linux-kernel - Re: [PATCH 0/3] further damage-control lack of clone scalability

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Message-ID: <aSMhj2Q6BTMNSsA4@casper.infradead.org>
Date: Sun, 23 Nov 2025 15:00:31 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: oleg@...hat.com, brauner@...nel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/3] further damage-control lack of clone scalability

On Sun, Nov 23, 2025 at 07:30:51AM +0100, Mateusz Guzik wrote:
> When spawning and killing threads in separate processes in parallel the
> primary bottleneck on the stock kernel is pidmap_lock, largely because
> of a back-to-back acquire in the common case.
> 
> Benchmark code at the end.
> 
> With this patchset alloc_pid() only takes the lock once and consequently
> alleviates the problem. While scalability improves, the lock remains the
> primary bottleneck by a large margin.
> 
> I believe idr is a poor choice for the task at hand to begin with, but
> sorting out that out beyond the scope of this patchset. At the same time
> any replacement would be best evaluated against a state where the
> above relock problem is fixed.

Good news!  The IDR is deprecated.  Bad news!  I'm not 100% sure that
the XArray is quite appropriate for this usecase.  I am opposed to
introducing more IDR APIs.  Have you looked at converting to the XArray?
Or do you have a better data structure in mind than the XArray?