Re: [PATCH v6 1/2] mm, memcg: cg2 memory{.swap,}.peak write handlers

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jul 29, 2024 at 10:37:42AM -0400, David Finkel wrote:
> Other mechanisms for querying the peak memory usage of either a process
> or v1 memory cgroup allow for resetting the high watermark. Restore
> parity with those mechanisms, but with a less racy API.
> 
> For example:
>  - Any write to memory.max_usage_in_bytes in a cgroup v1 mount resets
>    the high watermark.
>  - writing "5" to the clear_refs pseudo-file in a processes's proc
>    directory resets the peak RSS.
> 
> This change is an evolution of a previous patch, which mostly copied the
> cgroup v1 behavior, however, there were concerns about races/ownership
> issues with a global reset, so instead this change makes the reset
> filedescriptor-local.
> 
> Writing any non-empty string to the memory.peak and memory.swap.peak
> pseudo-files reset the high watermark to the current usage for
> subsequent reads through that same FD.
> 
> Notably, following Johannes's suggestion, this implementation moves the
> O(FDs that have written) behavior onto the FD write(2) path. Instead, on
> the page-allocation path, we simply add one additional watermark to
> conditionally bump per-hierarchy level in the page-counter.
> 
> Additionally, this takes Longman's suggestion of nesting the
> page-charging-path checks for the two watermarks to reduce the number of
> common-case comparisons.
> 
> This behavior is particularly useful for work scheduling systems that
> need to track memory usage of worker processes/cgroups per-work-item.
> Since memory can't be squeezed like CPU can (the OOM-killer has
> opinions), these systems need to track the peak memory usage to compute
> system/container fullness when binpacking workitems.
> 
> Most notably, Vimeo's use-case involves a system that's doing global
> binpacking across many Kubernetes pods/containers, and while we can use
> PSI for some local decisions about overload, we strive to avoid packing
> workloads too tightly in the first place. To facilitate this, we track
> the peak memory usage. However, since we run with long-lived workers (to
> amortize startup costs) we need a way to track the high watermark while
> a work-item is executing. Polling runs the risk of missing short spikes
> that last for timescales below the polling interval, and peak memory
> tracking at the cgroup level is otherwise perfect for this use-case.
> 
> As this data is used to ensure that binpacked work ends up with
> sufficient headroom, this use-case mostly avoids the inaccuracies
> surrounding reclaimable memory.
> 
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Suggested-by: Waiman Long <longman@redhat.com>
> Signed-off-by: David Finkel <davidf@vimeo.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>