linux-kernel - Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task

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Date:   Wed, 29 Apr 2020 13:00:58 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Scott Wood <swood@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task_switch()


On 28/04/20 23:33, Scott Wood wrote:
> On Tue, 2020-04-28 at 22:37 +0100, Valentin Schneider wrote:
>> On 28/04/20 06:02, Scott Wood wrote:
>> > Thus, newidle_balance() is entered with interrupts enabled, which allows
>> > (in the next patch) enabling interrupts when the lock is dropped.
>> >
>> > Signed-off-by: Scott Wood <swood@...hat.com>
>> > ---
>> >  kernel/sched/core.c  |  7 ++++---
>> >  kernel/sched/fair.c  | 45 ++++++++++++++++----------------------------
>> >  kernel/sched/sched.h |  6 ++----
>> >  3 files changed, 22 insertions(+), 36 deletions(-)
>> >
>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> > index 9a2fbf98fd6f..0294beb8d16c 100644
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>> > @@ -3241,6 +3241,10 @@ static struct rq *finish_task_switch(struct
>> > task_struct *prev)
>> >       }
>> >
>> >       tick_nohz_task_switch();
>> > +
>> > +	if (is_idle_task(current))
>> > +		newidle_balance();
>> > +
>>
>> This means we must go through a switch_to(idle) before figuring out we
>> could've switched to a CFS task, and do it then. I'm curious to see the
>> performance impact of that.
>
> Any particular benchmark I should try?
>

I'm going to be very original and suggest hackbench :-)

That would just be the first stop however, you would also want to try
something less wakeup-intensive, maybe sysbench and the like - I'm thinking
if you spawn ~1.5*nr_cpu_ids CPU-hogs, you'll hit that double switch fairly
easily.

And then there's always the big boys benchmarks like specjbb and co - I'd
suggest having a look at Mel's mmtests.