On Wed, Aug 31, 2022 at 11:02:49AM +0100, Mel Gorman wrote:

On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:

quoted

From: Kent Overstreet <kent.overstreet@linux.dev>

This patch adds lib/lazy-percpu-counter.c, which implements counters
that start out as atomics, but lazily switch to percpu mode if the
update rate crosses some threshold (arbitrarily set at 256 per second).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Why not use percpu_counter? It has a per-cpu counter that is synchronised
when a batch threshold (default 32) is exceeded and can explicitly sync
the counters when required assuming the synchronised count is only needed
when reading debugfs.

It doesn't switch from atomic mode to percpu mode when the update rate crosses a
threshold like lazy percpu counters does, it allocates all the percpu counters
up front - that makes it a non starter here.

Also, from my reading of the code... wtf is it even doing, and why would I use
it at all? This looks like old grotty code from ext3, it's not even using
this_cpu_add() - it does preempt_enable()/disable() just for adding to a local
percpu counter!

Noooooope.