linux-kernel - Re: [PATCH 1/2] sched: don't rebalance if attached on NULL domain

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Date:	Thu, 5 Mar 2009 02:03:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: don't rebalance if attached on NULL domain

On Thu, Mar 05, 2009 at 01:27:02AM +0100, Frederic Weisbecker wrote:
> Impact: fix function graph trace hang / drop pointless softirq on UP
> 
> While debugging a function graph trace hang on an old PII, I saw that it
> consumed most of its time on the timer interrupt.
> And the domain rebalancing softirq was the most concerned.
> 
> The timer interrupt calls trigger_load_balance() which will decide if it is
> worth to schedule a rebalancing softirq.
> 
> In case of builtin UP kernel, no problem arises because there is no
> domain question.
> 
> In case of builtin SMP kernel running on an SMP box, still no problem,
> the softirq will be raised each time we reach the next_balance time.
> 
> In case of builtin SMP kernel running on a UP box (most distros provide default SMP
> kernels, whatever the box you have), then the CPU is attached to the NULL sched domain.
> So a kind of unexpected behaviour happen:
> 
> trigger_load_balance() -> raises the rebalancing softirq
> later on softirq: run_rebalance_domains() -> rebalance_domains() where
> the for_each_domain(cpu, sd) is not taken because of the NULL domain we are attached at.
> Which means rq->next_balance is never updated.
> So on the next timer tick, we will enter trigger_load_balance() which will always reschedule()
> the rebalacing softirq:
> 
> if (time_after_eq(jiffies, rq->next_balance))
> 	raise_softirq(SCHED_SOFTIRQ);
> 
> So for each tick, we process this pointless softirq.
> 
> This patch fixes it by checking if we are attached to the null domain before raising the softirq,
> another possible fix would be to set the maximal possible JIFFIES value to rq->next_balance if we are
> attached to the NULL domain.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>


And speacking about the function graph hang, Reported-by: Ingo Molnar <mingo@...e.hu>

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