On Mon 11-04-16 16:39:21, Vlastimil Babka wrote:
> On 04/05/2016 01:25 PM, Michal Hocko wrote:
[...]
> >+/* Compaction has failed and it doesn't make much sense to keep retrying. */
> >+static inline bool compaction_failed(enum compact_result result)
> >+{
> >+ /* All zones where scanned completely and still not result. */
>
> Hmm given that try_to_compact_pages() uses a max() on results, then in fact
> it takes only one zone to get this. Others could have been also SKIPPED or
> DEFERRED. Is that what you want?
In short I didn't find any better way and still guarantee a some
guarantee of convergence. COMPACT_COMPLETE means that at least one zone
was completely scanned and led to no result. That zone would be
compact_suitable by definition. If I made DEFERRED or SKIPPED more
priorite (aka higher in the enum) then I could easily end up in a state
where all zones would return COMPACT_COMPLETE and few remaining would
just alternate returning their DEFFERED resp. SKIPPED. So while this
might sound like giving up too early I couldn't come up with anything
more specific that would lead to reliable results.
I am open to any suggestions of course.
[...]
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -3362,25 +3362,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> > if (page)
> > goto got_pg;
> >
> >- /* Checks for THP-specific high-order allocations */
> >- if (is_thp_gfp_mask(gfp_mask)) {
> >- /*
> >- * If compaction is deferred for high-order allocations, it is
> >- * because sync compaction recently failed. If this is the case
> >- * and the caller requested a THP allocation, we do not want
> >- * to heavily disrupt the system, so we fail the allocation
> >- * instead of entering direct reclaim.
> >- */
> >- if (compact_result == COMPACT_DEFERRED)
> >- goto nopage;
> >-
> >- /*
> >- * Compaction is contended so rather back off than cause
> >- * excessive stalls.
> >- */
> >- if(compact_result == COMPACT_CONTENDED)
> >- goto nopage;
> >- }
> >+ /*
> >+ * Checks for THP-specific high-order allocations and back off
> >+ * if the the compaction backed off
> >+ */
> >+ if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(compact_result))
> >+ goto nopage;
>
> The change of semantics for THP is not trivial here and should at least be
> discussed in changelog. CONTENDED and DEFERRED is only subset of
> compaction_withdrawn() as seen above.
True. My main motivation was to get rid of the compaction specific code
from the allocator path as much as possible. I can drop the above hunk
of course but I think we should get rid of these checks and make the
code simpler. To be honest I am not even sure those changes are really
measurable.
> Why is it useful to back off due to
> COMPACT_PARTIAL_SKIPPED (we were just unlucky in our starting position), but
> not due to COMPACT_COMPLETE (we have seen the whole zone but failed anyway)?
OK, that is a good remark. I could change that to:
if (is_thp_gfp_mask(gfp_mask) &&
(compaction_withdrawn(compact_result) || compaction_failed(compact_result))
> Why back off due to COMPACT_SKIPPED (not enough order-0 pages) without
> trying reclaim at least once, and then another async compaction, like
> before?
The idea was that COMPACT_SKIPPED wouldn't change after a single reclaim
round most of the time because a zone would have to get above
low_wmark + 1<<9 pages. So the only situation where it would matter would be if
we had some order-9 pages available hidden by the min wmark and we would
reclaim enough to get above the above gap. I am not sure this is what we
really want in the first place. Increase the reclaim stalls when we are
getting under memory pressure.
Thanks!
--
Michal Hocko
SUSE Labs
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