linux-kernel - Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to pv

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Date:	Wed, 20 Apr 2016 13:50:43 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument
 to pv_wait()

On 04/20/2016 10:19 AM, Peter Zijlstra wrote:
> On Wed, Apr 20, 2016 at 10:15:09PM +0800, Pan Xinhui wrote:
>> So there is such case that we search the whole hashtable and the lock is not found. :(
>> Waiman assume that if l = null, the lock is not stored. however the lock might be there actually.
>> But to avoid the worst case I just mentioned above, it can quickly finish the lookup.
>
>>>> +
>>>> +			/*
>>>> +			 * We try to locate the queue head pv_node by looking
>>>> +			 * up the hash table. If it is not found, use the
>>>> +			 * CPU in the previous node instead.
>>>> +			 */
>>>> +			hn = pv_lookup_hash(lock);
>>>> +			if (!hn)
>>>> +				hn = pn;
>>> This is potentially expensive... it does not explain why this lookup can
>>> fail etc.. nor mentioned that lock stealing caveat.
>>>
>> Yes, it's expensive. Normally, PPC phyp don't always need the correct
>> holder. That means current vcpu can just give up its slice.  There is
>> one lpar hvcall H_CONFER. I paste some spec below.
> Ok, so if we can indeed scan the _entire_ hashtable, then we really
> should not have that in common code. That's seriously expensive.

No, it should not scan the entire hashtable and there is no point in 
doing so.

Cheers,
Longman