Re: [PATCH 0/2] /proc/stat: Reduce irqs counting performance overhead

From: Waiman Long
Date: Mon Jan 07 2019 - 17:41:46 EST

On 01/07/2019 05:32 PM, Dave Chinner wrote:
> On Mon, Jan 07, 2019 at 10:12:56AM -0500, Waiman Long wrote:
>> As newer systems have more and more IRQs and CPUs available in their
>> system, the performance of reading /proc/stat frequently is getting
>> worse and worse.
> Because the "roll-your-own" per-cpu counter implementaiton has been
> optimised for low possible addition overhead on the premise that
> summing the counters is rare and isn't a performance issue. This
> patchset is a direct indication that this "summing is rare and can
> be slow" premise is now invalid.
> We have percpu counter infrastructure that trades off a small amount
> of addition overhead for zero-cost reading of the counter value.
> i.e. why not just convert this whole mess to percpu_counters and
> then just use percpu_counter_read_positive()? Then we just don't
> care how often userspace reads the /proc file because there is no
> summing involved at all...
> Cheers,
> Dave.

Yes, percpu_counter_read_positive() is cheap. However, you still need to
pay the price somewhere. In the case of percpu_counter, the update is
more expensive.

I would say the percentage of applications that will hit this problem is
small. But for them, this problem has some significant performance overhead.