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

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Message-ID: <1284196838.2251.12.camel@laptop>
Date:	Sat, 11 Sep 2010 11:20:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] generic-ipi: fix deadlock in __smp_call_function_single

On Fri, 2010-09-10 at 17:28 -0700, Andrew Morton wrote:
> Where is this scheduler bug?  Did it occur because someone didn't
> understand __smp_call_function_single()?  Or did it occur because the
> scheduler code is doing something which its implementors did not expect
> or intend? 


It comes from 83cd4fe2 (sched: Change nohz idle load balancing logic to
push model), where nohz_balance_kick() simply needs to kick the
designated driver into action.

I take it Venki assumed __smp_call_function_single() works like
smp_call_function_single() where you can use it for the local cpu as
well.

I guess we could do something like the below as well, which would be
slightly faster since we don't actually need to call raise_softirq()
since we already set it for self a bit earlier in order to have it do
the regular load-balance actions.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 kernel/sched_fair.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 9b5b4f8..c8ca1cb 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -3404,11 +3404,13 @@ static void nohz_balancer_kick(int cpu)
 	}
 
 	if (!cpu_rq(ilb_cpu)->nohz_balance_kick) {
-		struct call_single_data *cp;
-
 		cpu_rq(ilb_cpu)->nohz_balance_kick = 1;
-		cp = &per_cpu(remote_sched_softirq_cb, cpu);
-		__smp_call_function_single(ilb_cpu, cp, 0);
+
+		if (ilb_cpu != cpu) {
+			struct call_single_data *cp;
+			cp = &per_cpu(remote_sched_softirq_cb, cpu);
+			__smp_call_function_single(ilb_cpu, cp, 0);
+		}
 	}
 	return;
 }

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