Messages in this thread | | | Date | Tue, 7 Jan 2014 17:36:45 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] lib/percpu_counter.c: disable local irq when updating percpu couter |
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On Wed, 8 Jan 2014 09:12:19 +0800 Ming Lei <tom.leiming@gmail.com> wrote:
> Hi Andrew, > > >> --- a/lib/percpu_counter.c > >> +++ b/lib/percpu_counter.c > >> @@ -75,19 +75,19 @@ EXPORT_SYMBOL(percpu_counter_set); > >> void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch) > >> { > >> s64 count; > >> + unsigned long flags; > >> > >> - preempt_disable(); > >> + raw_local_irq_save(flags); > >> count = __this_cpu_read(*fbc->counters) + amount; > >> if (count >= batch || count <= -batch) { > >> - unsigned long flags; > >> - raw_spin_lock_irqsave(&fbc->lock, flags); > >> + raw_spin_lock(&fbc->lock); > >> fbc->count += count; > >> - raw_spin_unlock_irqrestore(&fbc->lock, flags); > >> + raw_spin_unlock(&fbc->lock); > >> __this_cpu_write(*fbc->counters, 0); > >> } else { > >> __this_cpu_write(*fbc->counters, count); > >> } > >> - preempt_enable(); > >> + raw_local_irq_restore(flags); > >> } > >> EXPORT_SYMBOL(__percpu_counter_add); > > > > Can this be made more efficient? > > > > The this_cpu_foo() documentation is fairly dreadful, but way down at > > the end of Documentation/this_cpu_ops.txt we find "this_cpu ops are > > interrupt safe". So I think this is a more efficient fix: > > > > --- a/lib/percpu_counter.c~a > > +++ a/lib/percpu_counter.c > > @@ -82,10 +82,10 @@ void __percpu_counter_add(struct percpu_ > > unsigned long flags; > > raw_spin_lock_irqsave(&fbc->lock, flags); > > fbc->count += count; > > + __this_cpu_sub(*fbc->counters, count); > > raw_spin_unlock_irqrestore(&fbc->lock, flags); > > - __this_cpu_write(*fbc->counters, 0); > > } else { > > - __this_cpu_write(*fbc->counters, count); > > + this_cpu_add(*fbc->counters, amount); > > } > > preempt_enable(); > > } > > > > It avoids the local_irq_disable() in the common case, when the CPU > > supports efficient this_cpu_add(). It will in rare race situations > > permit the cpu-local counter to exceed `batch', but that should be > > harmless. > > I am wondering if the above patch is more efficient, because: > > - raw_local_irq_save()/raw_local_irq_restore() should be cheaper > than preempt_enable() in theory
Don't think so - local_irq_disable() requires quite some internal synchronization in the CPU and is expensive. preempt_disable() is just an add+barrier, minus the add if the kernel is non-preemptable.
> - except for x86 and s390, other ARCHs have not their own implementation > of this_cpu_foo(), and the generic one just disables local interrupt > when operating the percpu variable.
Yup. But other CPUs should and will optimise their this_cpu implementations over time.
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