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

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Date:	Mon, 13 Sep 2010 11:02:21 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 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.

Perhaps the more appropriate patch would be(?):

Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
---
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 134f7ed..5b5aa97 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -3632,7 +3632,7 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
 	if (time_before(now, nohz.next_balance))
 		return 0;
 
-	if (!rq->nr_running)
+	if (rq->idle_at_tick)
 		return 0;
 
 	first_pick_cpu = atomic_read(&nohz.first_pick_cpu);


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