Re: [RFC][PATCH] tracing: Introduce restart_critical_timings()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-03-20 12:20, Steven Rostedt wrote:
> From: Steven Rostedt (Google) <rostedt@goodmis.org>
> 
> I'm debugging some latency issues on a Chromebook and the preemptirqsoff
> tracer hit this:
> 
> # tracer: preemptirqsoff
> #
> # preemptirqsoff latency trace v1.1.5 on 5.15.148-21853-g165fd2387469-dirty
> # --------------------------------------------------------------------
> # latency: 7813 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
> #    -----------------
> #    | task: init-1 (uid:0 nice:0 policy:0 rt_prio:0)
> #    -----------------
> #  => started at: rwsem_optimistic_spin
> #  => ended at:   rwsem_optimistic_spin
> #
> #
> #                    _------=> CPU#
> #                   / _-----=> irqs-off
> #                  | / _----=> need-resched
> #                  || / _---=> hardirq/softirq
> #                  ||| / _--=> preempt-depth
> #                  |||| / _-=> migrate-disable
> #                  ||||| /     delay
> #  cmd     pid     |||||| time  |   caller
> #     \   /        ||||||  \    |    /
>     <...>-1         1...1.    0us!: rwsem_optimistic_spin+0x20/0x194 <-rwsem_optimistic_spin+0x20/0x194
>     <...>-1         1.N.1. 7813us : rwsem_optimistic_spin+0x140/0x194 <-rwsem_optimistic_spin+0x140/0x194
>     <...>-1         1.N.1. 7814us+: tracer_preempt_on+0x4c/0x6a <-rwsem_optimistic_spin+0x140/0x194
>     <...>-1         1.N.1. 7824us : <stack trace>
>   => rwsem_optimistic_spin+0x140/0x194
>   => rwsem_down_write_slowpath+0xc9/0x3fe
>   => copy_process+0xd08/0x1b8a
>   => kernel_clone+0x94/0x256
>   => __x64_sys_clone+0x7a/0x9a
>   => do_syscall_64+0x51/0xa1
>   => entry_SYSCALL_64_after_hwframe+0x5c/0xc6
> 
> Which isn't a real issue, as it's in the rwsem_optimistic_spin() code that
> spins with interrupts enabled, preempt disabled, and checks for
> need_resched(). If it is true, it breaks out and schedules.
> 
> Hence, it's hiding real issues, because it can spin for a very long time
> and this is not the source of the latency I'm tracking down.
> 
> I would like to introduce restart_critical_timings() and place it in
> locations that have this behavior.

Is there any way you could move this to need_resched() rather than
sprinkle those everywhere ?

Thanks,

Mathieu

> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> 
> diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
> index 147feebd508c..e9f97f60bfc0 100644
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -145,6 +145,13 @@ do {						\
>   # define start_critical_timings() do { } while (0)
>   #endif
>   
> +/* Used in spins that check need_resched() with preemption disabled */
> +static inline void restart_critical_timings(void)
> +{
> +	stop_critical_timings();
> +	start_critical_timings();
> +}
> +
>   #ifdef CONFIG_DEBUG_IRQFLAGS
>   extern void warn_bogus_irq_restore(void);
>   #define raw_check_bogus_irq_restore()			\
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 2340b6d90ec6..fa7b43e53d20 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -27,6 +27,7 @@
>   #include <linux/export.h>
>   #include <linux/rwsem.h>
>   #include <linux/atomic.h>
> +#include <linux/irqflags.h>
>   #include <trace/events/lock.h>
>   
>   #ifndef CONFIG_PREEMPT_RT
> @@ -780,6 +781,7 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
>   		 */
>   		barrier();
>   
> +		restart_critical_timings();
>   		if (need_resched() || !owner_on_cpu(owner)) {
>   			state = OWNER_NONSPINNABLE;
>   			break;
> @@ -912,6 +914,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>   		 * a writer, need_resched() check needs to be done here.
>   		 */
>   		if (owner_state != OWNER_WRITER) {
> +			restart_critical_timings();
>   			if (need_resched())
>   				break;
>   			if (rt_task(current) &&

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com