linux-kernel - Re: [PATCH v1] sched: fix nohz idle load balancer issues

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Date:	Wed, 28 Sep 2011 09:45:01 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Venki Pallipadi <venki@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] sched: fix nohz idle load balancer issues

* Suresh Siddha <suresh.b.siddha@...el.com> [2011-09-27 16:49:36]:

> One of the reasons why we saw lib_cpu not idle is probably because that
> info was stale.
> 
> Consider this scenario.
> 
> a. got a tick when the cpu was busy, so idle_at_tick was not set
> b. cpu went idle
> c. same cpu got the kick IPI from other busy cpu
> d. and as it has idle_at_tick not set, it couldn't proceed with the nohz
> idle balance.

Good point ..we chould use idle_cpu() instead there ..

> I think we are mostly likely seeing the above mentioned scenario.
> 
> Also Vatsa, there is a deadlock associated by using
> __smp_call_funciton_single() in the nohz_balancer_kick(). So I am
> planning to remove the IPI that is used to kick the nohz balancer and
> instead use the resched_cpu logic to kick the nohz balancer.
> 
> I will post this patch mostly tomorrow. That patch will not use the
> idle_at_tick check in the nohz_idle_balance(). So that should address
> your issue in some cases if not most.

Ok ..would be glad to test your change ..I am however doubtfull if it
will eliminate rest of the issues I pointed out with nohz load balancer.

- vatsa
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