linux-kernel - Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning

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Message-ID: <55F721FC.4000504@hpe.com>
Date:	Mon, 14 Sep 2015 15:37:32 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning

On 09/14/2015 10:10 AM, Peter Zijlstra wrote:
> On Fri, Sep 11, 2015 at 02:37:38PM -0400, Waiman Long wrote:
>> In an overcommitted guest where some vCPUs have to be halted to make
>> forward progress in other areas, it is highly likely that a vCPU later
>> in the spinlock queue will be spinning while the ones earlier in the
>> queue would have been halted. The spinning in the later vCPUs is then
>> just a waste of precious CPU cycles because they are not going to
>> get the lock soon as the earlier ones have to be woken up and take
>> their turn to get the lock.
>>
>> This patch implements an adaptive spinning mechanism where the vCPU
>> will call pv_wait() if the following conditions are true:
>>
>>   1) the vCPU has not been halted before;
>>   2) the previous vCPU is not running.
> Why 1? For the mutex adaptive stuff we only care about the lock holder
> running, right?

The wait-early once logic was there because of the kick-ahead patch as I 
don't want a recently kicked vCPU near the head of the queue to go back 
to sleep too early. However, without kick-ahead, a woken up vCPU should 
now be at the queue head. Indeed, we can remove that check and simplify 
the logic.

BTW, the queue head vCPU at pv_wait_head_and_lock() doesn't wait early, 
it will spin the full threshold as there is no way for it to figure out 
if the lock holder is running or not.

Cheers,
Longman
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