Re: [PATCH v4 0/4] memcg: non-unified flushing for userspace stats

Waiman Long
2023-08-31T17:18:28Z

On 8/31/23 12:56, Yosry Ahmed wrote:
> Most memcg flushing contexts using "unified" flushing, where only one
> flusher is allowed at a time (others skip), and all flushers need to
> flush the entire tree. This works well with high concurrency, which
> mostly comes from in-kernel flushers (e.g. reclaim, refault, ..).
>
> For userspace reads, unified flushing leads to non-deterministic stats
> staleness and reading cost. This series clarifies and documents the
> differences between unified and non-unified flushing (patches 1 & 2),
> then opts userspace reads out of unified flushing (patch 3).
>
> This patch series is a follow up on the discussion in [1]. That was a
> patch that proposed that userspace reads wait for ongoing unified
> flushers to complete before returning. There were concerns about the
> latency that this introduces to userspace reads, especially with ongoing
> reports of expensive stat reads even with unified flushing. Hence, this
> series follows a different approach, by opting userspace reads out of
> unified flushing completely. The cost of userspace reads are now
> determinstic, and depend on the size of the subtree being read. This
> should fix both the *sometimes* expensive reads (due to flushing the
> entire tree) and occasional staless (due to skipping flushing).
>
> I attempted to remove unified flushing completely, but noticed that
> in-kernel flushers with high concurrency (e.g. hundreds of concurrent
> reclaimers). This sort of concurrency is not expected from userspace
> reads. More details about testing and some numbers in the last patch's
> changelog.
>
> v4 -> v5:
> - Fixed build error in the last patch with W=1 because of a missed
> 'static'.
>
> v4: https://lore.kernel.org/lkml/20230830175335.1536008-1-yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org/
>
> Yosry Ahmed (4):
> mm: memcg: properly name and document unified stats flushing
> mm: memcg: add a helper for non-unified stats flushing
> mm: memcg: let non-unified root stats flushes help unified flushes
> mm: memcg: use non-unified stats flushing for userspace reads
>
> include/linux/memcontrol.h | 8 +--
> mm/memcontrol.c | 106 +++++++++++++++++++++++++++----------
> mm/vmscan.c | 2 +-
> mm/workingset.c | 4 +-
> 4 files changed, 85 insertions(+), 35 deletions(-)
>
LGTM

Acked-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>