linux-kernel - Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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Message-ID: <CANN689GLde8ZY5yLadbgGrC3eagrY_quQRnTdD_nywP8m1McjA@mail.gmail.com>
Date:	Sun, 29 Sep 2013 18:11:13 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

On Sat, Sep 28, 2013 at 11:55 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> Btw, I really hate that thing. I think we should turn it back into a
> spinlock. None of what it protects needs a mutex or an rwsem.
>
> Because you guys talk about the regression of turning it into a rwsem,
> but nobody talks about the *original* regression.
>
> And it *used* to be a spinlock, and it was changed into a mutex back
> in 2011 by commit 2b575eb64f7a. That commit doesn't even have a reason
> listed for it, although my dim memory of it is that the reason was
> preemption latency.

I was wondering about that too. Regarding latencies, we used to have
unbounded latencies for anon_vma operations as the AVC chains could
get long under some workloads; now that we index the VMAs matching a
given anon_vma with an interval tree this particular source of
latencies should be gone. So yes, it could be worth trying to go back
to a non-sleeping lock.

That said, I am very scared of using rwlock_t here, and I would much
prefer we choose a fair lock (either spinlock or a new rwlock
implementation which guarantees not to starve any locker thread)

-- 
Michel Lespinasse
A program is never fully debugged until the last user dies.
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