> On Oct 7, 2022, at 1:19 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Oct 07, 2022 at 03:18:26AM +0000, Joel Fernandes wrote:
>>> On Tue, Oct 04, 2022 at 02:41:56AM +0000, Joel Fernandes (Google) wrote:
>>> From: Uladzislau Rezki <urezki@xxxxxxxxx>
>>>
>>> Slow boot time is seen on KVM running typical Linux distributions due to
>>> SCSI layer calling call_rcu(). Recent changes to save power may be
>>> causing this slowness. Using call_rcu_flush() fixes the issue and brings
>>> the boot time back to what it originally was. Convert it.
>>>
>>> Signed-off-by: Uladzislau Rezki <urezki@xxxxxxxxx>
>>> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>>
>> And I successfully setup Debian on KVM and verified that this fixes it, so
>> now I have a nice reproducible rig for my
>> 'lazy-callback-doing-a-wakeup-detector' (I wrote half the detector thanks to
>> ideas from Steve, and will finish the other half tomorrow or so).
>>
>> Tested-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> Looks like I can catch Vlad's issue with the below patch. Thoughts? Does this
> look reasonable for mainline? (I think so as it is self-contained and the
> debug option is default off, and could be useful down the line).
>
> [ 6.887033 ] rcu: *****************************************************
> [ 6.891242 ] rcu: RCU: A wake up has been detected from a lazy callback!
> [ 6.895377 ] rcu: The callback name is: scsi_eh_inc_host_failed
> [ 6.899084 ] rcu: The task it woke up is: scsi_eh_1 (61)
> [ 6.902405 ] rcu: This could cause performance issues! Check the stack.
> [ 6.906532 ] rcu: *****************************************************
>
>
> [ 17.127128 ] rcu: *****************************************************
> [ 17.131397 ] rcu: RCU: A wake up has been detected from a lazy callback!
> [ 17.135703 ] rcu: The callback name is: scsi_eh_inc_host_failed
> [ 17.139485 ] rcu: The task it woke up is: scsi_eh_1 (61)
> [ 17.142828 ] rcu: This could cause performance issues! Check the stack.
> [ 17.146962 ] rcu: *****************************************************
>
> And thanks to Steve for the binary search code.
>
> One thing I found is I have to ignore kworkers because there are times when a
> work item is queued from a callback and those callbacks don't seem to
> contribute to performance issues. So I am filtering these:
>
> [ 38.631724 ] rcu: The callback name is: thread_stack_free_rcu
> [ 38.635317 ] rcu: The task it woke up is: kworker/3:2 (143)
>
> [ 39.649332 ] rcu: The callback name is: delayed_put_task_struct
> [ 39.653037 ] rcu: The task it woke up is: kworker/0:1 (40)
>
> ---8<-----------------------
>
> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
> Subject: [PATCH] lazy wake debug
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> kernel/rcu/Kconfig | 7 ++
> kernel/rcu/lazy-debug.h | 149 ++++++++++++++++++++++++++++++++++++++++
> kernel/rcu/tree.c | 9 +++
> 3 files changed, 165 insertions(+)
> create mode 100644 kernel/rcu/lazy-debug.h
>
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index edd632e68497..08c06f739187 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -322,4 +322,11 @@ config RCU_LAZY
> To save power, batch RCU callbacks and flush after delay, memory
> pressure or callback list growing too big.
>
> +config RCU_LAZY_DEBUG
> + bool "RCU callback lazy invocation debugging"
> + depends on RCU_LAZY
> + default n
> + help
> + Debugging to catch issues caused by delayed RCU callbacks.
> +
> endmenu # "RCU Subsystem"
> diff --git a/kernel/rcu/lazy-debug.h b/kernel/rcu/lazy-debug.h
> new file mode 100644
> index 000000000000..fc1cc1cb89f0
> --- /dev/null
> +++ b/kernel/rcu/lazy-debug.h
> @@ -0,0 +1,149 @@
> +#include <linux/string.h>
> +#include <linux/spinlock.h>
> +
> +#ifdef CONFIG_RCU_LAZY_DEBUG
> +#include <linux/preempt.h>
> +#include <trace/events/sched.h>
> +
> +static DEFINE_PER_CPU(bool, rcu_lazy_cb_exec) = false;
> +static DEFINE_PER_CPU(void *, rcu_lazy_ip) = NULL;
> +
> +static DEFINE_RAW_SPINLOCK(lazy_funcs_lock);
> +
> +#define FUNC_SIZE 1024
And I know this array can overflow in the future so I will add checks for that in the code if we are going with this patch.
- Joel
> +static unsigned long lazy_funcs[FUNC_SIZE];
> +static int nr_funcs;
> +
> +static void __find_func(unsigned long ip, int *B, int *E, int *N)
> +{
> + unsigned long *p;
> + int b, e, n;
> +
> + b = n = 0;
> + e = nr_funcs - 1;
> +
> + while (b <= e) {
> + n = (b + e) / 2;
> + p = &lazy_funcs[n];
> + if (ip > *p) {
> + b = n + 1;
> + } else if (ip < *p) {
> + e = n - 1;
> + } else
> + break;
> + }
> +
> + *B = b;
> + *E = e;
> + *N = n;
> +
> + return;
> +}
> +
> +static bool lazy_func_exists(void* ip_ptr)
> +{
> + int b, e, n;
> + unsigned long flags;
> + unsigned long ip = (unsigned long)ip_ptr;
> +
> + raw_spin_lock_irqsave(&lazy_funcs_lock, flags);
> + __find_func(ip, &b, &e, &n);
> + raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> +
> + return b <= e;
> +}
> +
> +static int lazy_func_add(void* ip_ptr)
> +{
> + int b, e, n;
> + unsigned long flags;
> + unsigned long ip = (unsigned long)ip_ptr;
> +
> + raw_spin_lock_irqsave(&lazy_funcs_lock, flags);
> + if (nr_funcs >= FUNC_SIZE) {
> + raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> + return -1;
> + }
> +
> + __find_func(ip, &b, &e, &n);
> +
> + if (b > e) {
> + if (n != nr_funcs)
> + memmove(&lazy_funcs[n+1], &lazy_funcs[n],
> + (sizeof(*lazy_funcs) * (nr_funcs - n)));
> +
> + lazy_funcs[n] = ip;
> + nr_funcs++;
> + }
> +
> + raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> + return 0;
> +}
> +
> +static void rcu_set_lazy_context(void *ip_ptr)
> +{
> + bool *flag = this_cpu_ptr(&rcu_lazy_cb_exec);
> + *flag = lazy_func_exists(ip_ptr);
> +
> + if (*flag) {
> + *this_cpu_ptr(&rcu_lazy_ip) = ip_ptr;
> + } else {
> + *this_cpu_ptr(&rcu_lazy_ip) = NULL;
> + }
> +}
> +
> +static void rcu_reset_lazy_context(void)
> +{
> + bool *flag = this_cpu_ptr(&rcu_lazy_cb_exec);
> + *flag = false;
> +}
> +
> +static bool rcu_is_lazy_context(void)
> +{
> + return *(this_cpu_ptr(&rcu_lazy_cb_exec));
> +}
> +
> +static void
> +probe_waking(void *ignore, struct task_struct *p)
> +{
> + if (WARN_ON(!in_nmi() && !in_hardirq() && rcu_is_lazy_context())) {
> + pr_err("*****************************************************\n");
> + pr_err("RCU: A wake up has been detected from a lazy callback!\n");
> + pr_err("The callback name is: %ps\n", *this_cpu_ptr(&rcu_lazy_ip));
> + pr_err("The task it woke up is: %s (%d)\n", p->comm, p->pid);
> + pr_err("This could cause performance issues! Check the stack.\n");
> + pr_err("*****************************************************\n");
> + }
> +}
> +
> +static void rcu_lazy_debug_init(void)
> +{
> + int ret;
> + pr_info("RCU Lazy CB debugging is turned on, system may be slow.\n");
> +
> + ret = register_trace_sched_waking(probe_waking, NULL);
> + if (ret)
> + pr_info("RCU: Lazy debug ched_waking probe could not be registered.");
> +}
> +
> +#else
> +
> +static int lazy_func_add(void* ip_ptr)
> +{
> + return -1;
> +}
> +
> +
> +static void rcu_set_lazy_context(void *ip_ptr)
> +{
> +}
> +
> +static void rcu_reset_lazy_context(void)
> +{
> +}
> +
> +static void rcu_lazy_debug_init(void)
> +{
> +}
> +
> +#endif
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c20544c4aa29..ad8d4e52ae92 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -67,6 +67,7 @@
>
> #include "tree.h"
> #include "rcu.h"
> +#include "lazy-debug.h"
>
> #ifdef MODULE_PARAM_PREFIX
> #undef MODULE_PARAM_PREFIX
> @@ -2245,7 +2246,10 @@ static void rcu_do_batch(struct rcu_data *rdp)
>
> f = rhp->func;
> WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
> +
> + rcu_set_lazy_context(f);
> f(rhp);
> + rcu_reset_lazy_context();
>
> rcu_lock_release(&rcu_callback_map);
>
> @@ -2770,6 +2774,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy)
> }
>
> check_cb_ovld(rdp);
> +
> + if (lazy)
> + lazy_func_add(func);
> +
> if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy))
> return; // Enqueued onto ->nocb_bypass, so just leave.
> // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> @@ -4805,6 +4813,7 @@ void __init rcu_init(void)
> rcu_early_boot_tests();
>
> kfree_rcu_batch_init();
> + rcu_lazy_debug_init();
> rcu_bootup_announce();
> sanitize_kthread_prio();
> rcu_init_geometry();
> --
> 2.38.0.rc1.362.ged0d419d3c-goog
>
