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

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Message-ID: <20130928191339.GP18242@two.firstfloor.org>
Date:	Sat, 28 Sep 2013 21:13:39 +0200
From:	Andi Kleen <andi@...stfloor.org>
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>,
	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

> Of course, since then, we may well have screwed things up and now we
> sleep under it, but I still really think it was a mistake to do it in
> the first place.
> 
> So if the primary reason for this is really just that f*cking anon_vma
> lock, then I would seriously suggest:
> 
>  - turn it back into a spinlock (or rwlock_t, since we subsequently
> separated the read and write paths)

Yes please. spinlocks/rwlocks have so much nicer performance behavior than
rwsems/mutexes (which noone seems to fully understand)

We had also significant performance regressions from every such
spinning->sleeping change in the VM (this was just the latest)

And afaik anon_vma is usually hold short.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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