linux-kernel - Re: sched: Avoid SMT siblings in select_idle

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Date:	Mon, 20 Feb 2012 20:33:49 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <efault@....de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

* Peter Zijlstra <peterz@...radead.org> [2012-02-20 15:41:01]:

> On Fri, 2011-11-18 at 16:14 +0100, Mike Galbraith wrote:
> 
> > ---
> >  kernel/sched_fair.c |   10 ++--------
> >  1 file changed, 2 insertions(+), 8 deletions(-)
> > 
> > Index: linux-3.0-tip/kernel/sched_fair.c
> > ===================================================================
> > --- linux-3.0-tip.orig/kernel/sched_fair.c
> > +++ linux-3.0-tip/kernel/sched_fair.c
> > @@ -2276,17 +2276,11 @@ static int select_idle_sibling(struct ta
> >  		for_each_cpu_and(i, sched_domain_span(sd), tsk_cpus_allowed(p)) {
> >  			if (idle_cpu(i)) {
> >  				target = i;
> > +				if (sd->flags & SD_SHARE_CPUPOWER)
> > +					continue;
> >  				break;
> >  			}
> >  		}
> > -
> > -		/*
> > -		 * Lets stop looking for an idle sibling when we reached
> > -		 * the domain that spans the current cpu and prev_cpu.
> > -		 */
> > -		if (cpumask_test_cpu(cpu, sched_domain_span(sd)) &&
> > -		    cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
> > -			break;
> >  	}
> >  	rcu_read_unlock();
> 
> Mike, Suresh, did we ever get this sorted? I was looking at
> select_idle_sibling() and it looks like we dropped this.
> 
> Also, did anybody ever get an answer from a HW guy on why sharing stuff
> over SMT threads is so much worse than sharing it over proper cores? Its
> not like this workload actually does anything concurrently.
> 
> I was looking at this code due to vatsa wanting to do SD_BALANCE_WAKE.

>From a quick scan of that code, it seems to prefer selecting an idle cpu
in the same cache domain (vs selecting prev_cpu in absence of a core
that is fully idle). 

I can give that a try for my benchmark and see how much it helps. My
suspicion is it will not fully solve the problem I have on hand.

- vatsa

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