Messages in this thread Patch in this message | | | Date | Thu, 4 Jul 2019 18:00:55 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] cpu/hotplug: Cache number of online CPUs |
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----- On Jul 4, 2019, at 5:10 PM, Thomas Gleixner tglx@linutronix.de wrote:
> On Thu, 4 Jul 2019, Mathieu Desnoyers wrote: > >> ----- On Jul 4, 2019, at 4:42 PM, Thomas Gleixner tglx@linutronix.de wrote: >> >> > Revaluating the bitmap wheight of the online cpus bitmap in every >> > invocation of num_online_cpus() over and over is a pretty useless >> > exercise. Especially when num_online_cpus() is used in code pathes like the >> > IPI delivery of x86 or the membarrier code. >> > >> > Cache the number of online CPUs in the core and just return the cached >> > variable. >> > >> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> >> > --- >> > include/linux/cpumask.h | 16 +++++++--------- >> > kernel/cpu.c | 16 ++++++++++++++++ >> > 2 files changed, 23 insertions(+), 9 deletions(-) >> > >> > --- a/include/linux/cpumask.h >> > +++ b/include/linux/cpumask.h >> > @@ -95,8 +95,13 @@ extern struct cpumask __cpu_active_mask; >> > #define cpu_present_mask ((const struct cpumask *)&__cpu_present_mask) >> > #define cpu_active_mask ((const struct cpumask *)&__cpu_active_mask) >> > >> > +extern unsigned int __num_online_cpus; >> >> [...] >> >> > + >> > +void set_cpu_online(unsigned int cpu, bool online) >> > +{ >> > + lockdep_assert_cpus_held(); >> >> I don't think it is required that the cpu_hotplug lock is held >> when reading __num_online_cpus, right ? > > Errm, that's the update function. And this is better called from a hotplug > lock held region and not from some random crappy code.
Sure, this is fine to assume this lock is held for the update. It's the read-side I'm worried about (which does not hold the lock).
> >> I would have expected the increment/decrement below to be performed >> with a WRITE_ONCE(), and use a READ_ONCE() when reading the current >> value. > > What for? > > num_online_cpus() is racy today vs. CPU hotplug operations as > long as you don't hold the hotplug lock.
Fair point, AFAIU none of the loads performed within num_online_cpus() seem to rely on atomic nor volatile accesses. So not using a volatile access to load the cached value should not introduce any regression.
I'm concerned that some code may rely on re-fetching of the cached value between iterations of a loop. The lack of READ_ONCE() would let the compiler keep a lifted load within a register and never re-fetch, unless there is a cpu_relax() or a barrier() within the loop.
Thoughts ?
Thanks,
Mathieu
> > Thanks, > > tglx
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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