linux-kernel - Re: [PATCH] sched: Fix lost reschedule in __cond

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Date:	Sun, 14 Dec 2014 21:30:18 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Dave Jones <davej@...hat.com>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Fix lost reschedule in __cond_resched()

On Sun, Dec 14, 2014 at 11:50:20AM -0800, Linus Torvalds wrote:
> On Sun, Dec 14, 2014 at 10:04 AM, Frederic Weisbecker
> <fweisbec@...il.com> wrote:
> >
> > Such as:
> 
> So I like your patch, but quite frankly, can we go one step further?
> 
> Look at the callers of __schedule().
> 
> EVERY SINGLE ONE now has that loop around it that goes along the lines of
> 
>    do {
>       .. disable preemption somehow ..
>       __schedule();
>       ...enable preemption without scheduling ..
>    } while (need_resced());
> 
> except for one - the regular "schedule()" function.
> 
> Furthermore, look inside __schedule() itself: it has the same loop,
> except with a count of one.
> 
> So I would suggest going the extra mile, and
>  - remove the loop from __schedule() itself

That sounds like a good idea. Unless the loop inside __schedule()
is very frequent and sensitive enough to show visible overhead if we
force it to pass through the preemp_count_add/sub() and local_irq_*()
operations in the preempt_schedule_*() functions.

I suspect it's not, so I'm cooking that patch.

>  - add the same loop as everywhere else to "schedule()"

Right. I'm doing that too.

> IOW, just make this "you have to loop and disable preemption" thing be
> a rule that __schedule() can depend on.

Ok. It would be nice if we could have a common function that does the loop
and PREEMPT_ACTIVE increments. But the variable code is inside that loop
so that's only factorizable with a function pointer (no-go in that fast-path)
or a macro that would make things even worse and ugly.

So I think I'll just keep all those loops explicit.

Thanks.

>                  Linus
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