Re: [PATCH] lib/percpu_counter.c: disable local irq when updatingpercpu couter

From: Ming Lei
Date: Tue Jan 07 2014 - 23:31:24 EST

On Wed, Jan 8, 2014 at 11:29 AM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
> Hi Andrew,
> On Wed, Jan 8, 2014 at 9:36 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> 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
> Yes, it might be a little expensive on some CPUs, but should be
> arch-dependent(CPU inside things are involved)
>> an add+barrier, minus the add if the kernel is non-preemptable.
> IMO, generally, from software view, local_irq_save() only save the
> CPU's interrupt mask to the local variable 'flag', and sets irq mask
> to register, considered local variable can be thought to be in cache,
> so I think it might be cheaper than preempt_enable() because
> preempt counter may not be in cache.
> Also this_cpu_add() won't work in batch path(slow path), we still
> need to avoid interrupt coming between reading the percpu counter
> and resetting it, otherwise counts might be lost too, :-)

Sorry, I miss the __this_cpu_sub() in slow path, so it is correct, and
even preempt_enable() and preempt_disable() can be removed.

Ming Lei
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