Re: [PATCH v4] sched/fair: limit sched slice duration

[Mel Gorman]

On Fri, Jan 13, 2023 at 02:36:13PM +0100, Vincent Guittot wrote:
> In presence of a lot of small weight tasks like sched_idle tasks, normal
> or high weight tasks can see their ideal runtime (sched_slice) to increase
> to hundreds ms whereas it normally stays below sysctl_sched_latency.
> 
> 2 normal tasks running on a CPU will have a max sched_slice of 12ms
> (half of the sched_period). This means that they will make progress
> every sysctl_sched_latency period.
> 
> If we now add 1000 idle tasks on the CPU, the sched_period becomes
> 3006 ms and the ideal runtime of the normal tasks becomes 609 ms.
> It will even become 1500ms if the idle tasks belongs to an idle cgroup.
> This means that the scheduler will look for picking another waiting task
> after 609ms running time (1500ms respectively). The idle tasks change
> significantly the way the 2 normal tasks interleave their running time
> slot whereas they should have a small impact.
> 
> Such long sched_slice can delay significantly the release of resources
> as the tasks can wait hundreds of ms before the next running slot just
> because of idle tasks queued on the rq.
> 
> Cap the ideal_runtime to sysctl_sched_latency to make sure that tasks will
> regularly make progress and will not be significantly impacted by
> idle/background tasks queued on the rq.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Tested-By: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs