On 05/13/2016 03:38 PM, Michal Hocko wrote:
On Tue 10-05-16 09:36:01, Vlastimil Babka wrote:
During reclaim/compaction loop, it's desirable to get a final answer from
unsuccessful compaction so we can either fail the allocation or invoke the OOM
killer. However, heuristics such as deferred compaction or pageblock skip bits
can cause compaction to skip parts or whole zones and lead to premature OOM's,
failures or excessive reclaim/compaction retries.
To remedy this, we introduce a new direct compaction priority called
COMPACT_PRIO_SYNC_FULL, which instructs direct compaction to:
- ignore deferred compaction status for a zone
- ignore pageblock skip hints
- ignore cached scanner positions and scan the whole zone
- use MIGRATE_SYNC migration mode
I do not think we can do MIGRATE_SYNC because fallback_migrate_page
would trigger pageout and we are in the allocation path and so we
could blow up the stack.
Ah, I thought it was just waiting for the writeout to complete, and you
wanted to introduce another migrate mode to actually do the writeout.
But looks like I misremembered.
The new priority should get eventually picked up by should_compact_retry() and
this should improve success rates for costly allocations using __GFP_RETRY,
s@__GFP_RETRY@__GFP_REPEAT@
Ah thanks. Depending on the patch timing it might be __GFP_RETRY_HARD in
the end, right :)
such as hugetlbfs allocations, and reduce some corner-case OOM's for non-costly
allocations.
My testing has shown that even with the current implementation with
deferring, skip hints and cached positions had (close to) 100% success
rate even with close to OOM conditions.
Hmm, I thought you at one point said that ignoring skip hints was a
large improvement, because the current resetting of them is just too fuzzy.
I am wondering whether this strongest priority should be done only for
!costly high order pages. But we probably want less special cases
between costly and !costly orders.
Yeah, if somebody wants to retry hard, let him.
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
include/linux/compaction.h | 1 +
mm/compaction.c | 15 ++++++++++++---
2 files changed, 13 insertions(+), 3 deletions(-)
[...]
@@ -1631,7 +1639,8 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
ac->nodemask) {
enum compact_result status;
- if (compaction_deferred(zone, order)) {
+ if (prio > COMPACT_PRIO_SYNC_FULL
+ && compaction_deferred(zone, order)) {
rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
continue;
}
Wouldn't it be better to pull the prio check into compaction_deferred
directly? There are more callers and I am not really sure all of them
would behave consistently.
I'll check, thanks.
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