linux-kernel - Re: [PATCH] sched: Wholesale removal of sd

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Date:	Wed, 16 Feb 2011 14:23:03 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Nick Piggin <npiggin@...il.com>,
	Tim Chen <tim.c.chen@...el.com>, Alex Shi <alex.shi@...el.com>
Subject: Re: [PATCH] sched: Wholesale removal of sd_idle logic

* Venkatesh Pallipadi <venki@...gle.com> [2011-02-15 10:26:01]:

<snip>

> >> @@ -3386,10 +3363,6 @@ redo:
> >>                       sd->balance_interval *= 2;
> >>       }
> >>
> >> -     if (!ld_moved && !sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
> >> -         !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
> >> -             ld_moved = -1;
> >
> > I have not figured out where ld_moved is checked for -1 and why we
> > need to treat this as a special case.
> >
> 
> Return value of -1 was being consumed in rebalance domains() call
> to load_balance(). Returning -1 (instead of 0 in this case) makes
> rebalance_domains() to call higher domain load balancing
> with CPU_NOT_IDLE, when sibling is busy and even when there
> was no load pulled in.

Ok, so in rebalance_domain() we need not distinguish between 1 or -1
return code.  We can set idle = CPU_NOT_IDLE as long as we pull
a task.  With removal of sd_idle logic, the value of idle will remain
same for all domains.

Now in idle_balance() you have to check for return code > 0 in order
to reset this_rq->idle_stamp = 0.  But in the next check:

	if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
		/*
		 * We are going idle. next_balance may be set based on
		 * a busy processor. So reset next_balance.
		 */
		this_rq->next_balance = next_balance;
	}

Earlier, we would push the next_balance interval for busy sibling case
since pulled_task will be set to -1.  But now, with the removal of
sd_idle logic, the this_rq->next_balance will not be touched leading
to sooner rebalance.

In summary, the load_balance() will return 0 or 1, and the special
case of -1 is completely removed in the new code.  Thanks for the
clarification.

--Vaidy

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