Messages in this thread | | | Date | Mon, 3 Aug 2015 14:48:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 05/10] nohz: New tick dependency mask |
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On Fri, Jul 24, 2015 at 01:43:38PM -0400, Chris Metcalf wrote: > On 07/24/2015 01:16 PM, Frederic Weisbecker wrote: > >On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote: > >>On 07/23/2015 12:42 PM, Frederic Weisbecker wrote: > >>>+unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit, > >>>+ unsigned long *dep) > >>>+{ > >>>+ unsigned long prev; > >>>+ unsigned long old = *dep; > >>>+ unsigned long mask = BIT_MASK(bit); > >>>+ > >>>+ while ((prev = cmpxchg(dep, old, old | mask)) != old) { > >>>+ old = prev; > >>>+ cpu_relax(); > >>>+ } > >>>+ > >>>+ return prev; > >>>+} > >>Why not use set_bit() here? It is suitably atomic. > >Because I don't want to send an IPI if the CPU already had bits set in > >the dependency. > > > >Ideally I need something like test_and_set_bit() but which returns the > >whole previous value and not just the previous value of the bit. > > Ah, of course. Peter, maybe we need atomic_or_return() as part > of your new atomic_or/_and/_xor series? Certainly on tilegx, and > likely other architectures, we can do better than Frederic's > cmpxchg() loop.
No, atomic_or_return() would return the new value and is entirely pointless for the logic ops since they're not reversible (with the exception of xor).
What you'd need is atomic_fetch_or(), but we don't have any fetch_$op primitives at all. Introducing them might make sense, but it'll have to be a separate series.
Note that I have a fetch_or() macro in kernel/sched/core.c.
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