From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 4/4] sched: Account PREEMPT_ACTIVE context as atomic
Date: Mon, 2 Feb 2015 18:29:53 +0100 [thread overview]
Message-ID: <20150202172950.GC11054@lerouge> (raw)
In-Reply-To: <20150128154637.GI23038@twins.programming.kicks-ass.net>
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.
prev parent reply other threads:[~2015-02-02 17:30 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-28 0:24 [PATCH 0/4] sched: schedule/preempt optimizations and cleanups Frederic Weisbecker
2015-01-28 0:24 ` [PATCH 1/4] sched: Pull resched loop to __schedule() callers Frederic Weisbecker
2015-02-04 14:36 ` [tip:sched/core] " tip-bot for Frederic Weisbecker
2015-01-28 0:24 ` [RFC PATCH 2/4] sched: Use traced preempt count operations to toggle PREEMPT_ACTIVE Frederic Weisbecker
2015-01-28 1:42 ` Steven Rostedt
2015-01-28 13:59 ` Frederic Weisbecker
2015-01-28 15:04 ` Steven Rostedt
2015-01-28 15:42 ` Peter Zijlstra
2015-02-02 17:22 ` Frederic Weisbecker
2015-01-28 0:24 ` [PATCH 3/4] sched: Pull preemption disablement to __schedule() caller Frederic Weisbecker
2015-01-28 15:50 ` Peter Zijlstra
2015-02-02 17:53 ` Frederic Weisbecker
2015-02-03 10:53 ` Peter Zijlstra
2015-02-04 17:31 ` Frederic Weisbecker
2015-02-04 17:48 ` Peter Zijlstra
2015-01-28 0:24 ` [RFC PATCH 4/4] sched: Account PREEMPT_ACTIVE context as atomic Frederic Weisbecker
2015-01-28 15:46 ` Peter Zijlstra
2015-02-02 17:29 ` Frederic Weisbecker [this message]
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