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

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Message-ID: <20250819153459.2088221-1-joshua.hahnjy@gmail.com>
Date: Tue, 19 Aug 2025 08:34:58 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Chris Mason <clm@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.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 Mon, 18 Aug 2025 11:58:03 -0700 Joshua Hahn <joshua.hahnjy@...il.com> wrote:

Cc-ing Suren, who works at Google, not Gogle. Sorry for the typo!
Joshua

> While testing workloads with high sustained memory pressure on large machines
> (1TB memory, 316 CPUs), we saw an unexpectedly high number of softlockups.
> Further investigation showed that the lock in free_pcppages_bulk was being held
> for a long time, even being held while 2k+ pages were being freed.
> 
> Instead of holding the lock for the entirety of the freeing, check to see if
> the zone lock is contended every pcp->batch pages. If there is contention,
> relinquish the lock so that other processors have a change to grab the lock
> and perform critical work.
> 
> In our fleet, we have seen that performing batched lock freeing has led to
> significantly lower rates of softlockups, while incurring relatively small
> regressions (relative to the workload and relative to the variation).