Messages in this thread | | | Date | Wed, 20 Mar 2024 14:35:52 -0400 | Subject | Re: [RFC][PATCH] tracing: Introduce restart_critical_timings() | From | Mathieu Desnoyers <> |
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On 2024-03-20 13:58, Steven Rostedt wrote: > On Wed, 20 Mar 2024 13:15:39 -0400 > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote: > >>> 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 ? > > Because need_resched() itself does not mean it's going to schedule > immediately. I looked at a few locations that need_resched() is called. > Most are in idle code where the critical timings are already handled. > > I'm not sure I'd add it for places like mm/memory.c or drivers/md/bcache/btree.c. > > A lot of places look to use it more for PREEMPT_NONE situations as a open > coded cond_resched(). > > The main reason this one is particularly an issue, is that it spins as long > as the owner is still running. Which may be some time, as here it was 7ms.
What I think we should be discussing here is how calling need_resched() should interact with the latency tracked by critical timings.
AFAIU, when code explicitly calls need_resched() in a loop, there are two cases:
- need_resched() returns false: This means the loop can continue without causing long latency on the system. Technically we could restart the critical timings at this point.
- need_resched() returns true: This means the loop should exit quickly and call the scheduler. I would not reset the critical timings there, as whatever code is executed between need_resched() returning true and calling the scheduler is adding to latency.
Having stop/start critical timings around idle loops seems to just be an optimization over that.
As for mm and driver/md code, what is wrong with doing a critical timings reset when need_resched() returns false ? It would prevent a whole class of false-positives rather than playing whack-a-mole with those that pop up.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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