Re: [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

[Peter Zijlstra]

On Tue, Mar 23, 2021 at 02:44:57PM +0100, Vincent Guittot wrote:
> Hi Aurey,
> 
> On Tue, 16 Mar 2021 at 05:27, Li, Aubrey <aubrey.li@xxxxxxxxxxxxxxx> wrote:
> >
> > On 2021/2/24 16:15, Aubrey Li wrote:
> > > A long-tail load balance cost is observed on the newly idle path,
> > > this is caused by a race window between the first nr_running check
> > > of the busiest runqueue and its nr_running recheck in detach_tasks.
> > >
> > > Before the busiest runqueue is locked, the tasks on the busiest
> > > runqueue could be pulled by other CPUs and nr_running of the busiest
> > > runqueu becomes 1 or even 0 if the running task becomes idle, this
> > > causes detach_tasks breaks with LBF_ALL_PINNED flag set, and triggers
> > > load_balance redo at the same sched_domain level.
> > >
> > > In order to find the new busiest sched_group and CPU, load balance will
> > > recompute and update the various load statistics, which eventually leads
> > > to the long-tail load balance cost.
> > >
> > > This patch clears LBF_ALL_PINNED flag for this race condition, and hence
> > > reduces the long-tail cost of newly idle balance.
> >
> > Ping...
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Thanks!