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

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Date:	Sat, 28 Sep 2013 12:33:36 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	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

On Sat, Sep 28, 2013 at 12:21 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> If we do that then I suspect the next step will be queued rwlocks :-/ The
> current rwlock_t implementation is rather primitive by modern standards.
> (We'd probably have killed rwlock_t long ago if not for the
> tasklist_lock.)

Yeah, I'm not happy about or rwlocks. That's one lock that currently
is so broken that I think we could easily argue for making that one
queued.

Waiman had a qrwlock series that looked reasonable, and I think his
later versions were drop-in replacements (ie they automatically just
did the RightThing(tm) wrt interrupts taking a recursive read lock - I
objected to the first versions that required that to be stated
explicitly).

I think Waiman's patches (even the later ones) made the queued rwlocks
be a side-by-side implementation with the old rwlocks, and I think
that was just being unnecessarily careful. It might be useful for
testing to have a config option to switch between the two, but we
might as well go all the way.

The old rwlock's really have been a disappointment - they are slower
than spinlocks, and seldom/never end up scaling any better.  Their
main advantage was literally the irq behavior - allowing readers to
happen without the expense of worrying about irq's.

              Linus
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