Re: [PATCH v8 00/25] timer: Move from a push remote at enqueue to a pull at expiry model

[K Prateek Nayak]

Hello Anna-Maria,

On 10/4/2023 6:04 PM, Anna-Maria Behnsen wrote:
> [..snip..]
> 
> Ping Pong Oberservation
> ^^^^^^^^^^^^^^^^^^^^^^^
> 
> During testing on a mostly idle machine a ping pong game could be observed:
> a process_timeout timer is expired remotely on a non idle CPU. Then the CPU
> where the schedule_timeout() was executed to enqueue the timer comes out of
> idle and restarts the timer using schedule_timeout() and goes back to idle
> again. This is due to the fair scheduler which tries to keep the task on
> the CPU which it previously executed on.

Regarding above, are you referring to "wake_up_process(timeout->task)" in
"process_timeout()" ends up waking the task on an idle CPU instead of the
CPU where process_timeout() ran?

In which case, have you tried using the "WF_CURRENT_CPU" flag for the
wakeup? (landed upstream in v6.6-rc1) It is only used by wait queues in
kernel/sched/wait.c currently but perhaps we can have a
"wake_up_process_on_current_cpu()" that process_timeout() can call.

Something along the lines of:

	int wake_up_process_on_current_cpu(struct task_struct *p)
	{
		return try_to_wake_up(p, TASK_NORMAL, WF_CURRENT_CPU);
	}
	EXPORT_SYMBOL(wake_up_process_on_current_cpu);

Thoughts?

> 
> [..snip..]
> 

--
Thanks and Regards,
Prateek