Re: [PATCH 1/3] mm, swap: change back to use each swap device's percpu cluster

[Kairui Song <ryncsn@gmail.com>]

On Fri, Nov 14, 2025 at 9:05 AM Baoquan He <bhe@redhat.com> wrote:
> On 11/13/25 at 08:45pm, YoungJun Park wrote:
> > On Thu, Nov 13, 2025 at 02:07:59PM +0800, Kairui Song wrote:
> > > On Sun, Nov 9, 2025 at 8:54 PM Youngjun Park <youngjun.park@lge.com> wrote:
> > > >
> > > > This reverts commit 1b7e90020eb7 ("mm, swap: use percpu cluster as
> > > > allocation fast path").
> > > >
> > > > Because in the newly introduced swap tiers, the global percpu cluster
> > > > will cause two issues:
> > > > 1) it will cause caching oscillation in the same order of different si
> > > >    if two different memcg can only be allowed to access different si and
> > > >    both of them are swapping out.
> > > > 2) It can cause priority inversion on swap devices. Imagine a case where
> > > >    there are two memcg, say memcg1 and memcg2. Memcg1 can access si A, B
> > > >    and A is higher priority device. While memcg2 can only access si B.
> > > >    Then memcg 2 could write the global percpu cluster with si B, then
> > > >    memcg1 take si B in fast path even though si A is not exhausted.
> > > >
> > > > Hence in order to support swap tier, revert commit 1b7e90020eb7 to use
> > > > each swap device's percpu cluster.
> > > >
> > > > Co-developed-by: Baoquan He <bhe@redhat.com>
> > > > Suggested-by: Kairui Song <kasong@tencent.com>
> > > > Signed-off-by: Baoquan He <bhe@redhat.com>
> > > > Signed-off-by: Youngjun Park <youngjun.park@lge.com>
> > >
> > > Hi Youngjun, Baoquan, Thanks for the work on the percpu cluster thing.
> >
> > Hello Kairui,

...

> >
> > Yeah... The rotation rule has indeed changed. I remember the
> > discussion about rotation behavior:
> > https://lore.kernel.org/linux-mm/aPc3lmbJEVTXoV6h@yjaykim-PowerEdge-T330/
> >
> > After that discussion, I've been thinking about the rotation.
> > Currently, the requeue happens after every priority list traversal, and this logic
> > is easily affected by changes.
> > The rotation logic change behavior change is not not mentioned somtimes.
> > (as you mentioned in commit 1b7e90020eb7).
> >
> > I'd like to share some ideas and hear your thoughts:
> >
> > 1. Getting rid of the same priority requeue rule
> >    - same priority devices get priority - 1 or + 1 after requeue
> >      (more add or remove as needed to handle any overlapping priority appropriately)
> >
> > 2. Requeue only when a new cluster is allocated
> >    - Instead of requeueing after every priority list traversal, we
> >      requeue only when a cluster is fully used
> >    - This might have some performance impact, but the rotation behavior
> >      would be similar to the existing one (though slightly different due
> >      to synchronization and logic processing changes)
>
> 2) sounds better to me, and the logic and code change is simpler.
>
> Removing requeue may change behaviour. Swap devices of the same priority
> should be round robin to take.

I agree. We definitely need balancing between devices of the same
priority, cluster based rotation seems good enough.

And I'm thinking if we can have a better rotation mechanism? Maybe
plist isn't the best way to do rotation if we want to minimize the
cost of rotation.