From: Ingo Molnar <mingo@kernel.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
Matt Fleming <matt@codeblueprint.co.uk>,
Mike Galbraith <mgalbraith@suse.de>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] sched: Make schedstats a runtime tunable that is disabled by default v4
Date: Wed, 3 Feb 2016 13:49:21 +0100 [thread overview]
Message-ID: <20160203124921.GA28953@gmail.com> (raw)
In-Reply-To: <20160203113911.GP8337@techsingularity.net>
* Mel Gorman <mgorman@techsingularity.net> wrote:
> On Wed, Feb 03, 2016 at 12:28:49PM +0100, Ingo Molnar wrote:
> >
> > * Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > > Changelog since v3
> > > o Force enable stats during profiling and latencytop
> > >
> > > Changelog since V2
> > > o Print stats that are not related to schedstat
> > > o Reintroduce a static inline for update_stats_dequeue
> > >
> > > Changelog since V1
> > > o Introduce schedstat_enabled and address Ingo's feedback
> > > o More schedstat-only paths eliminated, particularly ttwu_stat
> > >
> > > schedstats is very useful during debugging and performance tuning but it
> > > incurs overhead. As such, even though it can be disabled at build time,
> > > it is often enabled as the information is useful. This patch adds a
> > > kernel command-line and sysctl tunable to enable or disable schedstats on
> > > demand. It is disabled by default as someone who knows they need it can
> > > also learn to enable it when necessary.
> > >
> > > The benefits are workload-dependent but when it gets down to it, the
> > > difference will be whether cache misses are incurred updating the shared
> > > stats or not. [...]
> >
> > Hm, which shared stats are those?
>
> Extremely poor phrasing on my part. The stats share a cache line and the impact
> partially depends on whether unrelated stats share a cache line or not during
> updates.
Yes, but the question is, are there true cross-CPU cache-misses? I.e. are there
any 'global' (or per node) counters that we keep touching and which keep
generating cache-misses?
> > I think we should really fix those as well: those shared stats should be
> > percpu collected as well, with no extra cache misses in any scheduler fast
> > path.
>
> I looked into that but converting those stats to per-cpu counters would incur
> sizable memory overhead. There are a *lot* of them and the basic structure for
> the generic percpu-counter is
>
> struct percpu_counter {
> raw_spinlock_t lock;
> s64 count;
> #ifdef CONFIG_HOTPLUG_CPU
> struct list_head list; /* All percpu_counters are on a list */
> #endif
> s32 __percpu *counters;
> };
We don't have to reuse percpu_counter().
> That's not taking the associated runtime overhead such as synchronising them.
Why do we have to synchronize them in the kernel? User-space can recover them on a
percpu basis and add them up if it wishes to. We can update the schedstat utility
to handle the more spread out fields as well.
Thanks,
Ingo
next prev parent reply other threads:[~2016-02-03 12:49 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-03 11:07 [PATCH 1/1] sched: Make schedstats a runtime tunable that is disabled by default v4 Mel Gorman
2016-02-03 11:28 ` Ingo Molnar
2016-02-03 11:39 ` Mel Gorman
2016-02-03 12:49 ` Ingo Molnar [this message]
2016-02-03 13:32 ` Mel Gorman
2016-02-03 14:56 ` Mel Gorman
2016-02-03 11:51 ` Srikar Dronamraju
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