linux-kernel - Re: [patch] sched: beef up wake

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Date:	Tue, 14 Jul 2015 16:17:46 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josef Bacik <jbacik@...com>, riel@...hat.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	kernel-team <Kernel-team@...com>
Subject: Re: [patch] sched: beef up wake_wide()

On Tue, 2015-07-14 at 16:07 +0200, Peter Zijlstra wrote:
> On Tue, Jul 14, 2015 at 03:49:17PM +0200, Mike Galbraith wrote:
> > On Tue, 2015-07-14 at 13:19 +0200, Peter Zijlstra wrote:
> > 
> > > OK, how about something like the below; it tightens things up by
> > > applying two rules:
> > > 
> > >  - We really should not continue looking for a balancing domain once
> > >    SD_LOAD_BALANCE is not set.
> > > 
> > >  - SD (balance) flags should really be set in a single contiguous range,
> > >    always starting at the bottom.
> > > 
> > > The latter means what if !want_affine and the (first) sd doesn't have
> > > BALANCE_WAKE set, we're done. Getting rid of (most of) that iteration
> > > junk you didn't like..
> > > 
> > > Hmm?
> > 
> > Yeah, that's better.  It's not big hairy deal either way, it just bugged
> > me to knowingly toss those cycles out the window ;-)
> > 
> > select_idle_sibling() looks kinda funny down there, but otoh when the
> > day comes (hah) that we can just balance, it's closer to the exit.
> 
> Right, not too pretty, does this look beter?

There's a buglet, I was just about to mention the inverse in the other.


> @@ -5041,17 +5037,17 @@ select_task_rq_fair(struct task_struct *
>  
>  		if (tmp->flags & sd_flag)
>  			sd = tmp;
> +		else if (!want_affine)
> +			break;
>  	}
>  
> -	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> -		prev_cpu = cpu;
> +	if (affine_sd) { /* Prefer affinity over any other flags */
> +		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> +			new_cpu = cpu;
>  
> -	if (sd_flag & SD_BALANCE_WAKE) {
> -		new_cpu = select_idle_sibling(p, prev_cpu);
> -		goto unlock;
> -	}
> +		new_cpu = select_idle_sibling(p, new_cpu);

We'll not look for a idle cpu when wake_wide() naks want_affine.

	-Mike

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