linux-kernel - Re: [PATCH] mm: reduce spinlock contention in release

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Message-ID: <YaTUR9WcGoOG4oLo@casper.infradead.org>
Date:   Mon, 29 Nov 2021 13:23:19 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Hao Lee <haolee.swjtu@...il.com>, Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>, vdavydov.dev@...il.com,
        Shakeel Butt <shakeelb@...gle.com>, cgroups@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: reduce spinlock contention in release_pages()

On Mon, Nov 29, 2021 at 09:39:16AM +0100, Michal Hocko wrote:
> On Fri 26-11-21 16:26:23, Hao Lee wrote:
> [...]
> > I will try Matthew's idea to use semaphore or mutex to limit the number of BE
> > jobs that are in the exiting path. This sounds like a feasible approach for
> > our scenario...
> 
> I am not really sure this is something that would be acceptable. Your
> problem is resource partitioning. Papering that over by a lock is not
> the right way to go. Besides that you will likely hit a hard question on
> how many tasks to allow to run concurrently. Whatever the value some
> workload will very likely going to suffer. We cannot assume admin to
> chose the right value because there is no clear answer for that. Not to
> mention other potential problems - e.g. even more priority inversions
> etc.

I don't see how we get priority inversions.  These tasks are exiting; at
the point they take the semaphore, they should not be holding any locks.
They're holding a resource (memory) that needs to be released, but a
task wanting to acquire memory must already be prepared to sleep.

I see this as being a thundering herd problem.  We have dozens, maybe
hundreds of tasks all trying to free their memory at once.  If we force
the herd to go through a narrow gap, they arrive at the spinlock in an
orderly manner.