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From: Peter Zijlstra <peterz@infradead.org>
To: "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc: mgorman@techsingularity.net, vincent.guittot@linaro.org,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	juri.lelli@redhat.com, valentin.schneider@arm.com,
	qais.yousef@arm.com, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com,
	tim.c.chen@linux.intel.com, benbjiang@gmail.com
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting
Date: Tue, 15 Dec 2020 08:59:11 +0100	[thread overview]
Message-ID: <20201215075911.GA3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <c4e31235-e1fb-52ac-99a8-ae943ee0de54@linux.intel.com>

On Tue, Dec 15, 2020 at 11:36:35AM +0800, Li, Aubrey wrote:
> On 2020/12/15 0:48, Peter Zijlstra wrote:
> > We compute the average cost of the total scan, but then use it as a
> > per-cpu scan cost when computing the scan proportion. Fix this by
> > properly computing a per-cpu scan cost.
> > 
> > This also fixes a bug where we would terminate early (!--nr, case) and
> > not account that cost at all.
> 
> I'm a bit worried this may introduce a regression under heavy load.
> The overhead of adding another cpu_clock() and calculation becomes 
> significant when sis_scan is throttled by nr.

The thing is, the code as it exists today makes no sense what so ever.
It's plain broken batshit.

We calculate the total scanning time (irrespective of how many CPUs we
touched), and then use that calculate the number of cpus to scan. That's
just daft.

After this patch we calculate the avg cost of scanning 1 cpu and use
that to calculate how many cpus to scan. Which is coherent and sane.

Maybe it can be improved, but that's a completely different thing.

  reply	other threads:[~2020-12-15  8:00 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-14 16:48 [RFC][PATCH 0/5] select_idle_sibling() wreckage Peter Zijlstra
2020-12-14 16:48 ` [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting Peter Zijlstra
2020-12-15  3:36   ` Li, Aubrey
2020-12-15  7:59     ` Peter Zijlstra [this message]
2020-12-15 11:45       ` Mel Gorman
2020-12-15 12:13       ` Li, Aubrey
2021-01-08 10:27       ` Mel Gorman
2021-01-08 13:01         ` Qais Yousef
2021-01-08 13:47           ` Mel Gorman
2021-01-08 13:41         ` Vincent Guittot
2021-01-08 14:40           ` Mel Gorman
2021-01-08 15:10             ` Vincent Guittot
2021-01-08 16:14               ` Mel Gorman
2021-01-11 14:36                 ` Vincent Guittot
2021-01-11 15:58                   ` Mel Gorman
2021-01-08 19:45               ` Peter Zijlstra
2021-01-09 14:12                 ` Mel Gorman
2021-01-11 14:39                 ` Vincent Guittot
2021-01-08 19:49               ` Peter Zijlstra
2021-01-11 14:52                 ` Vincent Guittot
2021-01-08 20:21         ` Peter Zijlstra
2021-01-09 13:59           ` Mel Gorman
2020-12-14 16:48 ` [RFC][PATCH 2/5] sched/fair: Make select_idle_cpu() proportional to cores Peter Zijlstra
2020-12-23 13:31   ` Vincent Guittot
2020-12-14 16:48 ` [RFC][PATCH 3/5] sched/fair: Remove select_idle_smt() Peter Zijlstra
2020-12-14 16:48 ` [RFC][PATCH 4/5] sched/fair: Merge select_idle_core/cpu() Peter Zijlstra
2020-12-14 16:48 ` [RFC][PATCH 5/5] sched/fair: SIS_PROP the idle core scan Peter Zijlstra
2020-12-16 12:59 ` [RFC][PATCH 0/5] select_idle_sibling() wreckage Li, Aubrey
2020-12-16 18:07   ` Vincent Guittot
2020-12-23 13:23     ` Vincent Guittot
2021-01-04 15:40       ` Mel Gorman

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