linux-kernel - Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

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Message-ID: <20190903200603.GW2349@hirez.programming.kicks-ass.net>
Date:   Tue, 3 Sep 2019 22:06:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

On Tue, Sep 03, 2019 at 12:18:47PM -0700, Linus Torvalds wrote:
> Now, if you can point to some particular field where that ordering
> makes sense for the particular case of "make it active on the
> runqueue" vs "look up the task from the runqueue using RCU", then I do
> think that the whole release->acquire consistency makes sense.
> 
> But it's not clear that such a field exists, particularly when this is
> in no way the *common* way to even get a task pointer, and other paths
> do *not* use the runqueue as the serialization point.

Even if we could find a case (and I'm not seeing one in a hurry), I
would try really hard to avoid adding extra barriers here and instead
make the consumer a little more complicated if at all possible.

The Power folks got rid of a SYNC (yes, more expensive than LWSYNC) from
their __switch_to() implementation and that had a measurable impact.

9145effd626d ("powerpc/64: Drop explicit hwsync in context switch")