Re: [PATCH v4 2/6] perf: Enqueue SIGTRAP always via task_work.

[Sebastian Andrzej Siewior]

On 2024-11-08 23:26:36 [+0100], Frederic Weisbecker wrote:
> > Please see
> > https://lore.kernel.org/all/1440816150.8932.123.camel@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > and the whole thread.

Thank you for this Oleg.

> > I don't think raw_spin_lock_irq + cmpxchg for each work is a good
> > idea, but quite possibly I misunderstood this change.
> 
> I did not realize there could be gazillion files released in a row. So there
> could be noticeable performance issues I guess...

I made a testcase to open 2M (2 *10^6) files and then exit. This led
task_work_run() run 2M + 3 callbacks (+ stdin/out/err) for the task.

Running 70 samples on the "orig" kernel:
- avg callback time 1.156.470,3 us
- 63 samples are starting with 11 (1,1 sec) avg: 1.128.046,7 us
- 6 samples are starting with 14 (1,4 sec) avg: 1.435.294,8us

Running 70 samples on the "patched" kernel:
- avg callback time 1.278.938,8 us
- 59 samples are starting with 12 (1,2 sec) avg: 1.230.189,1 us
- 10 samples are starting with 15 (1,5sec) avg: 1.555.934,5 us

With the extra lock the task_work_run() runtime extends by approximately
~122ms for the 2M invoked callbacks. 
The spike 1,1sec -> 1,4sec or 1,2sec -> 1,5 sec is due to context
switching (there are few cond_resched()/ might_sleep()).

It is not that bad, is it?

> Thanks.
> > 
> > Oleg.

Sebastian