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

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Message-ID: <5717C34D.4000104@hpe.com>
Date:	Wed, 20 Apr 2016 13:58:37 -0400
From:	Waiman Long <waiman.long@....com>
To:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	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 10:36 AM, Pan Xinhui wrote:
>
> On 2016年04月20日 22:19, 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.
>>
> Okay, I will try to add the holder lookup code in arch/...
>
> But I just come up with one idea,
> in __pv_queued_spin_unlock_slowpath()
> we will kick the node->cpu, who will become the holder soon.
> I think we can somehow record the node->cpu and use it in pv_wait_node :)
>
> thanks
> xinhui
>

That will make the code more complex. Unless you find it provide real 
performance improvement, I won't suggest doing that for the time being.

Cheers,
Longman