linux-kernel - Re: [RFC,v2 3/3] sched: ignore task_h_load for CPU_NEWLY

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Date:   Mon, 13 Feb 2017 14:51:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Uladzislau 2 Rezki <uladzislau2.rezki@...ymobile.com>
Subject: Re: [RFC,v2 3/3] sched: ignore task_h_load for CPU_NEWLY_IDLE

On Thu, Feb 09, 2017 at 07:54:05PM +0100, Uladzislau Rezki wrote:

> > Does this patch make an actual difference, if so how much and with
> > what workload?
> >
> Yes, it does. I see a slight improvement when it comes to frame drops
> (in my case drops per/two seconds). Basically a test case is left finger
> swipe on the display (21 times, duration is 2 seconds + 1 second sleep
> between iterations):
> 
> 0   Framedrops:  7    5
> 1   Framedrops:  5    3
> 2   Framedrops:  8    5
> 3   Framedrops:  4    5
> 4   Framedrops:  3    3
> 5   Framedrops:  6    4
> 6   Framedrops:  3    2
> 7   Framedrops:  3    4
> 8   Framedrops:  5    3
> 9   Framedrops:  3    3
> 10 Framedrops:  7    4
> 11 Framedrops:  3    4
> 12 Framedrops:  3    3
> 13 Framedrops:  3    3
> 14 Framedrops:  3    5
> 15 Framedrops:  7    3
> 16 Framedrops:  5    3
> 17 Framedrops:  3    2
> 18 Framedrops:  5    3
> 19 Framedrops:  4    3
> 20 Framedrops:  3    2
> 
> max is 8 vs 5; min is 2 vs 3.
> 
> As for applied load, it is not significant and i would say is "light".

So that is useful information that should have been in the Changelog.

OK, can you respin this patch with adjusted Changelog and taking Mike's
feedback?

Also, I worry about the effects of this on !PREEMPT kernels, the first
hunk (which explicitly states is about latency) should be under
CONFIG_PREEMPT to match the similar case we already have in
detach_tasks().

But your second hunk, which ignores the actual load of tasks in favour
of just moving _something_ already, is utterly dangerous if not coupled
with these two other conditions, so arguably that too should be under
CONFIG_PREEMPT.