Re: [RFC PATCH] mm: use precise current RSS for high-water accounting

[Mathieu Desnoyers]

On 2026-07-23 12:21, Nirmoy Das wrote:
> Hi Mathieu,
>
> On 2026-07-23 09:47, Mathieu Desnoyers wrote:
> > Did you compare this with my hierarchical percpu counters series ?
> >
> > https://lore.kernel.org/lkml/20260707131544.75906-1-mathieu.desnoyers@xxxxxxxxxxxx/
> >
> > I've sent it to Andrew two weeks ago but have not heard back from him
> > though. My series attack the same issue you are trying to solve without
> > increasing overhead when reading those counters.
>
> I tested the v20 series on a dual-socket system with 352 CPUs and
> 64 KiB pages.
>
> On the v7.2-rc4 test kernel without the series, getrusage03 failed
> 10/10:
>
>    TFAIL: child.children = 270336, expected 307200
>
> With v20, it also failed 10/10:
>
>    TFAIL: initial.children = 0, expected 102400
>    TFAIL: child.children = 262144, expected 307200
>
> In v20, get_mm_hiwater_rss() still calls get_mm_rss(), which uses an
> approximate counter read. The series did not help this test.

Correct, my hierarchical counters keep a approximation there, but is
improves the accuracy of the approximation (worse case inaccuracy
limited by the topology).

What is this test validating ? Does it actually matter for a real-life
use-case, or is it just the test being too strict ?

What is the overhead impact of your change from approximated counter
read to a precise sum on this /proc ABI ?

Thanks,

Mathieu

> Let me know if you want to me to try out something else.
>
> Thanks,
> Nirmoy


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com