linux-kernel - Re: [PATCH 04/11] qspinlock: Extract out the exchange of tail code word

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Date:	Wed, 18 Jun 2014 12:02:32 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Waiman Long <waiman.long@...com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, tglx@...utronix.de,
	mingo@...nel.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	paolo.bonzini@...il.com, boris.ostrovsky@...cle.com,
	paulmck@...ux.vnet.ibm.com, riel@...hat.com,
	torvalds@...ux-foundation.org, raghavendra.kt@...ux.vnet.ibm.com,
	david.vrabel@...rix.com, oleg@...hat.com, gleb@...hat.com,
	scott.norton@...com, chegu_vinod@...com,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 04/11] qspinlock: Extract out the exchange of tail code
 word

> >>However, I *do* agree with you that it's simpler to just squash this patch
> >>into 01/11.
> >Uh, did I say that? Oh I said why don't make it right the first time!
> >
> >I meant in terms of seperating the slowpath (aka the bytelock on the pending
> >bit) from the queue (MCS code). Or renaming the function to be called
> >'complex' instead of 'slowpath' as it is getting quite hairy.
> >
> >The #1 patch is nice by itself - as it lays out the foundation of the
> >MCS-similar code - and if Ingo decides he does not want this pending
> >byte-lock bit business - it can be easily reverted or dropped.
> 
> The pending bit code is needed for performance parity with ticket spinlock
> for light load. My own measurement indicates that the queuing overhead will
> cause the queue spinlock to be slower than ticket spinlock with 2-4
> contending tasks. The pending bit solves the performance problem with 2

Aha!

> contending tasks, leave only the 3-4 tasks cases being a bit slower than the
> ticket spinlock which should be more than compensated by its superior
> performance with heavy contention and slightly better performance with no
> contention.

That should be mentioned in the commit description as the rationale for
the patch "qspinlock: Add pending bit" and also in the code.

Thank you!
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