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

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Message-ID: <1329748861.2293.345.camel@twins>
Date:	Mon, 20 Feb 2012 15:41:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: sched: Avoid SMT siblings in select_idle_sibling() if possible

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.


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