Re: [PATCH v2 12/18] mm, compaction: more reliably increase direct compaction priority

[Vlastimil Babka]

On 06/01/2016 03:51 PM, Michal Hocko wrote:
> On Tue 31-05-16 15:08:12, Vlastimil Babka wrote:
> > During reclaim/compaction loop, compaction priority can be increased by the
> > should_compact_retry() function, but the current code is not optimal. Priority
> > is only increased when compaction_failed() is true, which means that compaction
> > has scanned the whole zone. This may not happen even after multiple attempts
> > with the lower priority due to parallel activity, so we might needlessly
> > struggle on the lower priority.
> >
> > We can remove these corner cases by increasing compaction priority regardless
> > of compaction_failed(). Examining further the compaction result can be
> > postponed only after reaching the highest priority. This is a simple solution
> > and we don't need to worry about reaching the highest priority "too soon" here,
> > because hen should_compact_retry() is called it means that the system is
> > already struggling and the allocation is supposed to either try as hard as
> > possible, or it cannot fail at all. There's not much point staying at lower
> > priorities with heuristics that may result in only partial compaction.
> >
> > The only exception here is the COMPACT_SKIPPED result, which means that
> > compaction could not run at all due to being below order-0 watermarks. In that
> > case, don't increase compaction priority, and check if compaction could proceed
> > when everything reclaimable was reclaimed. Before this patch, this was tied to
> > compaction_withdrawn(), but the other results considered there are in fact only
> > possible due to low compaction priority so we can ignore them thanks to the
> > patch. Since there are no other callers of compaction_withdrawn(), remove it.
>
> I agree with the change in general. I think that keeping compaction_withdrawn
> even with a single check is better because it abstracts the fact from a
> specific constant.

OK.

> Now that I think about that some more I guess you also want to update
> compaction_retries inside should_compact_retry as well, or at least
> update it only when we have reached the lowest priority. What do you
> think?

Makes sense, especially that after your suggestion, 
should_compact_retry() is not reached as long as should_reclaim_retry() 
returnes true. So I will do that.

> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> Other than that this makes sense
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!