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

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Message-ID: <20131121123906.GA20114@localhost.localdomain>
Date:	Thu, 21 Nov 2013 13:39:08 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Tony Luck <tony.luck@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.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>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [patch 4/6] ia64: Use preempt_schedule_irq

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 ?

So that's because the schedule point is on a common user and kernel return path?

If necessary, why not having a SCHEDULE_IRQ macro in ia64 that maps to either schedule or
preempt_schedule_irq() instead?

Unless that problem happens elsewhere as well?
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