Re: [PATCH] mm, memcg: cg2 memory{.swap,}.peak write handlers

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Jul 16, 2024 at 06:44:11AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jul 16, 2024 at 03:48:17PM +0200, Michal Hocko wrote:
> ...
> 
> The removal of resets was intentional. The problem was that it wasn't clear
> who owned those counters and there's no way of telling who reset what when.
> It was easy to accidentally end up with multiple entities that think they
> can get timed measurement by resetting.
> 
> So, in general, I don't think this is a great idea. There are shortcomings
> to how memory.peak behaves in that its meaningfulness quickly declines over
> time. This is expected and the rationale behind adding memory.peak, IIRC,
> was that it was difficult to tell the memory usage of a short-lived cgroup.
> 
> If we want to allow peak measurement of time periods, I wonder whether we
> could do something similar to pressure triggers - ie. let users register
> watchers so that each user can define their own watch periods. This is more
> involved but more useful and less error-inducing than adding reset to a
> single counter.

It's definitely a better user interface and I totally agree with you regarding
the shortcomings of the proposed interface with a global reset. But if you let
users to register a (potentially large) number of watchers, it might be quite
bad for the overall performance, isn't it? To mitigate it, we'll need to reduce
the accuracy of peak values. And then the question is why not just poll it
periodically from userspace?