Messages in this thread | | | Date | Mon, 12 Aug 2013 13:44:59 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [RFC] per-cpu preempt_count |
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On 08/12/2013 12:00 PM, Linus Torvalds wrote: > > Wrong. The thing is, the common case for preempt is to increment and > decrement the count, not testing it. Exactly because we do this for > spinlocks and for rcu read-locked regions. > > Now, what we *could* do is to say: > > - we will use the high bit of the preempt count for NEED_RESCHED > > - when we set/clear that high bit, we *always* use atomic sequences, > and we never change any of the other bits. > > - we will increment/decrement the other counters, we *only* do so on > the local CPU, and we don't use atomic accesses. > > Now, the downside of that is that *because* we don't use atomic > accesses for the inc/dec parts, the updates to the high bit can get > lost. But because the high bit updates are done with atomics, we know > that they won't mess up the actual counting bits, so at least the > count is never corrupted. > > And the NEED_RESCHED bit getting lost would be very unusual. That > clearly would *not* be acceptable for RT, but it it might be > acceptable for "in the unusual case where we want to preempt a thread > that was not preemtible, *and* we ended up having the extra unsual > case that preemption enable ended up missing the preempt bit, we don't > get preempted in a timely manner". It's probably impossible to ever > see in practice, and considering that for non-RT use the PREEMPT bit > is a "strong hint" rather than anything else, it sounds like it might > be acceptable. > > It is obviously *not* going to be acceptable for the RT people, > though, but since they do different code sequences _anyway_, that's > not really much of an issue. >
This seems more pain than need be if checking the count in the slow path is okay.
-hpa
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