linux-kernel - Re: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

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Date:	Thu, 9 Oct 2014 19:32:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kirill Tkhai <tkhai@...dex.ru>, Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched: schedule_tail() should disable preemption

On Thu, Oct 09, 2014 at 07:28:34PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 09, 2014 at 06:57:13PM +0200, Oleg Nesterov wrote:
> > > Your earlier proposal would penalize every
> > > !x86 arch by adding extra code to the scheduler core while they already
> > > automagically preserve their thread_info::preempt_count.
> > 
> > Sure, and it can't be even compiled on !x86.
> > 
> > But this is simple, just we need a new helper, preempt_count_restore(),
> > defined as nop in asm-generic/preempt.h. Well, perhaps another helper
> > makes sense, preempt_count_raw() which simply reads the counter, but
> > this is minor.
> > 
> > After the patch below we can remove ->saved_preempt_count. Including
> > init_task_preempt_count(), it is no longer needed after the change in
> > schedule_tail().
> 
> Ah, right, this makes more sense.
> 
> > @@ -2333,10 +2336,12 @@ context_switch(struct rq *rq, struct task_struct *prev,
> >  #endif
> >  
> >  	context_tracking_task_switch(prev, next);
> > +
> > +	pc = preempt_count();
> 
> The only problem here is that you can loose PREEMPT_NEED_RESCHED, I
> haven't thought about whether that is a problem here or not.

Also, if you make that preempt_count_save(), to mirror the restore, you
can both preserve PREEMPT_NEED_RESCHED and avoid emitting code on !x86.
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