linux-kernel - Re: [ia64] Question on __ARCH_WANT_UNLOCKED

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Date:	Fri, 16 Sep 2011 01:09:20 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Luck, Tony" <tony.luck@...el.com>, Ken Chen <kenchen@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [ia64] Question on __ARCH_WANT_UNLOCKED_CTXSW

On 09/13/11 11:59, Peter Zijlstra wrote:
> On Wed, 2011-08-24 at 13:46 -0700, Luck, Tony wrote:
>>> happen to remember what the perceived benefit of using
>>> __ARCH_WANT_UNLOCKED_CTXSW was about?
>>
>> No - digging around the code hasn't rung any bells for me either.
>>
>> Perhaps just general goodness for not holding a lock for
>> longer than we need to?  But that would imply some case where
>> someone else could do something useful by being able to grab
>> the lock when we drop it. About the only thing I can think
>> of is that it would allow tasks to be re-balanced just a
>> teeny bit earlier --- but re-balancing ought to be somewhat
>> rare, yes?
>
> Mostly yes, except remote wakeups, however that got a complete overhaul
> in 3.0. Instead of taking the remote rq->lock we now enqueue the task on
> a list and IPI the thing, then let the IPI do the remote enqueue and
> trigger the reschedule.
>
> So it might make sense to re-evaluate this on ia64 like Ken suggested..
> then again, who has a large ia64 box and is still willing to put time
> in?

Hum -- perhaps they'll come out of the woodwork if we just rip it out 
(if they exist)

- Paul
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