Message-ID: <553A598E.8080900@hp.com>
Date: Fri, 24 Apr 2015 10:56:14 -0400
From: Waiman Long <waiman.long@hp.com>
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To: Davidlohr Bueso <dave@stgolabs.net>
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>
Subject: Re: [PATCH v2] locking/rwsem: reduce spinlock contention in wakeup
 after up_read/up_write
References: <1429813449-19641-1-git-send-email-Waiman.Long@hp.com> <1429824690.10273.41.camel@stgolabs.net>
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On 04/23/2015 05:31 PM, Davidlohr Bueso wrote:
> It would be nice to not run into this by accident. Please CC all
> relevant parties ;)
>
> On Thu, 2015-04-23 at 14:24 -0400, Waiman Long wrote:
>> In up_read()/up_write(), 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.  This can be a problem especially for up_read()
>> where many readers can potentially call rwsem_wake() at more or less
>> the same time even though a single call should be enough. This will
>> cause contention in the wait_lock cacheline resulting in delay of
>> the up_read/up_write operations.
> Ok.
>
>> This patch makes the wait_lock taking and the call to __rwsem_do_wake()
>> optional if at least one spinning writer is present.
> But if the lock is taken by readers, like you suggest above, there
> cannot be any active spinners. We always block in these cases.

Yes, you are right. Will fix the log message.

Cheers,
Longman
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