Subject: + printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch added to -mm tree
To: will.deacon@xxxxxxx,jack@xxxxxxx,kay@xxxxxxxx,peterz@xxxxxxxxxxxxx
From: akpm@xxxxxxxxxxxxxxxxxxxx
Date: Fri, 02 May 2014 14:22:20 -0700
The patch titled
Subject: printk: print initial logbuf contents before re-enabling interrupts
has been added to the -mm tree. Its filename is
printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
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From: Will Deacon <will.deacon@xxxxxxx>
Subject: printk: print initial logbuf contents before re-enabling interrupts
When running on a hideously slow system (~10Mhz FPGA) with a bunch of
debug printk invocations on the timer interrupt path, we end up filling
the log buffer faster than we can drain it.
The reason is that console_unlock (which is responsible for moving
messages out of logbuf to hand over to the console driver) removes one
message at a time, briefly re-enabling interrupts between each of them.
If the interrupt path prints more than a single message, then we can
easily generate more messages than we can print for a regular, recurring
interrupt (e.g. a 1khz timer). This results in messages getting silently
dropped, leading to counter-intuitive, incomplete printk traces on the
console.
Rather than run the console_unlock loop with interrupts disabled (which
has obvious latency problems), this patch records the sequence number of
the last message in the log buffer after taking the logbuf_lock. We can
then print this fixed amount of work before re-enabling interrupts again,
making sure we keep up with ourself. Other CPUs could still potentially
flood the buffer, but there's little that we can do to protect against
that.
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Kay Sievers <kay@xxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/printk/printk.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff -puN kernel/printk/printk.c~printk-print-initial-logbuf-contents-before-re-enabling-interrupts kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-print-initial-logbuf-contents-before-re-enabling-interrupts
+++ a/kernel/printk/printk.c
@@ -2147,10 +2147,13 @@ void console_unlock(void)
again:
for (;;) {
struct printk_log *msg;
+ u64 console_end_seq;
size_t len;
int level;
raw_spin_lock_irqsave(&logbuf_lock, flags);
+ console_end_seq = log_next_seq;
+again_noirq:
if (seen_seq != log_next_seq) {
wake_klogd = true;
seen_seq = log_next_seq;
@@ -2195,6 +2198,12 @@ skip:
stop_critical_timings(); /* don't trace print latency */
call_console_drivers(level, text, len);
start_critical_timings();
+
+ if (console_seq < console_end_seq) {
+ raw_spin_lock(&logbuf_lock);
+ goto again_noirq;
+ }
+
local_irq_restore(flags);
}
console_locked = 0;
_
Patches currently in -mm which might be from will.deacon@xxxxxxx are
origin.patch
printk-print-initial-logbuf-contents-before-re-enabling-interrupts.patch
printk-report-dropping-of-messages-from-logbuf.patch
documentation-devicetree-bindings-add-documentation-for-the-apm-x-gene-soc-rtc-dts-binding.patch
drivers-rtc-add-apm-x-gene-soc-rtc-driver.patch
arm64-add-apm-x-gene-soc-rtc-dts-entry.patch
linux-next.patch
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