linux-kernel - Re: [PATCH v2] locking/rwsem: reduce spinlock contention in wakeup after up_read/up

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Date:	Fri, 24 Apr 2015 10:56:14 -0400
From:	Waiman Long <waiman.long@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2] locking/rwsem: reduce spinlock contention in wakeup
 after up_read/up_write

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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