linux-kernel - Re: [patch 4/6] ia64: Use preempt_schedule

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Date:	Thu, 21 Nov 2013 14:30:13 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Tony Luck <tony.luck@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [patch 4/6] ia64: Use preempt_schedule_irq

On Thu, 21 Nov 2013, Peter Zijlstra wrote:

> On Thu, Nov 21, 2013 at 12:41:44PM +0100, Thomas Gleixner wrote:
> > On Wed, 20 Nov 2013, Thomas Gleixner wrote:
> > > On Wed, 20 Nov 2013, Tony Luck wrote:
> > > > asmlinkage void __sched preempt_schedule_irq(void)
> > > > {
> > > >            schedule();
> > > > }
> > > > 
> > > > Or is life more complicated than that?
> > > 
> > > Hmm, I think I fubared that and you decided to ignore my patch :) 
> > > 
> > > Let me look at it tomorrow morning with full awake brain cells.
> > 
> > Ok, wrapped my brain around it. I tripped over the magic asm foo which
> > has a single need_resched check and schedule point for both sys call
> > return and interrupt return.
> > 
> > So you need the schedule_preempt_irq() for kernel preemption from
> > interrupt return while on a normal syscall preemption a schedule would
> > be sufficient. But using schedule_preempt_irq() is not harmful here in
> > any way. It just sets the preempt_active bit also in cases where it
> > would not be required. 
> > 
> > Even on preempt=n kernels adding the preempt_active bit is completely
> > harmless. So instead of having an extra function, moving the existing
> > one out of the ifdef PREEMPT looks like the sanest thing to do.
> > 
> > Peter, Ingo ?
> 
> Uhm, preempt_schedule_irq() assumes interrupts are disabled and
> explicitly returns with interrupts disabled again. Does the ia64
> callsite conform?

Yep.
 
> If so, schedule() would actually be actively wrong, because that will
> whinge when called with interrupts disabled, and will return with
> interrupts enabled.
> 
> Anyway, I don't object to the patch per se, but it might bloat a few
> !ia64 kernels for having to carry the extra text.
 
Well, we could get rid of quite some other sti/schedule/cli asm magic
all over the archs.

Thanks,

	tglx
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