linux-kernel - Re: [PATCH] mm/page_alloc: Occasionally relinquish zone lock in batch freeing

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Message-Id: <20250819144421.7a52f8df3f0fe5c315f90aa2@linux-foundation.org>
Date: Tue, 19 Aug 2025 14:44:21 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Chris Mason <clm@...com>,
 Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...le.com>,
 Michal Hocko <mhocko@...e.com>, Brendan Jackman <jackmanb@...gle.com>, Zi
 Yan <ziy@...dia.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 kernel-team@...a.com
Subject: Re: [PATCH] mm/page_alloc: Occasionally relinquish zone lock in
 batch freeing

On Tue, 19 Aug 2025 08:18:45 -0700 Joshua Hahn <joshua.hahnjy@...il.com> wrote:

> > Pretty this isn't.
> > 
> > Sigh, we do so much stuff here and in __free_one_page().
> > 
> > What sort of guarantee do we have that the contending task will be able
> > to get in and grab the spinlock in that tiny time window?
> 
> Thank you for pointing this out -- I don't think there is any guarantee.
> Kiryl suggested that I put a cond_resched() here, in order to guarantee that
> the contending tasks will be able to grab the spinlock. I think that's a great
> idea -- I'll make this change in v2.

cond_resched() might help because it takes more CPU cycles and expands
the window. A udelay() would of course do this more nicely.

But the contending task is already in state TASK_RUNNING so a
cond_resched() won't have any effect on it?

Also, callers hold pcp->lock, so cond_resched() cannot be called.

Sigh, I dunno, it's all very nasty.  I have vague memories of there
being a way of relinquishing a lock to some other task which is
spinning on that lock.  Or at least, a proposal.  Or I dreamed it. 
peterz would be a good person to ask.