Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is being made

[Vlastimil Babka]

On 10/8/21 15:53, Mel Gorman wrote:
> Memcg reclaim throttles on congestion if no reclaim progress is made.
> This makes little sense, it might be due to writeback or a host of
> other factors.
> 
> For !memcg reclaim, it's messy. Direct reclaim primarily is throttled
> in the page allocator if it is failing to make progress. Kswapd
> throttles if too many pages are under writeback and marked for
> immediate reclaim.
> 
> This patch explicitly throttles if reclaim is failing to make progress.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
...
> @@ -3769,6 +3797,16 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>  	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>  	set_task_reclaim_state(current, NULL);
>  
> +	if (!nr_reclaimed) {
> +		struct zoneref *z;
> +		pg_data_t *pgdat;
> +
> +		z = first_zones_zonelist(zonelist, sc.reclaim_idx, sc.nodemask);
> +		pgdat = zonelist_zone(z)->zone_pgdat;
> +
> +		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
> +	}

Is this necessary? AFAICS here we just returned from:

do_try_to_free_pages()
  shrink_zones()
   for_each_zone()...
     consider_reclaim_throttle()

Which already throttles when needed and using the appropriate pgdat, while
here we have to somewhat awkwardly assume the preferred one.

> +
>  	return nr_reclaimed;
>  }
>  #endif
>