linux-kernel - Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt

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Date:	Wed, 11 Sep 2013 09:35:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Len Brown <lenb@...nel.org>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt_count

On Wed, Sep 11, 2013 at 4:06 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Sep 11, 2013 at 10:25:30AM +0200, Peter Zijlstra wrote:
>> On Tue, Sep 10, 2013 at 06:59:57PM -0700, Andy Lutomirski wrote:
>
>> > It looks like the intel_idle code can get confused if TIF_NEED_RESCHED
>> > is set but the preempt resched bit is not -- the need_resched call
>> > between monitor and mwait won't notice TIF_NEED_RESCHED.
>> >
>> > Is this condition possible?
>>
>> Ah indeed, I'll have to go find all idle loops out there it seems. Now
>> if only they were easy to spot :/
>>
>> I was hoping the generic idle thing was good enough, apparently not
>> quite. Thanks for spotting that.
>
> OK, and the reason I didn't notice is that the entire TS_POLLING thing
> is completely wrecked so we'll get the interrupt anyway.
>

I bet that this improves cross-cpu wakeup latency, too -- the old code
would presumably wake up the cpu and then immediately interrupt it.

It might be nice to rename one or both of need_resched and
test_need_resched, though -- the difference is somewhat inscrutable.

--Andy
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