linux-kernel - Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

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Date:   Wed, 2 May 2018 10:43:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     mathieu.desnoyers@...icios.com, paulmck@...ux.vnet.ibm.com,
        boqun.feng@...il.com, luto@...capital.net, davejwatson@...com,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        pjt@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux@....linux.org.uk, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, Andrew Hunter <ahh@...gle.com>, andi@...stfloor.org,
        cl@...ux.com, bmaurer@...com, rostedt@...dmis.org,
        josh@...htriplett.org, torvalds@...ux-foundation.org,
        catalin.marinas@....com, will.deacon@....com,
        mtk.manpages@...il.com, Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

On Wed, May 02, 2018 at 03:53:47AM +0000, Daniel Colascione wrote:
> The usual approach to "better" is an "adaptive mutex". Such a thing, when
> it attempts to acquire a lock another thread owns, spins for some number of
> iterations, then falls back to futex. I guess that's a little better than
> immediately jumping to futex, but it's not optimal. We can still spin when
> the lock owner isn't scheduled, and the spin count is usually some guess
> (either specified manually or estimated statistically) that's not
> guaranteed to produce decent results. Even if we do pick a good spin count,
> we run a very good chance of under- or over-spinning on any given
> lock-acquire. We always want to sleep when spinning would be pointless.

Look for the FUTEX_LOCK patches from Waiman.