linux-kernel - Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

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Date:	Wed, 13 Apr 2016 11:53:26 -0400
From:	Waiman Long <waiman.long@....com>
To:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Boqun Feng <boqun.feng@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] powerpc: introduce {cmp}xchg for u8 and u16

On 04/13/2016 07:15 AM, Pan Xinhui wrote:
> Hello Peter,
>
> On 2016年04月12日 22:30, Peter Zijlstra wrote:
>
>>> I am working on the qspinlock implementation on PPC.
>>> Your and Waiman's patches are so nice. :)
>> Thanks!, last time I looked at PPC spinlocks they could not use things
>> like ticket locks because PPC might be a guest and fairness blows etc..
>>
>> You're making the qspinlock-paravirt thing work on PPC, or doing
>> qspinlock only for bare-metal PPC?
>>
> I am making the both work. :)
> qspinlock works on PPC now. I am preparing the patches and will send them out in next weeks :)

What of performance improvement are you seeing in PPC?

> The paravirt work is a little hard.
> currently, there are pv_wait() and pv_kick(). but only pv_kick has the parameter cpu(who will hold the lock as soon as the lock is unlocked).
> We need parameter cpu(who holds the lock now) in pv_wait,too.

That can be doable to a certain extent. However, if the current lock 
holder acquired the lock via the fastpath only. The CPU information is 
not logged anywhere. For a contended lock, the information should be there.

Cheers,
Longman