linux-kernel - Re: [PATCH 4/4] locking/qrwlock: Use direct MCS lock/unlock in slowpath

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Date:	Tue, 07 Jul 2015 17:59:59 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH 4/4] locking/qrwlock: Use direct MCS lock/unlock in slowpath

On 07/07/2015 07:24 AM, Peter Zijlstra wrote:
> On Mon, Jul 06, 2015 at 11:43:06AM -0400, Waiman Long wrote:
>> Lock waiting in the qrwlock uses the spinlock (qspinlock for x86)
>> as the waiting queue. This is slower than using MCS lock directly
>> because of the extra level of indirection causing more atomics to
>> be used as well as 2 waiting threads spinning on the lock cacheline
>> instead of only one.
> This needs a better explanation. Didn't we find with the qspinlock thing
> that the pending spinner improved performance on light loads?
>
> Taking it out seems counter intuitive, we could very much like these two
> the be the same.

Yes, for lightly loaded case, using raw_spin_lock should have an 
advantage. It is a different matter when the lock is highly contended. 
In this case, having the indirection in qspinlock will make it slower. I 
struggle myself as to whether to duplicate the locking code in qrwlock. 
So I send this patch out to test the water. I won't insist if you think 
this is not a good idea, but I do want to get the previous 2 patches in 
which should not be controversial.

Cheers,
Longman
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