Re: [RFC 1/1] mm: page_alloc: replace mm_percpu_wq with kthreads in drain_all_pages

[Suren Baghdasaryan]

On Mon, Mar 7, 2022 at 9:04 AM 'Michal Hocko' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
> On Thu 24-02-22 17:28:19, Suren Baghdasaryan wrote:
> > Sending as an RFC to confirm if this is the right direction and to
> > clarify if other tasks currently executed on mm_percpu_wq should be
> > also moved to kthreads. The patch seems stable in testing but I want
> > to collect more performance data before submitting a non-RFC version.
> >
> >
> > Currently drain_all_pages uses mm_percpu_wq to drain pages from pcp
> > list during direct reclaim. The tasks on a workqueue can be delayed
> > by other tasks in the workqueues using the same per-cpu worker pool.
> > This results in sizable delays in drain_all_pages when cpus are highly
> > contended.
>
> This is not about cpus being highly contended. It is about too much work
> on the WQ context.

Ack.

>
> > Memory management operations designed to relieve memory pressure should
> > not be allowed to block by other tasks, especially if the task in direct
> > reclaim has higher priority than the blocking tasks.
>
> Agreed here.
>
> > Replace the usage of mm_percpu_wq with per-cpu low priority FIFO
> > kthreads to execute draining tasks.
>
> This looks like a natural thing to do when WQ context is not suitable
> but I am not sure the additional resources is really justified. Large
> machines with a lot of cpus would create a lot of kernel threads. Can we
> do better than that?
>
> Would it be possible to have fewer workers (e.g. 1 or one per numa node)
> and it would perform the work on a dedicated cpu by changing its
> affinity? Or would that introduce an unacceptable overhead?

Not sure but I can try implementing per-node kthreads and measure the
performance of the reclaim path, comparing with the current and with
per-cpu approach.

>
> Or would it be possible to update the existing WQ code to use rescuer
> well before the WQ is completely clogged?
> --
> Michal Hocko
> SUSE Labs
>
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