linux-kernel - Re: [GIT PULL] sigqueue cache fix

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Date:   Wed, 07 Jul 2021 11:47:47 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] sigqueue cache fix

Linus,

On Mon, Jun 28 2021 at 12:02, Linus Torvalds wrote:
> And as the whole - and only - point of the sigqueue cache was a very
> subtle performance and latency issue, I don't think we want to use
> locks or atomics. It's why my revert commit suggests re-purposing the
> "percpu_cmpxchg()" functionality: that would likely be a good model at
> least for x86 and arm.
>
> But while we have "percpu_cmpxchg()", I don't think we currently don't
> really have that kind of operation where it
>
>  (a) works on a non-percpu variable (but we can force casts, I guess)
>
>  (b) has "acquire" semantics
>
> We do have the *atomic* cmpxchg with acquire semantics, but atomics
> are rather more expensive than we'd probably really want.

first of all, sorry for being non-responsive on this. I'll have a fresh
look at this with your comments in mind and be more careful.

Thanks,

        tglx