linux-kernel - Re: [External] Re: [PATCH] cpuset: introduce non-blocking cpuset.mems setting option

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Message-ID: <CACSyD1PhM=U1bxqYeZXHojSRWWPB3Y7j30jLLykjRzLuQQzn2Q@mail.gmail.com>
Date: Tue, 24 Jun 2025 16:11:01 +0800
From: Zhongkun He <hezhongkun.hzk@...edance.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Waiman Long <llong@...hat.com>, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, muchun.song@...ux.dev
Subject: Re: [External] Re: [PATCH] cpuset: introduce non-blocking cpuset.mems
 setting option

()

On Thu, Jun 19, 2025 at 8:10 PM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Thu, Jun 19, 2025 at 11:49:58AM +0800, Zhongkun He <hezhongkun.hzk@...edance.com> wrote:
> > In our scenario, when we shrink the allowed cpuset.mems —for example,
> > from nodes 1, 2, 3 to just nodes 2,3—there may still be a large number of pages
> > residing on node 1. Currently, modifying cpuset.mems triggers synchronous memory
> > migration, which results in prolonged and unacceptable service downtime under
> > cgroup v2. This behavior has become a major blocker for us in adopting
> > cgroup v2.
> >
> > Tejun suggested adding an interface to control the migration rate, and
> > I plan to try that later.
>
> It sounds unnecessarily not work-conserving and in principle adding
> cond_resched()s (or eventually having a preemptible kernel) should
> achieve the same. Or how would that project onto service metrics?
> (But I'm not familiar with this migration path, thus I was asking about
> the contention points.)

The cond_resched() is already there, please have a look in
migrate_pages_batch().

The issue(contention ) lies in the fact that, during page migration, the PTE
is replaced with a migration_entry(). If a task attempts to access such a page,
it will be blocked in migration_entry_wait() until the migration completes.
When a large number of hot pages are involved, this can cause significant
service disruption due to prolonged blocking.

Thanks
Zhongkun

>
> > However, we believe that the cpuset.migrate interface in cgroup v1 is
> > also sufficient for our use case and is easier to work with.  :)
>
> Too easy I think, it'd make cpuset.mems only "advisory" constraint. (I
> know it could be justified too but perhaps not as a solution to costly
> migrations.)
>
> Michal