Messages in this thread | | | Date | Mon, 2 Feb 2015 18:29:53 +0100 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH 4/4] sched: Account PREEMPT_ACTIVE context as atomic |
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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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