x10/0x2b
[processor]
[ 119.505028] [<ffffffffa000164a>] acpi_idle_enter_c1+0x5c/0x81
[processor]
[ 119.505035] [<ffffffff8164e83a>] cpuidle_enter_state+0x4a/0xe0
[ 119.505041] [<ffffffff8164e96e>] cpuidle_idle_call+0x9e/0x160
[ 119.505046] [<ffffffff8106092d>] ? __atomic_notifier_call_chain+0xd/0x10
[ 119.505052] [<ffffffff8100af49>] arch_cpu_idle+0x9/0x30
[ 119.505057] [<ffffffff81073241>] cpu_startup_entry+0x91/0x180
[ 119.505063] [<ffffffff81b4be73>] start_secondary+0x1a0/0x1a4
Should these patches be merged into kernel 3.10 line?
And maybe also to 3.12? Or is there any other better solution?
Best regards,
Alex
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 74826b1e2529..3c6d5aec501a 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2621,18 +2621,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Format: <bool> (1/Y/y=enable, 0/N/n=disable)
default: disabled
- printk.offload_chars=
+ printk.offload_chars= [KNL]
Printing to console can be relatively slow especially
in case of serial console. When there is intensive
printing happening from several cpus (as is the case
during boot), a cpu can be spending significant time
(seconds or more) doing printing. To avoid softlockups,
lost interrupts, and similar problems other cpus
- will take over printing after the currently printing
- cpu has printed 'printk.offload_chars' characters.
- Higher value means possibly longer interrupt and other
- latencies but lower overhead of printing due to handing
- over of printing.
+ will take over printing (if CONFIG_PRINTK_OFFLOAD=y)
+ after the currently printing cpu has printed
+ 'printk.offload_chars' characters. Higher value means
+ possibly longer interrupt and other latencies but
+ lower overhead of printing due to handing over of
+ printing.
Format: <number> (0 = disabled)
default: 1000
diff --git a/init/Kconfig b/init/Kconfig
index 009a797dd242..45aa7368d92f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1343,6 +1343,20 @@ config PRINTK
very difficult to diagnose system problems, saying N here is
strongly discouraged.
+config PRINTK_OFFLOAD
+ default y
+ bool "Enable support for offloading printing to different CPU"
+ depends on PRINTK
+ help
+ Printing to console can be relatively slow especially in case of
+ serial console. On large machines when there is intensive printing
+ happening from several cpus (as is the case during boot), a cpu can
+ be spending significant time (seconds or more) doing printing. To
+ avoid softlockups, lost interrupts, and similar problems other cpus
+ will take over printing after the currently printing cpu has printed
+ certain number of characters (tunable via 'printk.offload_chars'
+ kernel parameter).
+
config BUG
bool "BUG() support" if EXPERT
default y
diff --git a/kernel/printk.c b/kernel/printk.c
index 1c0577383af5..c3ad3b834f68 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -103,6 +103,7 @@ enum {
};
static long printk_handover_state;
+#ifdef CONFIG_PRINTK_OFFLOAD
/*
* Number of kernel threads for offloading printing. We need at least two so
* that they can hand over printing from one to another one and thus switch
@@ -116,6 +117,7 @@ static DEFINE_MUTEX(printk_kthread_mutex);
/* Wait queue printing kthreads sleep on when idle */
static DECLARE_WAIT_QUEUE_HEAD(print_queue);
+#endif /* CONFIG_PRINTK_OFFLOAD */
#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_lock_dep_map = {
@@ -284,6 +286,7 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
+#ifdef CONFIG_PRINTK_OFFLOAD
static int offload_chars_set(const char *val, const struct kernel_param *kp);
static struct kernel_param_ops offload_chars_ops = {
.set = offload_chars_set,
@@ -302,6 +305,7 @@ module_param_cb(offload_chars, &offload_chars_ops, &printk_offload_chars,
S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(offload_chars, "offload printing to console to a different"
" cpu after this number of characters");
+#endif
/* human readable text of the record */
static char *log_text(const struct printk_log *msg)
@@ -2021,6 +2025,7 @@ int console_trylock(void)
}
EXPORT_SYMBOL(console_trylock);
+#ifdef CONFIG_PRINTK_OFFLOAD
/*
* This is a version of console_lock() which spins to acquire console_sem.
* It is only for use by threads that take care of flushing printk buffer so
@@ -2052,6 +2057,7 @@ static int console_lock_try_spin(void)
mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);
return 1;
}
+#endif
int is_console_locked(void)
{
@@ -2087,6 +2093,7 @@ out:
raw_spin_unlock_irqrestore(&logbuf_lock, flags);
}
+#ifdef CONFIG_PRINTK_OFFLOAD
/*
* Returns true iff there is other cpu waiting to take over printing. This
* function also takes are of setting PRINTK_HANDOVER_B if we want to hand over
@@ -2113,6 +2120,7 @@ static bool cpu_stop_printing(int printed_chars)
}
return false;
}
+#endif
/**
* console_unlock - unlock the console system
@@ -2155,10 +2163,12 @@ again:
size_t len;
int level;
+#ifdef CONFIG_PRINTK_OFFLOAD
if (cpu_stop_printing(printed_chars)) {
hand_over = true;
break;
}
+#endif
raw_spin_lock_irqsave(&logbuf_lock, flags);
if (seen_seq != log_next_seq) {
@@ -2576,6 +2586,7 @@ int unregister_console(struct console *console)
}
EXPORT_SYMBOL(unregister_console);
+#ifdef CONFIG_PRINTK_OFFLOAD
/* Kthread which takes over printing from a CPU which asks for help */
static int printing_task(void *arg)
{
@@ -2638,6 +2649,7 @@ static int offload_chars_set(const char *val, const struct kernel_param *kp)
mutex_unlock(&printk_kthread_mutex);
return 0;
}
+#endif /* CONFIG_PRINTK_OFFLOAD */
static int __init printk_late_init(void)
{
@@ -2650,9 +2662,11 @@ static int __init printk_late_init(void)
}
hotcpu_notifier(console_cpu_notify, 0);
+#ifdef CONFIG_PRINTK_OFFLOAD
mutex_lock(&printk_kthread_mutex);
printk_start_offload_kthreads();
mutex_unlock(&printk_kthread_mutex);
+#endif
return 0;
}
--
1.8.1.4
diff --git a/kernel/printk.c b/kernel/printk/printk.c
index bd7ee2a9f960..7a8ffd89875c 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -249,9 +249,6 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
-/* cpu currently holding logbuf_lock */
-static volatile unsigned int logbuf_cpu = UINT_MAX;
-
/* human readable text of the record */
static char *log_text(const struct printk_log *msg)
{
@@ -1332,36 +1329,22 @@ static inline int can_use_console(unsigned int cpu)
* messages from a 'printk'. Return true (and with the
* console_lock held, and 'console_locked' set) if it
* is successful, false otherwise.
- *
- * This gets called with the 'logbuf_lock' spinlock held and
- * interrupts disabled. It should return with 'lockbuf_lock'
- * released but interrupts still disabled.
*/
static int console_trylock_for_printk(unsigned int cpu)
- __releases(&logbuf_lock)
{
- int retval = 0, wake = 0;
-
- if (console_trylock()) {
- retval = 1;
-
- /*
- * If we can't use the console, we need to release
- * the console semaphore by hand to avoid flushing
- * the buffer. We need to hold the console semaphore
- * in order to do this test safely.
- */
- if (!can_use_console(cpu)) {
- console_locked = 0;
- wake = 1;
- retval = 0;
- }
- }
- logbuf_cpu = UINT_MAX;
- raw_spin_unlock(&logbuf_lock);
- if (wake)
+ if (!console_trylock())
+ return 0;
+ /*
+ * If we can't use the console, we need to release the console
+ * semaphore by hand to avoid flushing the buffer. We need to hold the
+ * console semaphore in order to do this test safely.
+ */
+ if (!can_use_console(cpu)) {
+ console_locked = 0;
up(&console_sem);
- return retval;
+ return 0;
+ }
+ return 1;
}
int printk_delay_msec __read_mostly;
@@ -1494,6 +1477,9 @@ asmlinkage int vprintk_emit(int facility, int level,
unsigned long flags;
int this_cpu;
int printed_len = 0;
+ /* cpu currently holding logbuf_lock in this function */
+ static volatile unsigned int logbuf_cpu = UINT_MAX;
+
boot_delay_msec(level);
printk_delay();
@@ -1609,13 +1595,12 @@ asmlinkage int vprintk_emit(int facility, int level,
}
printed_len += text_len;
+ logbuf_cpu = UINT_MAX;
+ raw_spin_unlock(&logbuf_lock);
/*
* Try to acquire and then immediately release the console semaphore.
* The release will print out buffers and wake up /dev/kmsg and syslog()
* users.
- *
- * The console_trylock_for_printk() function will release 'logbuf_lock'
- * regardless of whether it actually gets the console semaphore or not.
*/
if (console_trylock_for_printk(this_cpu))
console_unlock();
--
1.8.1.4
diff --git a/include/linux/console.h b/include/linux/console.h
index 7571a16bd653..c61c169f85b3 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -150,6 +150,7 @@ extern int console_trylock(void);
extern void console_unlock(void);
extern void console_conditional_schedule(void);
extern void console_unblank(void);
+extern void console_flush(void);
extern struct tty_driver *console_device(int *);
extern void console_stop(struct console *);
extern void console_start(struct console *);
diff --git a/kernel/printk.c b/kernel/printk.c
index 8d981b2b5bb1..1c0577383af5 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -2306,6 +2306,28 @@ struct tty_driver *console_device(int *index)
}
/*
+ * Wait until all messages accumulated in the printk buffer are printed to
+ * console. Note that as soon as this function returns, new messages may be
+ * added to the printk buffer by other CPUs.
+ */
+void console_flush(void)
+{
+ bool retry;
+ unsigned long flags;
+
+ while (1) {
+ raw_spin_lock_irqsave(&logbuf_lock, flags);
+ retry = console_seq != log_next_seq;
+ raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+ if (!retry || console_suspended)
+ break;
+ /* Cycle console_sem to wait for outstanding printing */
+ console_lock();
+ console_unlock();
+ }
+}
+
+/*
* Prevent further output on the passed console device so that (for example)
* serial drivers can disable console output before suspending a port, and can
* re-enable output afterwards.
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 84571e09c907..14ac740e0c7f 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -21,5 +21,6 @@
#include <linux/smpboot.h>
#include <linux/atomic.h>
+#include <linux/console.h>
/*
* Structure to determine completion condition and record errors. May
@@ -574,6 +575,14 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
return ret;
}
+ /*
+ * If there are lots of outstanding messages, printing them can take a
+ * long time and all cpus would be spinning waiting for the printing to
+ * finish thus triggering NMI watchdog, RCU lockups etc. Wait for the
+ * printing here to avoid these.
+ */
+ console_flush();
+
/* Set the initial state and stop all online cpus. */
set_state(&msdata, MULTI_STOP_PREPARE);
return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
--
1.8.1.4
diff --git a/kernel/printk.c b/kernel/printk.c
index cb7e06850eb5..8d981b2b5bb1 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -109,6 +109,10 @@ static long printk_handover_state;
* CPUs.
*/
#define PRINTING_TASKS 2
+/* Pointers to printing kthreads */
+static struct task_struct *printing_kthread[PRINTING_TASKS];
+/* Serialization of changes to printk_offload_chars and kthread creation */
+static DEFINE_MUTEX(printk_kthread_mutex);
/* Wait queue printing kthreads sleep on when idle */
static DECLARE_WAIT_QUEUE_HEAD(print_queue);
@@ -280,6 +284,12 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
+static int offload_chars_set(const char *val, const struct kernel_param *kp);
+static struct kernel_param_ops offload_chars_ops = {
+ .set = offload_chars_set,
+ .get = param_get_uint,
+};
+
/*
* How many characters can we print in one call of printk before asking
* other cpus to continue printing. 0 means infinity. Tunable via
@@ -288,7 +298,7 @@ static u32 log_buf_len = __LOG_BUF_LEN;
*/
static unsigned int __read_mostly printk_offload_chars = 1000;
-module_param_named(offload_chars, printk_offload_chars, uint,
+module_param_cb(offload_chars, &offload_chars_ops, &printk_offload_chars,
S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(offload_chars, "offload printing to console to a different"
" cpu after this number of characters");
@@ -2571,32 +2581,56 @@ static int printing_task(void *arg)
return 0;
}
-static int __init printk_late_init(void)
+static void printk_start_offload_kthreads(void)
{
- struct console *con;
int i;
struct task_struct *task;
- for_each_console(con) {
- if (!keep_bootcon && con->flags & CON_BOOT) {
- printk(KERN_INFO "turn off boot console %s%d\n",
- con->name, con->index);
- unregister_console(con);
- }
- }
- hotcpu_notifier(console_cpu_notify, 0);
-
- /* Does any handover of printing have any sence? */
- if (num_possible_cpus() <= 1)
- return 0;
-
+ /* Does handover of printing make any sense? */
+ if (printk_offload_chars == 0 || num_possible_cpus() <= 1)
+ return;
for (i = 0; i < PRINTING_TASKS; i++) {
+ if (printing_kthread[i])
+ continue;
task = kthread_run(printing_task, NULL, "print/%d", i);
if (IS_ERR(task)) {
pr_err("printk: Cannot create printing thread: %ld\n",
PTR_ERR(task));
}
+ printing_kthread[i] = task;
}
+}
+
+static int offload_chars_set(const char *val, const struct kernel_param *kp)
+{
+ int ret;
+
+ /* Protect against parallel change of printk_offload_chars */
+ mutex_lock(&printk_kthread_mutex);
+ ret = param_set_uint(val, kp);
+ if (ret) {
+ mutex_unlock(&printk_kthread_mutex);
+ return ret;
+ }
+ printk_start_offload_kthreads();
+ mutex_unlock(&printk_kthread_mutex);
+ return 0;
+}
+
+static int __init printk_late_init(void)
+{
+ struct console *con;
+
+ for_each_console(con) {
+ if (!keep_bootcon && con->flags & CON_BOOT) {
+ unregister_console(con);
+ }
+ }
+ hotcpu_notifier(console_cpu_notify, 0);
+
+ mutex_lock(&printk_kthread_mutex);
+ printk_start_offload_kthreads();
+ mutex_unlock(&printk_kthread_mutex);
return 0;
}
--
1.8.1.4
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 4dae9cbe9259..bd7ee2a9f960 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -206,8 +206,7 @@ struct printk_log {
};
/*
- * The logbuf_lock protects kmsg buffer, indices, counters. It is also
- * used in interesting ways to provide interlocking in console_unlock();
+ * The logbuf_lock protects kmsg buffer, indices, counters.
*/
static DEFINE_RAW_SPINLOCK(logbuf_lock);
--
1.8.1.4
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 7116fda7077f..74826b1e2529 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2621,6 +2621,21 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Format: <bool> (1/Y/y=enable, 0/N/n=disable)
default: disabled
+ printk.offload_chars=
+ Printing to console can be relatively slow especially
+ in case of serial console. When there is intensive
+ printing happening from several cpus (as is the case
+ during boot), a cpu can be spending significant time
+ (seconds or more) doing printing. To avoid softlockups,
+ lost interrupts, and similar problems other cpus
+ will take over printing after the currently printing
+ cpu has printed 'printk.offload_chars' characters.
+ Higher value means possibly longer interrupt and other
+ latencies but lower overhead of printing due to handing
+ over of printing.
+ Format: <number> (0 = disabled)
+ default: 1000
+
printk.time= Show timing data prefixed to each printk message line
Format: <bool> (1/Y/y=enable, 0/N/n=disable)
diff --git a/kernel/printk.c b/kernel/printk.c
index 91c554e027c5..cb7e06850eb5 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -45,6 +45,7 @@
#include <linux/poll.h>
#include <linux/irq_work.h>
#include <linux/utsname.h>
+#include <linux/kthread.h>
#include <asm/uaccess.h>
@@ -87,6 +88,31 @@ static DEFINE_SEMAPHORE(console_sem);
struct console *console_drivers;
EXPORT_SYMBOL_GPL(console_drivers);
+/* State machine for handing over printing */
+enum {
+ /*
+ * Set by the holder of console_sem if currently printing task wants to
+ * hand over printing. Cleared before console_sem is released.
+ */
+ PRINTK_HANDOVER_B,
+ /*
+ * Set if there's someone spinning on console_sem to take over printing.
+ * Cleared after acquiring console_sem.
+ */
+ PRINTK_CONSOLE_SPIN_B,
+};
+static long printk_handover_state;
+
+/*
+ * Number of kernel threads for offloading printing. We need at least two so
+ * that they can hand over printing from one to another one and thus switch
+ * CPUs.
+ */
+#define PRINTING_TASKS 2
+
+/* Wait queue printing kthreads sleep on when idle */
+static DECLARE_WAIT_QUEUE_HEAD(print_queue);
+
#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_lock_dep_map = {
.name = "console_lock"
@@ -254,6 +280,19 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;
+/*
+ * How many characters can we print in one call of printk before asking
+ * other cpus to continue printing. 0 means infinity. Tunable via
+ * printk.offload_chars kernel parameter. Our default 1000 means about
+ * 0.1s maximum latency due to printing.
+ */
+static unsigned int __read_mostly printk_offload_chars = 1000;
+
+module_param_named(offload_chars, printk_offload_chars, uint,
+ S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(offload_chars, "offload printing to console to a different"
+ " cpu after this number of characters");
+
/* human readable text of the record */
static char *log_text(const struct printk_log *msg)
{
@@ -1942,6 +1981,7 @@ void console_lock(void)
if (console_suspended)
return;
console_locked = 1;
+ printk_handover_state = 0;
console_may_schedule = 1;
mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);
}
@@ -1964,12 +2004,45 @@ int console_trylock(void)
return 0;
}
console_locked = 1;
+ printk_handover_state = 0;
console_may_schedule = 0;
mutex_acquire(&console_lock_dep_map, 0, 1, _RET_IP_);
return 1;
}
EXPORT_SYMBOL(console_trylock);
+/*
+ * This is a version of console_lock() which spins to acquire console_sem.
+ * It is only for use by threads that take care of flushing printk buffer so
+ * that they can be sure they are not preempted while waiting for console_sem.
+ *
+ * The function returns 1 if we acquired console_sem, 0 if we failed (either
+ * someone else is already spinning, someone acquired console_sem, or console
+ * is suspended).
+ */
+static int console_lock_try_spin(void)
+{
+ WARN_ON_ONCE(preemptible());
+ /* Someone already spinning? Don't waste cpu time... */
+ if (test_and_set_bit(PRINTK_CONSOLE_SPIN_B, &printk_handover_state))
+ return 0;
+ while (down_trylock(&console_sem)) {
+ /* Someone else took console_sem? */
+ if (!test_bit(PRINTK_CONSOLE_SPIN_B, &printk_handover_state))
+ return 0;
+ cpu_relax();
+ }
+ printk_handover_state = 0;
+ if (console_suspended) {
+ up(&console_sem);
+ return 0;
+ }
+ console_locked = 1;
+ console_may_schedule = 0;
+ mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);
+ return 1;
+}
+
int is_console_locked(void)
{
return console_locked;
@@ -2004,15 +2077,44 @@ out:
raw_spin_unlock_irqrestore(&logbuf_lock, flags);
}
+/*
+ * Returns true iff there is other cpu waiting to take over printing. This
+ * function also takes are of setting PRINTK_HANDOVER_B if we want to hand over
+ * printing to some other cpu.
+ */
+static bool cpu_stop_printing(int printed_chars)
+{
+ /* Oops? Print everything now to maximize chances user will see it */
+ if (oops_in_progress)
+ return false;
+ if (!printk_offload_chars || printed_chars < printk_offload_chars)
+ return false;
+ /* Someone is spinning on console_sem? Give away to him. */
+ if (test_bit(PRINTK_CONSOLE_SPIN_B, &printk_handover_state))
+ return true;
+ if (!test_bit(PRINTK_HANDOVER_B, &printk_handover_state)) {
+ set_bit(PRINTK_HANDOVER_B, &printk_handover_state);
+ /*
+ * Paired with barrier in prepare_to_wait_exclusive() in
+ * printing_task()
+ */
+ smp_mb();
+ wake_up(&print_queue);
+ }
+ return false;
+}
+
/**
* console_unlock - unlock the console system
*
* Releases the console_lock which the caller holds on the console system
* and the console driver list.
*
- * While the console_lock was held, console output may have been buffered
- * by printk(). If this is the case, console_unlock(); emits
- * the output prior to releasing the lock.
+ * While the console_lock was held, console output may have been buffered by
+ * printk(). If this is the case, console_unlock() emits the output prior to
+ * releasing the lock. However we need not write all the data in the buffer if
+ * we would hog the CPU for too long. In such case we try to hand over printing
+ * to a different cpu.
*
* If there is output waiting, we wake /dev/kmsg and syslog() users.
*
@@ -2025,6 +2127,8 @@ void console_unlock(void)
unsigned long flags;
bool wake_klogd = false;
bool retry;
+ bool hand_over = false;
+ int printed_chars = 0;
if (console_suspended) {
up(&console_sem);
@@ -2041,6 +2145,11 @@ again:
size_t len;
int level;
+ if (cpu_stop_printing(printed_chars)) {
+ hand_over = true;
+ break;
+ }
+
raw_spin_lock_irqsave(&logbuf_lock, flags);
if (seen_seq != log_next_seq) {
wake_klogd = true;
@@ -2054,8 +2163,10 @@ again:
console_prev = 0;
}
skip:
- if (console_seq == log_next_seq)
+ if (console_seq == log_next_seq) {
+ raw_spin_unlock(&logbuf_lock);
break;
+ }
msg = log_from_idx(console_idx);
if (msg->flags & LOG_NOCONS) {
@@ -2086,31 +2197,39 @@ skip:
stop_critical_timings(); /* don't trace print latency */
call_console_drivers(level, text, len);
start_critical_timings();
+ printed_chars += len;
local_irq_restore(flags);
}
- console_locked = 0;
- mutex_release(&console_lock_dep_map, 1, _RET_IP_);
/* Release the exclusive_console once it is used */
if (unlikely(exclusive_console))
exclusive_console = NULL;
- raw_spin_unlock(&logbuf_lock);
-
+ /* Save modification of printk_handover_state in the common fast path */
+ if (test_bit(PRINTK_HANDOVER_B, &printk_handover_state))
+ clear_bit(PRINTK_HANDOVER_B, &printk_handover_state);
+ console_locked = 0;
+ mutex_release(&console_lock_dep_map, 1, _RET_IP_);
up(&console_sem);
/*
- * Someone could have filled up the buffer again, so re-check if there's
- * something to flush. In case we cannot trylock the console_sem again,
- * there's a new owner and the console_unlock() from them will do the
- * flush, no worries.
+ * Subtlety: We have interrupts disabled iff hand_over == false (to
+ * save one cli/sti pair in the fast path.
*/
- raw_spin_lock(&logbuf_lock);
- retry = console_seq != log_next_seq;
- raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+ if (!hand_over) {
+ /*
+ * Someone could have filled up the buffer again, so re-check
+ * if there's something to flush. In case we cannot trylock the
+ * console_sem again, there's a new owner and the
+ * console_unlock() from them will do the flush, no worries.
+ */
+ raw_spin_lock(&logbuf_lock);
+ retry = console_seq != log_next_seq;
+ raw_spin_unlock_irqrestore(&logbuf_lock, flags);
- if (retry && console_trylock())
- goto again;
+ if (retry && console_trylock())
+ goto again;
+ }
if (wake_klogd)
wake_up_klogd();
@@ -2148,6 +2267,7 @@ void console_unblank(void)
console_lock();
console_locked = 1;
+ printk_handover_state = 0;
console_may_schedule = 0;
for_each_console(c)
if ((c->flags & CON_ENABLED) && c->unblank)
@@ -2424,9 +2544,38 @@ int unregister_console(struct console *console)
}
EXPORT_SYMBOL(unregister_console);
+/* Kthread which takes over printing from a CPU which asks for help */
+static int printing_task(void *arg)
+{
+ DEFINE_WAIT(wait);
+
+ while (1) {
+ prepare_to_wait_exclusive(&print_queue, &wait,
+ TASK_INTERRUPTIBLE);
+ if (!test_bit(PRINTK_HANDOVER_B, &printk_handover_state))
+ schedule();
+ finish_wait(&print_queue, &wait);
+ /*
+ * We don't want to be scheduled away once we got the CPU (that
+ * would be especially problematic if we hold console_sem at
+ * that moment since noone else could print to console). So
+ * disable preemption and spin on console_sem. We shouldn't
+ * spin for long since printing CPU drops console_sem as soon
+ * as it notices there is someone spinning on it.
+ */
+ preempt_disable();
+ if (console_lock_try_spin())
+ console_unlock();
+ preempt_enable();
+ }
+ return 0;
+}
+
static int __init printk_late_init(void)
{
struct console *con;
+ int i;
+ struct task_struct *task;
for_each_console(con) {
if (!keep_bootcon && con->flags & CON_BOOT) {
@@ -2434,6 +2583,19 @@ static int __init printk_late_init(void)
}
}
hotcpu_notifier(console_cpu_notify, 0);
+
+ /* Does any handover of printing have any sence? */
+ if (num_possible_cpus() <= 1)
+ return 0;
+
+ for (i = 0; i < PRINTING_TASKS; i++) {
+ task = kthread_run(printing_task, NULL, "print/%d", i);
+ if (IS_ERR(task)) {
+ pr_err("printk: Cannot create printing thread: %ld\n",
+ PTR_ERR(task));
+ }
+ }
+
return 0;
}
late_initcall(printk_late_init);
--
1.8.1.4
diff --git a/kernel/printk.c b/kernel/printk.c
index 7a8ffd89875c..56649edfae9c 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -1314,10 +1314,9 @@ static int have_callable_console(void)
/*
* Can we actually use the console at this time on this cpu?
*
- * Console drivers may assume that per-cpu resources have
- * been allocated. So unless they're explicitly marked as
- * being able to cope (CON_ANYTIME) don't call them until
- * this CPU is officially up.
+ * Console drivers may assume that per-cpu resources have been allocated. So
+ * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
+ * call them until this CPU is officially up.
*/
static inline int can_use_console(unsigned int cpu)
{
@@ -1330,8 +1329,10 @@ static inline int can_use_console(unsigned int cpu)
* console_lock held, and 'console_locked' set) if it
* is successful, false otherwise.
*/
-static int console_trylock_for_printk(unsigned int cpu)
+static int console_trylock_for_printk(void)
{
+ unsigned int cpu = smp_processor_id();
+
if (!console_trylock())
return 0;
/*
@@ -1501,7 +1502,8 @@ asmlinkage int vprintk_emit(int facility, int level,
*/
if (!oops_in_progress && !lockdep_recursing(current)) {
recursion_bug = 1;
- goto out_restore_irqs;
+ local_irq_restore(flags);
+ return 0;
}
zap_locks();
}
@@ -1597,17 +1599,22 @@ asmlinkage int vprintk_emit(int facility, int level,
logbuf_cpu = UINT_MAX;
raw_spin_unlock(&logbuf_lock);
+ lockdep_on();
+ local_irq_restore(flags);
+
+ /*
+ * Disable preemption to avoid being preempted while holding
+ * console_sem which would prevent anyone from printing to console
+ */
+ preempt_disable();
/*
* Try to acquire and then immediately release the console semaphore.
* The release will print out buffers and wake up /dev/kmsg and syslog()
* users.
*/
- if (console_trylock_for_printk(this_cpu))
+ if (console_trylock_for_printk())
console_unlock();
-
- lockdep_on();
-out_restore_irqs:
- local_irq_restore(flags);
+ preempt_enable();
return printed_len;
}
--
1.8.1.4
diff --git a/kernel/printk.c b/kernel/printk.c
index 56649edfae9c..91c554e027c5 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -68,6 +68,9 @@ int console_printk[4] = {
DEFAULT_CONSOLE_LOGLEVEL, /* default_console_loglevel */
};
+/* Deferred messaged from sched code are marked by this special level */
+#define SCHED_MESSAGE_LOGLEVEL -2
+
/*
* Low level drivers may need that to know if they can schedule in
* their unblank() callback or not. So let's export it.
@@ -206,7 +209,9 @@ struct printk_log {
};
/*
- * The logbuf_lock protects kmsg buffer, indices, counters.
+ * The logbuf_lock protects kmsg buffer, indices, counters. This can be taken
+ * within the scheduler's rq lock. It must be released before calling
+ * console_unlock() or anything else that might wake up a process.
*/
static DEFINE_RAW_SPINLOCK(logbuf_lock);
@@ -1473,14 +1478,19 @@ asmlinkage int vprintk_emit(int facility, int level,
static int recursion_bug;
static char textbuf[LOG_LINE_MAX];
char *text = textbuf;
- size_t text_len;
+ size_t text_len = 0;
enum log_flags lflags = 0;
unsigned long flags;
int this_cpu;
int printed_len = 0;
+ bool in_sched = false;
/* cpu currently holding logbuf_lock in this function */
static volatile unsigned int logbuf_cpu = UINT_MAX;
+ if (level == SCHED_MESSAGE_LOGLEVEL) {
+ level = -1;
+ in_sched = true;
+ }
boot_delay_msec(level);
printk_delay();
@@ -1527,7 +1537,12 @@ asmlinkage int vprintk_emit(int facility, int level,
* The printf needs to come first; we need the syslog
* prefix which might be passed-in as a parameter.
*/
- text_len = vscnprintf(text, sizeof(textbuf), fmt, args);
+ if (in_sched)
+ text_len = scnprintf(text, sizeof(textbuf),
+ KERN_WARNING "[sched_delayed] ");
+
+ text_len += vscnprintf(text + text_len,
+ sizeof(textbuf) - text_len, fmt, args);
/* mark and strip a trailing newline */
if (text_len && text[text_len-1] == '\n') {
@@ -1602,6 +1617,10 @@ asmlinkage int vprintk_emit(int facility, int level,
lockdep_on();
local_irq_restore(flags);
+ /* If called from the scheduler, we can not call up(). */
+ if (in_sched)
+ return printed_len;
+
/*
* Disable preemption to avoid being preempted while holding
* console_sem which would prevent anyone from printing to console
@@ -2423,21 +2442,19 @@ late_initcall(printk_late_init);
/*
* Delayed printk version, for scheduler-internal messages:
*/
-#define PRINTK_BUF_SIZE 512
-
#define PRINTK_PENDING_WAKEUP 0x01
-#define PRINTK_PENDING_SCHED 0x02
+#define PRINTK_PENDING_OUTPUT 0x02
static DEFINE_PER_CPU(int, printk_pending);
-static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
static void wake_up_klogd_work_func(struct irq_work *irq_work)
{
int pending = __this_cpu_xchg(printk_pending, 0);
- if (pending & PRINTK_PENDING_SCHED) {
- char *buf = __get_cpu_var(printk_sched_buf);
- printk(KERN_WARNING "[sched_delayed] %s", buf);
+ if (pending & PRINTK_PENDING_OUTPUT) {
+ /* If trylock fails, someone else is doing the printing */
+ if (console_trylock())
+ console_unlock();
}
if (pending & PRINTK_PENDING_WAKEUP)
@@ -2461,21 +2478,15 @@ void wake_up_klogd(void)
int printk_sched(const char *fmt, ...)
{
- unsigned long flags;
va_list args;
- char *buf;
int r;
- local_irq_save(flags);
- buf = __get_cpu_var(printk_sched_buf);
-
va_start(args, fmt);
- r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args);
+ r = vprintk_emit(0, SCHED_MESSAGE_LOGLEVEL, NULL, 0, fmt, args);
va_end(args);
- __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
+ __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
- local_irq_restore(flags);
return r;
}
--
1.8.1.4
