netdev - Re: [PATCH v2 2/3] mm: page_counter: rearrange struct page

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Date:   Wed, 24 Aug 2022 22:21:50 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Michal Koutný <mkoutny@...e.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Oliver Sang <oliver.sang@...el.com>, lkp@...ts.01.org,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm: page_counter: rearrange struct page_counter
 fields

On Wed, 24 Aug 2022 21:41:42 -0700 Shakeel Butt <shakeelb@...gle.com> wrote:

> > Did you evaluate the effects of using a per-cpu counter of some form?
> 
> Do you mean per-cpu counter for usage or something else?

percpu_counter, perhaps.  Or some hand-rolled thing if that's more suitable.

> The usage
> needs to be compared against the limits and accumulating per-cpu is
> costly particularly on larger machines,

Well, there are tricks one can play.  For example, only run
__percpu_counter_sum() when `usage' is close to its limit.  

I'd suggest flinging together a prototype which simply uses
percpu_counter_read() all the time.  If the performance testing results
are sufficiently promising, then look into the accuracy issues.