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

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Message-ID: <alpine.DEB.2.02.1311211230030.30673@ionos.tec.linutronix.de>
Date:	Thu, 21 Nov 2013 12:41:44 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Tony Luck <tony.luck@...il.com>
cc:	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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [patch 4/6] ia64: Use preempt_schedule_irq

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 ?

Thanks,

	tglx
------
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c180860..0c59642 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2660,6 +2660,7 @@ asmlinkage void __sched notrace preempt_schedule(void)
 	} while (need_resched());
 }
 EXPORT_SYMBOL(preempt_schedule);
+#endif /* CONFIG_PREEMPT */
 
 /*
  * this is the entry point to schedule() from kernel preemption
@@ -2693,8 +2694,6 @@ asmlinkage void __sched preempt_schedule_irq(void)
 	exception_exit(prev_state);
 }
 
-#endif /* CONFIG_PREEMPT */
-
 int default_wake_function(wait_queue_t *curr, unsigned mode, int wake_flags,
 			  void *key)
 {




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