Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg

From: Vincent Guittot <vincent.guittot@linaro.org>
To: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Mike Galbraith <efault@gmx.de>, Paul Turner <pjt@google.com>,
	Chris Mason <clm@fb.com>,
	kernel-team@fb.com
Subject: Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg
Date: Wed, 26 Apr 2017 12:21:52 +0200	[thread overview]
Message-ID: <CAKfTPtC92nVXCH3QX-Qqf5R5gD58pk2=S_OpwiTao5y16g84Xw@mail.gmail.com> (raw)
In-Reply-To: <20170425210810.GB20255@wtj.duckdns.org>

On 25 April 2017 at 23:08, Tejun Heo <tj@kernel.org> wrote:
> On Tue, Apr 25, 2017 at 11:49:41AM -0700, Tejun Heo wrote:
>> > I have run a quick test with your patches and schbench on my platform.
>> > I haven't been able to reproduce your regression but my platform is
>> > quite different from yours (only 8 cores without SMT)
>> > But most importantly, the parent cfs_rq->runnable_load_avg never
>> > reaches 0 (or almost 0) when it is idle. Instead, it still has a
>> > runnable_load_avg (this is not due to rounding computation) whereas
>> > runnable_load_avg should be 0
>>
>> Heh, let me try that out.  Probably a silly mistake somewhere.
>
> This is from the follow-up patch.  I was confused.  Because we don't
> propagate decays, we still should decay the runnable_load_avg;
> otherwise, we end up accumulating errors in the counter.  I'll drop
> the last patch.

Ok, the runnable_load_avg goes back to 0 when I drop patch 3. But i
see  runnable_load_avg sometimes significantly higher than load_avg
which is normally not possible as load_avg = runnable_load_avg +
sleeping task's load_avg

Then, I just have the opposite behavior on my platform. I see a
increase of latency at p99 with your patches.
My platform is a hikey : 2x4 cores ARM and I have used schbench -m 2
-t 4 -s 10000 -c 15000 -r 30 so I have 1 worker thread per CPU which
is similar to what you are doing on your platform

With v4.11-rc8. I have run 10 times the test and get consistent results
schbench -m 2 -t 4 -s 10000 -c 15000 -r 30
Latency percentiles (usec)
50.0000th: 255
75.0000th: 350
90.0000th: 454
95.0000th: 489
*99.0000th: 539
99.5000th: 585
99.9000th: 10224
min=0, max=13567

With your patches i see an increase of the latency for p99. I run 10
times the test too and half tests show latency increase like below
schbench$ ./schbench -m 2 -t 4 -s 10000 -c 15000 -r 30
Latency percentiles (usec)
50.0000th: 216
75.0000th: 295
90.0000th: 395
95.0000th: 444
*99.0000th: 2034
99.5000th: 5960
99.9000th: 12240
min=0, max=14744

>
> Thanks.
>
> --
> tejun