linux-kernel - Re: [RFC PATCH for 4.16 02/21] rseq: Introduce restartable sequences system call (v12)

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Message-ID: <20171214194853.GE3326@worktop>
Date:   Thu, 14 Dec 2017 20:48:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.16 02/21] rseq: Introduce restartable sequences
 system call (v12)

On Thu, Dec 14, 2017 at 12:50:13PM -0600, Christopher Lameter wrote:
> Ultimately I wish fast increments like done by this_cpu_inc() could be
> implemented in an efficient way on non x86 platforms that do not have
> cheap instructions like that.

So the problem isn't migration; for that we could wrap the operation in
preempt_disable() which is not more expensive than rseq would be. And a
lot more deterministic.

The problem instead is interrupts, which can result in nested load-store
operations, and that comes apart. This then means having to disable
interrupts over these things and _that_ is expensive.