Message-ID: <1430911095.1940.28.camel@stgolabs.net>
Subject: Re: [PATCH v4 1/2] locking/rwsem: reduce spinlock contention in
 wakeup after up_read/up_write
From: Davidlohr Bueso <dave@stgolabs.net>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, Jason Low <jason.low2@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Douglas Hatch <doug.hatch@hp.com>
Date: Wed, 06 May 2015 04:18:15 -0700
In-Reply-To: <1430428337-16802-2-git-send-email-Waiman.Long@hp.com>
References: <1430428337-16802-1-git-send-email-Waiman.Long@hp.com>
	 <1430428337-16802-2-git-send-email-Waiman.Long@hp.com>
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On Thu, 2015-04-30 at 17:12 -0400, Waiman Long wrote:
> In up_write()/up_read(), rwsem_wake() will be called whenever it
> detects that some writers/readers are waiting. The rwsem_wake()
> function will take the wait_lock and call __rwsem_do_wake() to do the
> real wakeup.  For a heavily contended rwsem, doing a spin_lock() on
> wait_lock will cause further contention on the heavily contended rwsem
> cacheline resulting in delay in the completion of the up_read/up_write
> operations.
> 
> This patch makes the wait_lock taking and the call to __rwsem_do_wake()
> optional if at least one spinning writer is present. The spinning
> writer will be able to take the rwsem and call rwsem_wake() later
> when it calls up_write(). With the presence of a spinning writer,
> rwsem_wake() will now try to acquire the lock using trylock. If that
> fails, it will just quit.
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

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