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From: Balbir Singh <bsingharora@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@kernel.org, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, pbonzini@redhat.com, maz@kernel.org,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	riel@surriel.com, hannes@cmpxchg.org
Subject: Re: [PATCH 0/6] sched,delayacct: Some cleanups
Date: Fri, 7 May 2021 22:38:10 +1000	[thread overview]
Message-ID: <20210507123810.GB4236@balbir-desktop> (raw)
In-Reply-To: <YJOzUAg30LZWSHcI@hirez.programming.kicks-ass.net>

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.

  reply	other threads:[~2021-05-07 12:38 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05 10:59 Peter Zijlstra
2021-05-05 10:59 ` [PATCH 1/6] delayacct: Use sched_clock() Peter Zijlstra
2021-05-05 14:40   ` Rik van Riel
2021-05-06 13:59   ` Johannes Weiner
2021-05-06 14:17     ` Peter Zijlstra
2021-05-06 15:17       ` Johannes Weiner
2021-05-07 12:40   ` Balbir Singh
2021-05-12 10:43   ` Mel Gorman
2021-05-05 10:59 ` [PATCH 2/6] sched: Rename sched_info_{queued,dequeued} Peter Zijlstra
2021-05-05 14:39   ` Rik van Riel
2021-05-06 13:59   ` Johannes Weiner
2021-05-10  8:45   ` Balbir Singh
2021-05-12 10:49   ` Mel Gorman
2021-05-05 10:59 ` [PATCH 3/6] sched: Simplify sched_info_on() Peter Zijlstra
2021-05-06 14:03   ` Johannes Weiner
2021-05-12 11:10   ` Mel Gorman
2021-05-12 11:32     ` Peter Zijlstra
2021-05-12 12:51       ` Mel Gorman
2021-05-05 10:59 ` [PATCH 4/6] kvm: Select SCHED_INFO instead of TASK_DELAY_ACCT Peter Zijlstra
2021-05-05 11:37   ` Paolo Bonzini
2021-05-06 14:38   ` Marc Zyngier
2021-05-07 12:42   ` Balbir Singh
2021-05-12 11:11   ` Mel Gorman
2021-05-05 10:59 ` [PATCH 5/6] delayacct: Add static_branch in scheduler hooks Peter Zijlstra
2021-05-06 14:05   ` Johannes Weiner
2021-05-10  8:42   ` Balbir Singh
2021-05-12 11:13   ` Mel Gorman
2021-05-05 10:59 ` [PATCH 6/6] [RFC] delayacct: Default disabled Peter Zijlstra
2021-05-12 11:35   ` Mel Gorman
2021-05-05 22:29 ` [PATCH 0/6] sched,delayacct: Some cleanups Balbir Singh
2021-05-06  9:13   ` Peter Zijlstra
2021-05-07 12:38     ` Balbir Singh [this message]
2021-05-12 11:34       ` Mel Gorman
2021-05-12 11:38         ` Peter Zijlstra
2021-05-12 12:23         ` Paul Wise
2021-05-12 13:00           ` Mel Gorman
2021-05-13  1:29             ` Paul Wise
2021-06-25  0:50         ` Paul Wise
2021-05-07  9:05 ` Thomas Gleixner
2021-05-10  7:08 ` Ingo Molnar
2021-05-10 12:05 ` [PATCH 7/6] delayacct: Add sysctl to enable at runtime Peter Zijlstra
2021-05-10 12:06   ` Peter Zijlstra
2021-05-12 11:40   ` Mel Gorman

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