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Wed, 2 May 2018 04:44:54 -0400 Date: Wed, 2 May 2018 10:43:28 +0200 From: Peter Zijlstra To: Daniel Colascione Cc: mathieu.desnoyers@efficios.com, paulmck@linux.vnet.ibm.com, boqun.feng@gmail.com, luto@amacapital.net, davejwatson@fb.com, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, pjt@google.com, Andrew Morton , linux@arm.linux.org.uk, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, Andrew Hunter , andi@firstfloor.org, cl@linux.com, bmaurer@fb.com, rostedt@goodmis.org, josh@joshtriplett.org, torvalds@linux-foundation.org, catalin.marinas@arm.com, will.deacon@arm.com, mtk.manpages@gmail.com, Joel Fernandes Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences Message-ID: <20180502084328.GE12180@hirez.programming.kicks-ass.net> References: <20180430224433.17407-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 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.