Re: [PATCH] mm/memcontrol: Export memcg->watermark via sysfs for v2 memcg

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

On Thu, May 05, 2022 at 05:13:30AM -0700, Ganesan Rajagopal wrote:
> v1 memcg exports memcg->watermark as "memory.mem_usage_in_bytes" in
> sysfs. This is missing for v2 memcg though "memory.current" is exported.
> There is no other easy way of getting this information in Linux.
> getrsuage() returns ru_maxrss but that's the max RSS of a single process
> instead of the aggregated max RSS of all the processes. Hence, expose
> memcg->watermark as "memory.watermark" for v2 memcg.
> 
> Signed-off-by: Ganesan Rajagopal <rganesan-nzgTgzXrdUbQT0dZR+AlfA@public.gmane.org>

This wasn't initially added to cgroup2 because its usefulness is very
specific: it's (mostly) useless on limited cgroups, on long-running
cgroups, and on cgroups that are recycled for multiple jobs. And I
expect these categories apply to the majority of cgroup usecases.

However, for the situation where you want to measure the footprint of
a short-lived, unlimited one-off cgroup, there really is no good
alternative. And it's a legitimate usecase. It doesn't cost much to
maintain this info. So I think we should go ahead with this patch.

But please add a blurb to Documentation/admin-guide/cgroup-v2.rst.