Re: [PATCH v4 3/4] mm: memcg: let non-unified root stats flushes help unified flushes

[Michal Hocko <mhocko@suse.com>]

On Thu 31-08-23 16:56:10, Yosry Ahmed wrote:
> Unified flushing of memcg stats keeps track of the magnitude of pending
> updates, and only allows a flush if that magnitude exceeds a threshold.
> It also keeps track of the time at which ratelimited flushing should be
> allowed as flush_next_time.
> 
> A non-unified flush on the root memcg has the same effect as a unified
> flush, so let it help unified flushing by resetting pending updates and
> kicking flush_next_time forward. Move the logic into the common
> do_stats_flush() helper, and do it for all root flushes, unified or
> not.

I have hard time to follow why we really want/need this. Does this cause
any observable changes to the behavior?

> 
> There is a subtle change here, we reset stats_flush_threshold before a
> flush rather than after a flush. This probably okay because:
> 
> (a) For flushers: only unified flushers check stats_flush_threshold, and
> those flushers skip anyway if there is another unified flush ongoing.
> Having them also skip if there is an ongoing non-unified root flush is
> actually more consistent.
> 
> (b) For updaters: Resetting stats_flush_threshold early may lead to more
> atomic updates of stats_flush_threshold, as we start updating it
> earlier. This should not be significant in practice because we stop
> updating stats_flush_threshold when it reaches the threshold anyway. If
> we start early and stop early, the number of atomic updates remain the
> same. The only difference is the scenario where we reset
> stats_flush_threshold early, start doing atomic updates early, and then
> the periodic flusher kicks in before we reach the threshold. In this
> case, we will have done more atomic updates. However, since the
> threshold wasn't reached, then we did not do a lot of updates anyway.
> 
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/memcontrol.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8c046feeaae7..94d5a6751a9e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -647,6 +647,11 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>   */
>  static void do_stats_flush(struct mem_cgroup *memcg)
>  {
> +	/* for unified flushing, root non-unified flushing can help as well */
> +	if (mem_cgroup_is_root(memcg)) {
> +		WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
> +		atomic_set(&stats_flush_threshold, 0);
> +	}
>  	cgroup_rstat_flush(memcg->css.cgroup);
>  }
>  
> @@ -665,11 +670,8 @@ static void do_unified_stats_flush(void)
>  	    atomic_xchg(&stats_unified_flush_ongoing, 1))
>  		return;
>  
> -	WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
> -
>  	do_stats_flush(root_mem_cgroup);
>  
> -	atomic_set(&stats_flush_threshold, 0);
>  	atomic_set(&stats_unified_flush_ongoing, 0);
>  }
>  
> -- 
> 2.42.0.rc2.253.gd59a3bf2b4-goog

-- 
Michal Hocko
SUSE Labs