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

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Message-ID: <20131001082547.GB21793@gmail.com>
Date:	Tue, 1 Oct 2013 10:25:47 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.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


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Oct 01, 2013 at 09:28:15AM +0200, Ingo Molnar wrote:
>
> > That I mostly agree with, except that without a serious usecase do we 
> > have a guarantee that bugs in fancies queueing in rwsems gets ironed 
> > out?
> 
> Methinks mmap_sem is still a big enough lock to work out a locking 
> primitive :-)

I mean the AIM7 usecase probably falls away - we need to find another one 
that shows the inefficiencies.

> In fact, try something like this from userspace:
> 
> n-threads:
> 
>   pthread_mutex_lock(&mutex);
>   foo = mmap();
>   pthread_mutex_lock(&mutex);
> 
>   /* work */
> 
>   pthread_mutex_unlock(&mutex);
>   munma(foo);
>   pthread_mutex_unlock(&mutex);
> 
> vs
> 
> n-threads:
> 
>   foo = mmap();
>   /* work */
>   munmap(foo);
> 
> 
> I've had reports that the former was significantly faster than the
> latter.

That looks like a legitimate pattern that ought to trigger in many apps. 
Would be nice to turn this into a:

	perf bench mm thread-create

testcase or so.

Thanks,

	Ingo
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