linux-kernel - Re: [PATCH v2] sched: Warn on long periods of pending need

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Date:   Wed, 24 Mar 2021 15:36:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>, linux-doc@...r.kernel.org,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] sched: Warn on long periods of pending need_resched

On Wed, Mar 24, 2021 at 01:39:16PM +0000, Mel Gorman wrote:

> > Yeah, lets say I was pleasantly surprised to find it there :-)
> > 
> 
> Minimally, lets move that out before it gets kicked out. Patch below.

OK, stuck that in front.

> > > Moving something like sched_min_granularity_ns will break a number of
> > > tuning guides as well as the "tuned" tool which ships by default with
> > > some distros and I believe some of the default profiles used for tuned
> > > tweak kernel.sched_min_granularity_ns
> > 
> > Yeah, can't say I care. I suppose some people with PREEMPT=n kernels
> > increase that to make their server workloads 'go fast'. But I'll
> > absolutely suck rock on anything desktop.
> > 
> 
> Broadly speaking yes and despite the lack of documentation, enough people
> think of that parameter when tuning for throughput vs latency depending on
> the expected use of the machine.  kernel.sched_wakeup_granularity_ns might
> get tuned if preemption is causing overscheduling. Same potentially with
> kernel.sched_min_granularity_ns and kernel.sched_latency_ns. That said, I'm
> struggling to think of an instance where I've seen tuning recommendations
> properly quantified other than the impact on microbenchmarks but I
> think there will be complaining if they disappear. I suspect that some
> recommended tuning is based on "I tried a number of different values and
> this seemed to work reasonably well".

Right, except that due to that scaling thing, you'd have to re-evaluate
when you change machine.

Also, do you have any inclination on the perf difference we're talking
about? (I should probably ask Google and not you...)

> kernel.sched_schedstats probably should not depend in SCHED_DEBUG because
> it has value for workload analysis which is not necessarily about debugging
> per-se. It might simply be informing whether another variable should be
> tuned or useful for debugging applications rather than the kernel.

Dubious, if you're that far down the rabit hole, you're dang near
debugging.

> As an aside, I wonder how often SCHED_DEBUG has been enabled simply
> because LATENCYTOP selects it -- no idea offhand why LATENCYTOP even
> needs SCHED_DEBUG.

Perhaps schedstats used to rely on debug? I can't remember. I don't
think I've used latencytop in at least 10 years. ftrace and perf sorta
killed the need for it.

> > These knobs really shouldn't have been as widely available as they are.
> > 
> 
> Probably not. Worse, some of the tuning is probably based on "this worked
> for workload X 10 years ago so I'll just keep doing that"

That sounds like an excellent reason to disrupt ;-)

> > And guides, well, the writes have to earn a living too, right.
> > 
> 
> For most of the guides I've seen they either specify values without
> explaining why or just describe roughly what the parameter does and it's
> not always that accurate a description.

Another good reason.

> > > Whether there are legimiate reasons to modify those values or not,
> > > removing them may generate fun bug reports.
> > 
> > Which I'll close with -EDONTCARE, userspace has to cope with
> > SCHED_DEBUG=n in any case.
> 
> True but removing the throughput vs latency parameters is likely to
> generate a lot of noise even if the reasons for tuning are bad ones.
> Some definitely should not be depending on SCHED_DEBUG, others may
> need to be moved to debugfs one patch at a time so they can be reverted
> individually if complaining is excessive and there is a legiminate reason
> why it should be tuned. It's possible that complaining will be based on
> a workload regression that really depended on tuned changing parameters.

The way I've done it, you can simply re-instate the systl table entry
and it'll work again, except for the entries that had a custom handler.

I'm ready to disrupt :-)