linux-kernel - Re: [PATCH] sched/fair: ensure tasks spreading in LLC during LB

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Message-ID: <20201110125506.GJ2594@hirez.programming.kicks-ass.net>
Date:   Tue, 10 Nov 2020 13:55:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org, riel@...riel.com, clm@...com,
        hannes@...xchg.org
Subject: Re: [PATCH] sched/fair: ensure tasks spreading in LLC during LB

On Mon, Nov 02, 2020 at 11:24:57AM +0100, Vincent Guittot wrote:
> schbench shows latency increase for 95 percentile above since:
>   commit 0b0695f2b34a ("sched/fair: Rework load_balance()")
> 
> Align the behavior of the load balancer with the wake up path, which tries
> to select an idle CPU which belongs to the LLC for a waking task.
> 
> calculate_imbalance() will use nr_running instead of the spare
> capacity when CPUs share resources (ie cache) at the domain level. This
> will ensure a better spread of tasks on idle CPUs.
> 
> Running schbench on a hikey (8cores arm64) shows the problem:
> 
> tip/sched/core :
> schbench -m 2 -t 4 -s 10000 -c 1000000 -r 10
> Latency percentiles (usec)
> 	50.0th: 33
> 	75.0th: 45
> 	90.0th: 51
> 	95.0th: 4152
> 	*99.0th: 14288
> 	99.5th: 14288
> 	99.9th: 14288
> 	min=0, max=14276
> 
> tip/sched/core + patch :
> schbench -m 2 -t 4 -s 10000 -c 1000000 -r 10
> Latency percentiles (usec)
> 	50.0th: 34
> 	75.0th: 47
> 	90.0th: 52
> 	95.0th: 78
> 	*99.0th: 94
> 	99.5th: 94
> 	99.9th: 94
> 	min=0, max=94
> 
> Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
> Reported-by: Chris Mason <clm@...com>
> Suggested-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!