linux-kernel - Re: [RFC PATCH 57/86] coccinelle: script to remove cond

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Date:   Mon, 20 Nov 2023 21:16:19 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     paulmck@...nel.org
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
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        Julia Lawall <Julia.Lawall@...ia.fr>,
        Nicolas Palix <nicolas.palix@...g.fr>
Subject: Re: [RFC PATCH 57/86] coccinelle: script to remove cond_resched()


Paul E. McKenney <paulmck@...nel.org> writes:

> On Tue, Nov 07, 2023 at 03:07:53PM -0800, Ankur Arora wrote:
>> Rudimentary script to remove the straight-forward subset of
>> cond_resched() and allies:
>>
>> 1)  if (need_resched())
>> 	  cond_resched()
>>
>> 2)  expression*;
>>     cond_resched();  /* or in the reverse order */
>>
>> 3)  if (expression)
>> 	statement
>>     cond_resched();  /* or in the reverse order */
>>
>> The last two patterns depend on the control flow level to ensure
>> that the complex cond_resched() patterns (ex. conditioned ones)
>> are left alone and we only pick up ones which are only minimally
>> related the neighbouring code.
>
> This series looks to get rid of stall warnings for long in-kernel
> preempt-enabled code paths, which is of course a very good thing.
> But removing all of the cond_resched() calls can actually increase
> scheduling latency compared to the current CONFIG_PREEMPT_NONE=y state,
> correct?

Not necessarily.

If TIF_NEED_RESCHED_LAZY is set, then we let the current task finish
before preempting. If that task runs for arbitrarily long (what Thomas
calls the hog problem) -- currently we allow them to run for upto one
extra tick (which might shorten/become a tunable.)

If TIF_NEED_RESCHED is set, then it gets folded the same it does now
and preemption happens at the next safe preemption point.

So, I guess the scheduling latency would always be bounded but how much
latency a task would incur would be scheduler policy dependent.

This is early days, so the policy (or really the rest of it) isn't set
in stone but having two levels of preemption -- immediate and
deferred -- does seem to give the scheduler greater freedom of policy.

Btw, are you concerned about the scheduling latencies in general or the
scheduling latency of a particular set of tasks?

> If so, it would be good to take a measured approach.  For example, it
> is clear that a loop that does a cond_resched() every (say) ten jiffies
> can remove that cond_resched() without penalty, at least in kernels built
> with either CONFIG_NO_HZ_FULL=n or CONFIG_PREEMPT=y.  But this is not so
> clear for a loop that does a cond_resched() every (say) ten microseconds.

True. Though both of those loops sound bad :).

Yeah, and as we were discussing offlist, the question is the comparative
density of preempt_dec_and_test() is true vs calls to cond_resched().

And if they are similar then we could replace cond_resched() quiescence
reporting with ones in preempt_enable() (as you mention elsewhere in the
thread.)


Thanks

--
ankur