linux-kernel - Re: [PATCH 2/2] locking/qrwlock: Don't contend with readers when setting _QW

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Message-ID: <20150609120420.GV3644@twins.programming.kicks-ass.net>
Date:	Tue, 9 Jun 2015 14:04:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 2/2] locking/qrwlock: Don't contend with readers when
 setting _QW_WAITING

On Mon, Jun 08, 2015 at 06:20:44PM -0400, Waiman Long wrote:
> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
> in queue_write_lock_slowpath() will contend with incoming readers
> causing possibly extra cmpxchg() operations that are wasteful. This
> patch changes the code to do a byte cmpxchg() to eliminate contention
> with new readers.

This is very narrow, would not the main cost still be the cacheline
transfers?

Do you have any numbers to back this? I would feel much better about
this if there's real numbers attached.
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