linux-kernel - Re: [RFC PATCH 4/4] sched: Account PREEMPT

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Date:	Mon, 2 Feb 2015 18:29:53 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 4/4] sched: Account PREEMPT_ACTIVE context as atomic

On Wed, Jan 28, 2015 at 04:46:37PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 28, 2015 at 01:24:12AM +0100, Frederic Weisbecker wrote:
> > PREEMPT_ACTIVE implies non-preemptible context and thus atomic context
> > despite what in_atomic*() APIs reports about it. These functions
> > shouldn't ignore this value like they are currently doing.
> > 
> > It appears that these APIs were ignoring PREEMPT_ACTIVE in order to
> > ease the check in schedule_debug(). Meanwhile it is sufficient to rely
> > on PREEMPT_ACTIVE in order to disable preemption in __schedule().
> > 
> > So lets fix the in_atomic*() APIs and simplify the preempt count ops
> > on __schedule() callers.
> 
> So what I think the history is here is that PREEMPT_ACTIVE is/was seen
> as a flag, protecting recursion, not so much a preempt-disable.
> 
> By doing this, you loose that separation.

Indeed, preemption disablement is a side effet.

> 
> Note that (at least on x86) we have another flag in the preempt count.
> 
> And I don't think the generated code really changes, the only difference
> is the value added/subtracted and that's an encoded immediate I think.

Right the resulting code isn't optimized at all with this patch. Only the C code
was deemed to be more simple but actually it isn't since we are abusing a side
effect property.

I'm dropping this patch then.

Thanks.
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