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

From: Dave Chinner
Date: Mon Jan 07 2019 - 17:32:22 EST

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...


Dave Chinner