Re: [RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset_refault()

Johannes Weiner
2023-03-23T16:00:43Z

On Thu, Mar 23, 2023 at 04:00:36AM +0000, Yosry Ahmed wrote:
> In workingset_refault(), we call mem_cgroup_flush_stats_delayed() to
> flush stats within an RCU read section and with sleeping disallowed.
> Move the call to mem_cgroup_flush_stats_delayed() above the RCU read
> section and allow sleeping to avoid unnecessarily performing a lot of
> work without sleeping.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>
> A lot of code paths call into workingset_refault(), so I am not
> generally sure at all whether it's okay to sleep in all contexts or not.
> Feedback here would be very helpful.
>
> ---
> mm/workingset.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 042eabbb43f6..410bc6684ea7 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
> unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
> eviction <<= bucket_order;
>
> + /* Flush stats (and potentially sleep) before holding RCU read lock */
> + mem_cgroup_flush_stats_delayed(true);

Btw, it might be a good time to rename this while you're in the
area. delayed suggests this is using a delayed_work, but this is
actually sometimes flushing directly from the callsite.

What it's doing is ratelimited calls. A better name would be:

mem_cgroup_flush_stats_ratelimited()