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From: Mel Gorman <mgorman@techsingularity.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Li, Aubrey" <aubrey.li@linux.intel.com>,
	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 11:45:06 +0000	[thread overview]
Message-ID: <20201215114506.GB3371@techsingularity.net> (raw)
In-Reply-To: <20201215075911.GA3040@hirez.programming.kicks-ass.net>

On Tue, Dec 15, 2020 at 08:59:11AM +0100, Peter Zijlstra wrote:
> 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.

Which makes it very hard to reason about or change in a "safe" manner as
all sorts of counter-intuitive effects occur.

The series is queued and running and takes 1-2 days. I haven't reviewed
the patches properly (holiday) but it'll be interesting to get some
provisional data at least.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2020-12-15 11:46 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
2020-12-15 11:45       ` Mel Gorman [this message]
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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