linux-kernel - Re: [PATCH] mm: remove unintentional voluntary preemption in get_mmap_lock

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Date:   Sun, 20 Aug 2023 14:59:07 +0200
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: remove unintentional voluntary preemption in get_mmap_lock_carefully

On Sun, 20 Aug 2023 at 14:47, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But without that odd ifdef, I think it's fine.

Another option might be to just move the might_sleep() to the top, and
do it unconditionally. If the trylock fails, the overhead of possibly
doing a cond_resched() is kind of moot.

IOW, the main problem here is not that it causes a scheduling point
(if the kernel isn't preemptable), it seems to be just that we
unnecessarily schedule in a place with the mm lock is held, so it
unnecessarily causes possible lock contention for writers.

With the per-vma locking catching most cases, does any of this even matter?

Mateusz - on that note: I'm wondering what made you see this as a
problem. The case you quote doesn't actually seem to be threaded, so
the vm lock contention shouldn't actually matter there.

Does it schedule away? Sure. But only if "needs_resched" is set, so it
doesn't seem to be a *bad* thing per se.

It might just be that this particular scheduling point ends up being a
common one on that load, and with those kernel config options (ie
PREEMPT_VOLUNTARY)?

              Linus