linux-kernel - Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

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Date:	Tue, 14 Jul 2015 21:24:13 -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>
Subject: Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

On 07/13/2015 08:02 AM, Peter Zijlstra wrote:
> On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote:
>> @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node)
>>   			pv_wait(&pn->state, vcpu_halted);
>>
>>   		/*
>> -		 * Reset the vCPU state to avoid unncessary CPU kicking
>> +		 * Reset the state except when vcpu_hashed is set.
>>   		 */
>> -		WRITE_ONCE(pn->state, vcpu_running);
>> +		cmpxchg(&pn->state, vcpu_halted, vcpu_running);
> Why? Suppose we did get advanced into the hashed state, and then get a
> (spurious) wakeup, this means we'll observe our ->locked == 1 condition
> and fall out of pv_wait_node().
>
> We'll then enter pv_wait_head(), which with your modification:
>
>> @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>>   {
>>   	struct pv_node *pn = (struct pv_node *)node;
>>   	struct __qspinlock *l = (void *)lock;
>> -	struct qspinlock **lp = NULL;
>> +	struct qspinlock **lp;
>>   	int loop;
>>
>> +	/*
>> +	 * Initialize lp to a non-NULL value if it has already been in the
>> +	 * pv_hashed state so that pv_hash() won't be called again.
>> +	 */
>> +	lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1
>> +						   : NULL;
>>   	for (;;) {
>> +		WRITE_ONCE(pn->state, vcpu_running);
> Will instantly and unconditionally write vcpu_running.
>
>

This code is kind of complicated. I am going to get rid of the current 
tri-state setup, and switch to a separate sync variable for defer kicking.

Cheers,
Longman
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