linux-kernel - Re: [PATCH 0/1] sched: fix the PREEMPT_ACTIVE check in __trace_sched_switch

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Message-ID: <20141008080016.GB10832@worktop.programming.kicks-ass.net>
Date:	Wed, 8 Oct 2014 10:00:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] sched: fix the PREEMPT_ACTIVE check in
 __trace_sched_switch_state()

On Tue, Oct 07, 2014 at 09:50:46PM +0200, Oleg Nesterov wrote:
> And note that another caller of task_preempt_count(), set_cpu(), is
> fine but it doesn't really need this helper.
> 
> And afaics we do not need ->saved_preempt_count at all, the trivial
> patch below makes it unnecessary, we can kill it and all its users.
> 
> Not only this will simplify the code, this will make (well, almost)
> the per-cpu preempt counter arch-agnostic.
> 
> Or I missed something?

Two things, per-cpu isn't always faster on some archs, and load-store
archs have problems with PREEMPT_NEED_RESCHED, although arguably you can
do per-cpu preempt count without that.

> Do you think this makes sense? If yes, I'll try to make the patches.

It penalizes everything but x86 I think. There is no other arch that has
per-cpu preempt count atm.
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