Message-ID: <20181127205737.GI16136@redhat.com>
Date:   Tue, 27 Nov 2018 15:57:37 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     rong.a.chen@...el.com, Michal Hocko <mhocko@...e.com>,
        s.priebe@...fihost.ag, alex.williamson@...hat.com,
        mgorman@...hsingularity.net, zi.yan@...rutgers.edu,
        Vlastimil Babka <vbabka@...e.cz>, rientjes@...gle.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        lkp@...org
Subject: Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3%
 regression

Hi Linus,

On Tue, Nov 27, 2018 at 09:08:50AM -0800, Linus Torvalds wrote:
> On Mon, Nov 26, 2018 at 10:24 PM kernel test robot
> <rong.a.chen@...el.com> wrote:
> >
> > FYI, we noticed a -61.3% regression of vm-scalability.throughput due
> > to commit ac5b2c18911f ("mm: thp: relax __GFP_THISNODE for
> > MADV_HUGEPAGE mappings")
> 
> Well, that's certainly noticeable and not good.

Noticed this email too.

This difference can only happen with defrag=always, and that's not the
current upstream default.

        thp_enabled: always
        thp_defrag: always
        ^^^^^^^^^^^^^^^^^^ emulates MADV_HUGEPAGE always set

> Andrea, I suspect it might be causing fights with auto numa migration..

That MADV_HUGEPAGE causes flights with NUMA balancing is not great
indeed, qemu needs NUMA locality too, but then the badness caused by
__GFP_THISNODE was a larger regression in the worst case for qemu.

When the kernel wants to allocate a THP from node A, if there are no
THP generated on node A but there are in node B, they'll be picked from
node B now.

__GFP_THISNODE previously prevented any THP to be allocated from any
node except A. This introduces a higher chance of initial misplacement
which NUMA balancing will correct over time, but it should only apply
to long lived allocations under MADV_HUGEPAGE. Perhaps the workload
here uses short lived allocations and sets defrag=always which is not
optimal to begin with?

The motivation of the fix, is that the previous code invoked reclaim
with __GFP_THISNODE set. That looked insecure and such behavior should
only have been possible under a mlock/mempolicy
capability. __GFP_THISNODE is like a transient but still privileged
mbind for reclaim.

Before the fix, __GFP_THISNODE would end up swapping out everything
from node A to free 4k pages from node A, despite perhaps there were
gigabytes of memory free in node B. That caused severe regression to
threaded workloads whose memory spanned more than one NUMA node. So
again going back doesn't sounds great for NUMA in general.

The vmscalability test is most certainly not including any
multithreaded process whose memory doesn't fit in a single NUMA node
or we'd see also the other side of the tradeoff. It'd be nice to add
such a test to be sure that the old __GFP_THISNODE behavior won't
happen again for apps that don't fit in a single node.

> Lots more system time, but also look at this:
> 
> >    1122389 ±  9%     +17.2%    1315380 ±  4%  proc-vmstat.numa_hit
> >     214722 ±  5%     +21.6%     261076 ±  3%  proc-vmstat.numa_huge_pte_updates
> >    1108142 ±  9%     +17.4%    1300857 ±  4%  proc-vmstat.numa_local
> >     145368 ± 48%     +63.1%     237050 ± 17%  proc-vmstat.numa_miss
> >     159615 ± 44%     +57.6%     251573 ± 16%  proc-vmstat.numa_other
> >     185.50 ± 81%   +8278.6%      15542 ± 40%  proc-vmstat.numa_pages_migrated
> 
> Should the commit be reverted? Or perhaps at least modified?

I proposed two solutions, the other one required a new minor feature:
__GFP_ONLY_COMPACT. The other solution wouldn't regress like
above. The THP utilization ratio would decrease though (it had margin
for improvement though).

Kirill preferred the __GFP_ONLY_COMPACT, I was mostly neutral because
it's a tradeoff.

So the short term alternative again would be the alternate patch that
does __GFP_THISNODE|GFP_ONLY_COMPACT appended below.

There's no particular problem in restricting only compaction to the
local node and to skip reclaim entirely during a THP allocation as
long as reclaim is skipped entirely.

David implemented a hardcoded version of GFP_COMPACTONLY too which was
runtime equivalent, but it was hardcoded for THP only the allocator,
but it looks less flexible to hardcode it for THP.

The current fix you merged is simpler overall and puts us back to a
"stable" state without introducing new (minor) features.

The below is for further review of the potential alternative (which
has still margin for improvement).

=======
From: Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH 1/2] mm: thp: consolidate policy_nodemask call

Just a minor cleanup.

Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 mm/mempolicy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 01f1a14facc4..d6512ef28cde 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2026,6 +2026,8 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 		goto out;
 	}
 
+	nmask = policy_nodemask(gfp, pol);
+
 	if (unlikely(IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hugepage)) {
 		int hpage_node = node;
 
@@ -2043,7 +2045,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 						!(pol->flags & MPOL_F_LOCAL))
 			hpage_node = pol->v.preferred_node;
 
-		nmask = policy_nodemask(gfp, pol);
 		if (!nmask || node_isset(hpage_node, *nmask)) {
 			mpol_cond_put(pol);
 			page = __alloc_pages_node(hpage_node,
@@ -2052,7 +2053,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 		}
 	}
 
-	nmask = policy_nodemask(gfp, pol);
 	preferred_nid = policy_node(gfp, pol, node);
 	page = __alloc_pages_nodemask(gfp, order, preferred_nid, nmask);
 	mpol_cond_put(pol);


From: Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH 2/2] mm: thp: fix transparent_hugepage/defrag = madvise || always

qemu uses MADV_HUGEPAGE which allows direct compaction (i.e.
__GFP_DIRECT_RECLAIM is set).

The problem is that direct compaction combined with the NUMA
__GFP_THISNODE logic in mempolicy.c is telling reclaim to swap very
hard the local node, instead of failing the allocation if there's no
THP available in the local node.

Such logic was ok until __GFP_THISNODE was added to the THP allocation
path even with MPOL_DEFAULT.

The idea behind the __GFP_THISNODE addition, is that it is better to
provide local memory in PAGE_SIZE units than to use remote NUMA THP
backed memory. That largely depends on the remote latency though, on
threadrippers for example the overhead is relatively low in my
experience.

The combination of __GFP_THISNODE and __GFP_DIRECT_RECLAIM results in
extremely slow qemu startup with vfio, if the VM is larger than the
size of one host NUMA node. This is because it will try very hard to
unsuccessfully swapout get_user_pages pinned pages as result of the
__GFP_THISNODE being set, instead of falling back to PAGE_SIZE
allocations and instead of trying to allocate THP on other nodes (it
would be even worse without vfio type1 GUP pins of course, except it'd
be swapping heavily instead).

It's very easy to reproduce this by setting
transparent_hugepage/defrag to "always", even with a simple memhog.

1) This can be fixed by retaining the __GFP_THISNODE logic also for
   __GFP_DIRECT_RELCAIM by allowing only one compaction run. Not even
   COMPACT_SKIPPED (i.e. compaction failing because not enough free
   memory in the zone) should be allowed to invoke reclaim.

2) An alternative is not use __GFP_THISNODE if __GFP_DIRECT_RELCAIM
   has been set by the caller (i.e. MADV_HUGEPAGE or
   defrag="always"). That would keep the NUMA locality restriction
   only when __GFP_DIRECT_RECLAIM is not set by the caller. So THP
   will be provided from remote nodes if available before falling back
   to PAGE_SIZE units in the local node, but an app using defrag =
   always (or madvise with MADV_HUGEPAGE) supposedly prefers that.

These are the results of 1) (higher GB/s is better).

Finished: 30 GB mapped, 10.188535s elapsed, 2.94GB/s
Finished: 34 GB mapped, 12.274777s elapsed, 2.77GB/s
Finished: 38 GB mapped, 13.847840s elapsed, 2.74GB/s
Finished: 42 GB mapped, 14.288587s elapsed, 2.94GB/s

Finished: 30 GB mapped, 8.907367s elapsed, 3.37GB/s
Finished: 34 GB mapped, 10.724797s elapsed, 3.17GB/s
Finished: 38 GB mapped, 14.272882s elapsed, 2.66GB/s
Finished: 42 GB mapped, 13.929525s elapsed, 3.02GB/s

These are the results of 2) (higher GB/s is better).

Finished: 30 GB mapped, 10.163159s elapsed, 2.95GB/s
Finished: 34 GB mapped, 11.806526s elapsed, 2.88GB/s
Finished: 38 GB mapped, 10.369081s elapsed, 3.66GB/s
Finished: 42 GB mapped, 12.357719s elapsed, 3.40GB/s

Finished: 30 GB mapped, 8.251396s elapsed, 3.64GB/s
Finished: 34 GB mapped, 12.093030s elapsed, 2.81GB/s
Finished: 38 GB mapped, 11.824903s elapsed, 3.21GB/s
Finished: 42 GB mapped, 15.950661s elapsed, 2.63GB/s

This is current upstream (higher GB/s is better).

Finished: 30 GB mapped, 8.821632s elapsed, 3.40GB/s
Finished: 34 GB mapped, 341.979543s elapsed, 0.10GB/s
Finished: 38 GB mapped, 761.933231s elapsed, 0.05GB/s
Finished: 42 GB mapped, 1188.409235s elapsed, 0.04GB/s

vfio is a good test because by pinning all memory it avoids the
swapping and reclaim only wastes CPU, a memhog based test would
created swapout storms and supposedly show a bigger stddev.

What is better between 1) and 2) depends on the hardware and on the
software. Virtualization EPT/NTP gets a bigger boost from THP as well
than host applications.

This commit implements 1).

Reported-by: Alex Williamson <alex.williamson@...hat.com>
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 include/linux/gfp.h | 18 ++++++++++++++++++
 mm/mempolicy.c      | 12 +++++++++++-
 mm/page_alloc.c     |  4 ++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index a6afcec53795..3c04d5d90e6d 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -44,6 +44,7 @@ struct vm_area_struct;
 #else
 #define ___GFP_NOLOCKDEP	0
 #endif
+#define ___GFP_ONLY_COMPACT	0x1000000u
 /* If the above are modified, __GFP_BITS_SHIFT may need updating */
 
 /*
@@ -178,6 +179,21 @@ struct vm_area_struct;
  *   definitely preferable to use the flag rather than opencode endless
  *   loop around allocator.
  *   Using this flag for costly allocations is _highly_ discouraged.
+ *
+ * __GFP_ONLY_COMPACT: Only invoke compaction. Do not try to succeed
+ * the allocation by freeing memory. Never risk to free any
+ * "PAGE_SIZE" memory unit even if compaction failed specifically
+ * because of not enough free pages in the zone. This only makes sense
+ * only in combination with __GFP_THISNODE (enforced with a
+ * VM_WARN_ON), to restrict the THP allocation in the local node that
+ * triggered the page fault and fallback into PAGE_SIZE allocations in
+ * the same node. We don't want to invoke reclaim because there may be
+ * plenty of free memory already in the local node. More importantly
+ * there may be even plenty of free THP available in remote nodes so
+ * we should allocate those if something instead of reclaiming any
+ * memory in the local node. Implementation detail: set ___GFP_NORETRY
+ * too so that ___GFP_ONLY_COMPACT only needs to be checked in a slow
+ * path.
  */
 #define __GFP_IO	((__force gfp_t)___GFP_IO)
 #define __GFP_FS	((__force gfp_t)___GFP_FS)
@@ -187,6 +203,8 @@ struct vm_area_struct;
 #define __GFP_RETRY_MAYFAIL	((__force gfp_t)___GFP_RETRY_MAYFAIL)
 #define __GFP_NOFAIL	((__force gfp_t)___GFP_NOFAIL)
 #define __GFP_NORETRY	((__force gfp_t)___GFP_NORETRY)
+#define __GFP_ONLY_COMPACT	((__force gfp_t)(___GFP_NORETRY | \
+						 ___GFP_ONLY_COMPACT))
 
 /*
  * Action modifiers
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index d6512ef28cde..6bf839f20dcc 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2047,8 +2047,18 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 
 		if (!nmask || node_isset(hpage_node, *nmask)) {
 			mpol_cond_put(pol);
+			/*
+			 * We restricted the allocation to the
+			 * hpage_node so we must use
+			 * __GFP_ONLY_COMPACT to allow at most a
+			 * compaction attempt and not ever get into
+			 * reclaim or it'll swap heavily with
+			 * transparent_hugepage/defrag = always (or
+			 * madvise under MADV_HUGEPAGE).
+			 */
 			page = __alloc_pages_node(hpage_node,
-						gfp | __GFP_THISNODE, order);
+						  gfp | __GFP_THISNODE |
+						  __GFP_ONLY_COMPACT, order);
 			goto out;
 		}
 	}
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a790ef4be74e..01a5c2bd0860 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4144,6 +4144,10 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 			 */
 			if (compact_result == COMPACT_DEFERRED)
 				goto nopage;
+			if (gfp_mask & __GFP_ONLY_COMPACT) {
+				VM_WARN_ON(!(gfp_mask & __GFP_THISNODE));
+				goto nopage;
+			}
 
 			/*
 			 * Looks like reclaim/compaction is worth trying, but

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