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

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Message-ID: <p5dssax5ac6ndongdhp4bvnmy3gz4pswdxoogbmy66bhk5zqzg@bfurmve7htwi>
Date: Tue, 1 Jul 2025 10:16:18 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Zhongkun He <hezhongkun.hzk@...edance.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 Tue, Jun 24, 2025 at 04:11:01PM +0800, Zhongkun He <hezhongkun.hzk@...edance.com> wrote:
> The cond_resched() is already there, please have a look in
> migrate_pages_batch().

Thanks, this is enlightening.

> 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.

migration_entry_wait() waits only for a single page (folio?) to be
migrated. How can the number of pages affect the disruption? Or do you
mean that these individual waits add up and the service is generally
slowed down by that? If the migration was spread out over longer time,
the cummulative slowdown would be the same.

Thanks,
Michal

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