Re: [PATCH v2 4/4] psi: Optimize task switch inside shared cgroups

[Johannes Weiner]

On Wed, Mar 03, 2021 at 11:46:59AM +0800, Chengming Zhou wrote:
> The commit 36b238d57172 ("psi: Optimize switching tasks inside shared
> cgroups") only update cgroups whose state actually changes during a
> task switch only in task preempt case, not in task sleep case.
> 
> We actually don't need to clear and set TSK_ONCPU state for common cgroups
> of next and prev task in sleep case, that can save many psi_group_change
> especially when most activity comes from one leaf cgroup.
> 
> sleep before:
> psi_dequeue()
>   while ((group = iterate_groups(prev)))  # all ancestors
>     psi_group_change(prev, .clear=TSK_RUNNING|TSK_ONCPU)
> psi_task_switch()
>   while ((group = iterate_groups(next)))  # all ancestors
>     psi_group_change(next, .set=TSK_ONCPU)
> 
> sleep after:
> psi_dequeue()
>   nop
> psi_task_switch()
>   while ((group = iterate_groups(next)))  # until (prev & next)
>     psi_group_change(next, .set=TSK_ONCPU)
>   while ((group = iterate_groups(prev)))  # all ancestors
>     psi_group_change(prev, .clear=common?TSK_RUNNING:TSK_RUNNING|TSK_ONCPU)
> 
> When a voluntary sleep switches to another task, we remove one call of
> psi_group_change() for every common cgroup ancestor of the two tasks.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>