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SubjectRe: [patch for-5.3 0/4] revert immediate fallback to remote hugepages
Let me revive this thread as there was no follow up.

On Mon 09-09-19 21:30:20, Michal Hocko wrote:
[...]
> I believe it would be the best to start by explaining why we do not see
> the same problem with order-0 requests. We do not enter the slow path
> and thus the memory reclaim if there is any other node to pass through
> watermakr as well right? So essentially we are relying on kswapd to keep
> nodes balanced so that allocation request can be satisfied from a local
> node. We do have kcompactd to do background compaction. Why do we want
> to rely on the direct compaction instead? What is the fundamental
> difference?

I am especially interested about this part. The more I think about this
the more I am convinced that the underlying problem really is in the pre
mature fallback in the fast path. Does the almost-patch below helps your
workload? It effectively reduces the fast path for higher order
allocations to the local/requested node. The justification is that
watermark check might be too strict for those requests as it is primary
order-0 oriented. Low watermark target simply has no meaning for the
higher order requests AFAIU. The min-low gap is giving kswapd a chance
to balance and be more local node friendly while we do not have anything
like that in compaction.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ff5484fdbdf9..09036cf55fca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4685,7 +4685,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
{
struct page *page;
unsigned int alloc_flags = ALLOC_WMARK_LOW;
- gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
+ gfp_t fastpath_mask, alloc_mask; /* The gfp_t that was actually used for allocation */
struct alloc_context ac = { };

/*
@@ -4698,7 +4698,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
}

gfp_mask &= gfp_allowed_mask;
- alloc_mask = gfp_mask;
+ fastpath_mask = alloc_mask = gfp_mask;
if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags))
return NULL;

@@ -4710,8 +4710,17 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
*/
alloc_flags |= alloc_flags_nofragment(ac.preferred_zoneref->zone, gfp_mask);

- /* First allocation attempt */
- page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
+ /*
+ * First allocation attempt. If we have a high order allocation then do not fall
+ * back to a remote node just based on the watermark check on the requested node
+ * because compaction might easily free up a requested order and then it would be
+ * better to simply go to the slow path.
+ * TODO: kcompactd should help here but nobody has woken it up unless we hit the
+ * slow path so we might need some tuning there as well.
+ */
+ if (order && (gfp_mask & __GFP_DIRECT_RECLAIM))
+ fastpath_mask |= __GFP_THISNODE;
+ page = get_page_from_freelist(fastpath_mask, order, alloc_flags, &ac);
if (likely(page))
goto out;

--
Michal Hocko
SUSE Labs
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