linux-kernel - Re: [PATCH] generic-ipi: fix deadlock in __smp_call_function

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Message-ID: <1284451427.2275.462.camel@laptop>
Date:	Tue, 14 Sep 2010 10:03:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] generic-ipi: fix deadlock in __smp_call_function_single

On Mon, 2010-09-13 at 11:02 -0700, Suresh Siddha wrote:
> On Sat, 2010-09-11 at 09:42 -0700, Venkatesh Pallipadi wrote:
> > Also, as we don't have rq lock around this point, it seems possible
> > that the CPU that was busy and wants to kick idle load balance on
> > remote CPU, could have become idle and nominated itself as idle load
> > balancer.
> 
> A busy cpu (currently running something -- one task on the rq atleast)
> can't become idle in the middle of trigger_load_balance().
> 
> What might be happening is similar what you said but the opposite of it.
> 
> cpu-x is idle which is also ilb_cpu
> got a scheduler tick during idle
> and the nohz_kick_needed() in trigger_load_balance() checks for
> rq_x->nr_running which might not be zero (because of someone waking a
> task on this rq etc) and this leads to the situation of the cpu-x
> sending a kick to itself.

So what patches are we going to merge?

I share Heiko's opinion on that its somewhat surprising to have
__smp_call_function_single() differ in this detail from
smp_call_function_single() and think that merging his patch would be
good in that respect. But Andrew seemed to have reservations.

We can also merge either my or Suresh's patch (which I think makes
sense, but is kinda subtle) to avoid the needless self kick.

Hmm?
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