linux-kernel - Re: [RFC PATCH 03/30] Lazy percpu counters

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Message-ID: <20220831162030.hzgzhxu3qn6g3k5r@moria.home.lan>
Date:   Wed, 31 Aug 2022 12:20:30 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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Subject: Re: [RFC PATCH 03/30] Lazy percpu counters

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:
> > From: Kent Overstreet <kent.overstreet@...ux.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@...ux.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.