Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

From: Dietmar Eggemann
Date: Mon Oct 10 2016 - 09:54:49 EST

On 10/10/16 13:29, Vincent Guittot wrote:
> On 10 October 2016 at 12:01, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
>> On Sun, 09 Oct, at 11:39:27AM, Wanpeng Li wrote:
>>> The difference between this patch and Peterz's is your patch have a
>>> delta since activate_task()->enqueue_task() does do update_rq_clock(),
>>> so why don't have the delta will cause low cpu machines (4 or 8) to
>>> regress against your another reply in this thread?
>> Both my patch and Peter's patch cause issues with low cpu machines. In
>> <20161004201105.GP16071@xxxxxxxxxxxxxxxxxxx> I said,
>> "This patch causes some low cpu machines (4 or 8) to regress. It turns
>> out they regress with my patch too."
>> Have I misunderstood your question?
>> I ran out of time to investigate this last week, though I did try all
>> proposed patches, including Vincent's, and none of them produced wins
>> across the board.
> I have tried to reprocude your issue on my target an hikey board (ARM
> based octo cores) but i failed to see a regression with commit
> 7dc603c9028e. Neverthless, i can see tasks not been well spread

Wasn't this about the two patches mentioned in this thread? The one from
Matt using 'se->sum_exec_runtime' in the if condition in
enqueue_entity_load_avg() and Peterz's conditional call to
update_rq_clock(rq) in enqueue_task()?

> during fork as you mentioned. So I have studied a bit more the
> spreading issue during fork last week and i have a new version of my
> proposed patch that i'm going to send soon. With this patch, i can see
> a good spread of tasks during the fork sequence and some kind of perf
> improvement even if it's bit difficult as the variance is quite
> important with hackbench test so it's mainly an improvement of
> repeatability of the result

Hikey (ARM64 2x4 cpus) board: cpufreq: performance, cpuidle: disabled

Performance counter stats for 'perf bench sched messaging -g 20 -l 500'
(10 runs):

(1) tip/sched/core: commit 447976ef4fd0

5.902209533 seconds time elapsed ( +- 0.31% )

(2) tip/sched/core + original patch on the 'sched/fair: Do not decay
new task load on first enqueue' thread (23/09/16)

5.919933030 seconds time elapsed ( +- 0.44% )

(3) tip/sched/core + Peter's ENQUEUE_NEW patch on the 'sched/fair: Do
not decay new task load on first enqueue' thread (28/09/16)

5.970195534 seconds time elapsed ( +- 0.37% )

Not sure if we can call this a regression but it also shows no
performance gain.

>> I should get a bit further this week.
>> Vincent, Dietmar, did you guys ever get around to submitting your PELT
>> tracepoint patches? Getting some introspection into the scheduler's
> My tarcepoint are not in a shape to be submitted and would need a
> cleanup as some are more hacks for debugging than real trace events.
> Nevertheless, i can push them on a git branch if they can be useful
> for someone

We carry two trace events locally, one for PELT on se and one for
cfs_rq's (I have to add the runnable bits here) which work for
__update_load_avg(), attach_entity_load_avg() and
detach_entity_load_avg(). I could post them but so far mainline has been
reluctant to see the need for PELT related trace events ...