linux-kernel - Re: [PATCH v3 2/2] pid: only take pidmap

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Message-ID: <CAGudoHEsq=1YDYYmN3uu_HMoVz0Rts1w=bkOYNP3084PsSdOSg@mail.gmail.com>
Date: Tue, 16 Dec 2025 19:19:53 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Tycho Andersen <tycho@...nel.org>
Cc: oleg@...hat.com, brauner@...nel.org, linux-kernel@...r.kernel.org, 
	akpm@...ux-foundation.org, linux-mm@...ck.org, willy@...radead.org
Subject: Re: [PATCH v3 2/2] pid: only take pidmap_lock once on alloc

On Tue, Dec 16, 2025 at 6:09 PM Tycho Andersen <tycho@...nel.org> wrote:
> In a particularly pathological case I can get a substantial
> improvement in kernel CPU time with this patch:
>
> Before:   Time (mean ± σ):     53.007 s ±  0.148 s    [User: 430.891 s, System: 12406.459 s]
> After :   Time (mean ± σ):     45.624 s ±  0.394 s    [User: 435.021 s, System: 7692.072 s]
>
> across 10 different boots. Feel free to add
>
> Tested-by: Tycho Andersen (AMD) <tycho@...nel.org>
>

thanks for testing

> > +                      * (the routine unfortunately returns void, so we have no idea if it got anywhere).
>
> Is it worth changing the return type? The underlying function looks
> like it knows whether the allocation was actually successful...
>

willy says IDR is a stale api and the intent is to move consumers off
of it. the loop is already a result of not changing the api, see v1
which add preload for more than one buffer.

Whatever replacement should definitely indicate something like this if
preload exists for its case.