linux-kernel - Re: [PATCH v3] sched: balance_cpu to consider other cpus in its group as target of (pinned) task

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Message-ID: <20120621034059.GA9748@linux.vnet.ibm.com>
Date:	Thu, 21 Jun 2012 09:10:59 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Prashanth Nageshappa <prashanth@...ux.vnet.ibm.com>,
	mingo@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	roland@...nel.org, efault@....de, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v3] sched: balance_cpu to consider other cpus in its
 group as target of (pinned) task

* Peter Zijlstra <peterz@...radead.org> [2012-06-20 19:38:21]:

> On Tue, 2012-06-19 at 17:43 +0530, Prashanth Nageshappa wrote:
> > T2 will starve eternally in this case. The same
> > scenario can arise in presence of non-rt tasks as well (say we replace F1 with
> > high irq load or with a very high priority SCHED_OTHER task that can't move out
> > of C2). 
> 
> Uhm, no. In the case where both F1 and T2 are SCHED_OTHER starvation is
> impossible.
> 
> What can happen with pure SCHED_OTHER affinities is being less fair than
> desired.

Right ..sorry should have made that more explicit in the description.

> Anyway, I took the patch with a few minor edits

Thanks!

> -- ie. we don't need to
> reset loop_break, its never changed (same for the ALL_PINNED patch you
> sent).

Hmm ..I can see loop_break being incremented here:

                /* take a breather every nr_migrate tasks */
                if (env->loop > env->loop_break) {
                        env->loop_break += sched_nr_migrate_break;
                        env->flags |= LBF_NEED_BREAK;
                        goto out;
                }

As a result, when we redo with a different src_cpu, both loop and
loop_break could be at non-default values.  Am I missing something here?

- vatsa

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