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

[Mel Gorman]

On Thu, Oct 14, 2021 at 02:31:17PM +0200, Vlastimil Babka wrote:
> 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.
> 

Yes, you're right, consider_reclaim_throttle not only throttles on the
appropriate pgdat but takes priority into account.

Well spotted!

-- 
Mel Gorman
SUSE Labs