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

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Date:	Wed, 28 Jan 2015 16:46:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 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.

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.
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