linux-kernel - Re: [PATCH 0/6] sched,delayacct: Some cleanups

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Date:   Fri, 7 May 2021 22:38:10 +1000
From:   Balbir Singh <bsingharora@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, pbonzini@...hat.com, maz@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        riel@...riel.com, hannes@...xchg.org
Subject: Re: [PATCH 0/6] sched,delayacct: Some cleanups

On Thu, May 06, 2021 at 11:13:52AM +0200, Peter Zijlstra wrote:
> On Thu, May 06, 2021 at 08:29:40AM +1000, Balbir Singh wrote:
> > On Wed, May 05, 2021 at 12:59:40PM +0200, Peter Zijlstra wrote:
> > > Hi,
> > > 
> > > Due to:
> > > 
> > >   https://lkml.kernel.org/r/0000000000001d43ac05c0f5c6a0@google.com
> > > 
> > > and general principle, delayacct really shouldn't be using ktime (pvclock also
> > > really shouldn't be doing what it does, but that's another story). This lead me
> > > to looking at the SCHED_INFO, SCHEDSTATS, DELAYACCT (and PSI) accounting hell.
> > > 
> > > The rest of the patches are an attempt at simplifying all that a little. All
> > > that crud is enabled by default for distros which is leading to a death by a
> > > thousand cuts.
> > > 
> > > The last patch is an attempt at default disabling DELAYACCT, because I don't
> > > think anybody actually uses that much, but what do I know, there were no ill
> > > effects on my testbox. Perhaps we should mirror
> > > /proc/sys/kernel/sched_schedstats and provide a delayacct sysctl for runtime
> > > frobbing.
> > >
> > 
> > There are tools like iotop that use delayacct to display information. 
> 
> Right, but how many actual people use that? Does that justify saddling
> the whole sodding world with the overhead?
>

Not sure I have that data.
 
> > When the
> > code was checked in, we did run SPEC* back in the day 2006 to find overheads,
> > nothing significant showed. Do we have any date on the overhead your seeing?
> 
> I've not looked, but having it disabled saves that per-task allocation
> and that spinlock in delayacct_end() for iowait wakeups and a bunch of
> cache misses ofcourse.
> 
> I doubt SPEC is a benchmark that tickles those paths much if at all.
> 
> The thing is; we can't just keep growing more and more stats, that'll
> kill us quite dead.


I don't disagree, we've had these around for a while and I know of users
that use these stats to find potential starvation. I am OK with default
disabled. I suspect distros will have the final say.

Balbir Singh.