Re: [RFC 06/13] mm, thp: remove __GFP_NORETRY from khugepaged and madvised allocations

Vlastimil Babka
2016-05-20T13:57:13Z

On 05/18/2016 05:24 PM, Michal Hocko wrote:
>>
>> So the following patch attempts what you suggest, if I understand you
>> correctly. GFP_TRANSHUGE includes all possible flag, and then they are
>> removed as needed. I don't really think it helps code readability
>> though.
>
> yeah it is ugly has _hell_. I do not think this deserves too much time
> to discuss as the flag is mostly internal but one last proposal would be
> to define different THP allocations context explicitly. Some callers
> would still need some additional meddling but maybe it would be slightly
> better to read. Dunno. Anyway if you think this is not really an
> improvement then I won't insist on any change to your original patch.
> ---
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 570383a41853..e7926b466107 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -255,9 +255,14 @@ struct vm_area_struct;
> #define GFP_DMA32 __GFP_DMA32
> #define GFP_HIGHUSER (GFP_USER | __GFP_HIGHMEM)
> #define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
> -#define GFP_TRANSHUGE ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
> +
> +/* Optimistic or latency sensitive THP allocation - page fault path */
> +#define GFP_TRANSHUGE_LIGHT ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
> __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN) & \
> ~__GFP_RECLAIM)
> +/* More serious THP allocation request - kcompactd */
> +#define GFP_TRANSHUGE (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM) & \
> + ~__GFP_NORETRY

[...]

OK I took the core idea and arrived at the following. I think it could
work and the amount of further per-site modifications to GFP_TRANSHUGE*
is reduced, but if anyone think it's overkill to have two
GFP_TRANSHUGE*, I will just return to the original patch.

>From 48ddb10e96fd9741a9eb3be9672c13589db7239a Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Wed, 4 May 2016 13:40:03 +0200
Subject: [PATCH] mm, thp: remove __GFP_NORETRY from khugepaged and madvised
allocations

After the previous patch, we can distinguish costly allocations that should be
really lightweight, such as THP page faults, with __GFP_NORETRY. This means we
don't need to recognize khugepaged allocations via PF_KTHREAD anymore. We can
also change THP page faults in areas where madvise(MADV_HUGEPAGE) was used to
try as hard as khugepaged, as the process has indicated that it benefits from
THP's and is willing to pay some initial latency costs.

We can also make the flags handling less cryptic by distinguishing
GFP_TRANSHUGE_LIGHT (no reclaim at all, default mode in page fault) from
GFP_TRANSHUGE (only direct reclaim, khugepaged default). Adding __GFP_NORETRY
or __GFP_KSWAPD_RECLAIM is done where needed.

The patch effectively changes the current GFP_TRANSHUGE users as follows:

* get_huge_zero_page() - the zero page lifetime should be relatively long and
it's shared by multiple users, so it's worth spending some effort on it.
We use GFP_TRANSHUGE, and __GFP_NORETRY is not added. This also restores
direct reclaim to this allocation, which was unintentionally removed by
commit e4a49efe4e7e ("mm: thp: set THP defrag by default to madvise and add
a stall-free defrag option")

* alloc_hugepage_khugepaged_gfpmask() - this is khugepaged, so latency is not
an issue. So if khugepaged "defrag" is enabled (the default), do reclaim
via GFP_TRANSHUGE without __GFP_NORETRY. We can remove the PF_KTHREAD check
from page alloc.
As a side-effect, khugepaged will now no longer check if the initial
compaction was deferred or contended. This is OK, as khugepaged sleep times
between collapsion attemps are long enough to prevent noticeable disruption,
so we should allow it to spend some effort.

* migrate_misplaced_transhuge_page() - already was masking out __GFP_RECLAIM,
so just convert to GFP_TRANSHUGE_LIGHT which is equivalent.

* alloc_hugepage_direct_gfpmask() - vma's with VM_HUGEPAGE (via madvise) are
now allocating without __GFP_NORETRY. Other vma's keep using __GFP_NORETRY
if direct reclaim/compaction is at all allowed (by default it's allowed only
for madvised vma's). The rest is conversion to GFP_TRANSHUGE(_LIGHT).

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
include/linux/gfp.h | 14 ++++++++------
include/trace/events/mmflags.h | 1 +
mm/huge_memory.c | 27 +++++++++++++++------------
mm/migrate.c | 2 +-
mm/page_alloc.c | 6 ++----
tools/perf/builtin-kmem.c | 1 +
6 files changed, 28 insertions(+), 23 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 570383a41853..1dfca27df492 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -238,9 +238,11 @@ struct vm_area_struct;
* are expected to be movable via page reclaim or page migration. Typically,
* pages on the LRU would also be allocated with GFP_HIGHUSER_MOVABLE.
*
- * GFP_TRANSHUGE is used for THP allocations. They are compound allocations
- * that will fail quickly if memory is not available and will not wake
- * kswapd on failure.
+ * GFP_TRANSHUGE and GFP_TRANSHUGE_LIGHT are used for THP allocations. They are
+ * compound allocations that will generally fail quickly if memory is not
+ * available and will not wake kswapd/kcompactd on failure. The _LIGHT
+ * version does not attempt reclaim/compaction at all and is by default used
+ * in page fault path, while the non-light is used by khugepaged.
*/
#define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
#define GFP_KERNEL (__GFP_RECLAIM | __GFP_IO | __GFP_FS)
@@ -255,9 +257,9 @@ struct vm_area_struct;
#define GFP_DMA32 __GFP_DMA32
#define GFP_HIGHUSER (GFP_USER | __GFP_HIGHMEM)
#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
-#define GFP_TRANSHUGE ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
- __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN) & \
- ~__GFP_RECLAIM)
+#define GFP_TRANSHUGE_LIGHT ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
+ __GFP_NOMEMALLOC| __GFP_NOWARN) & ~__GFP_RECLAIM)
+#define GFP_TRANSHUGE (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM)

/* Convert GFP flags to their corresponding migrate type */
#define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 43cedbf0c759..5a81ab48a2fb 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -11,6 +11,7 @@

#define __def_gfpflag_names \
{(unsigned long)GFP_TRANSHUGE, "GFP_TRANSHUGE"}, \
+ {(unsigned long)GFP_TRANSHUGE_LIGHT, "GFP_TRANSHUGE_LIGHT"}, \
{(unsigned long)GFP_HIGHUSER_MOVABLE, "GFP_HIGHUSER_MOVABLE"},\
{(unsigned long)GFP_HIGHUSER, "GFP_HIGHUSER"}, \
{(unsigned long)GFP_USER, "GFP_USER"}, \
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 87f09dc986ab..aa87db8c7f8f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -882,29 +882,32 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
}

/*
- * If THP is set to always then directly reclaim/compact as necessary
- * If set to defer then do no reclaim and defer to khugepaged
+ * If THP defrag is set to always then directly reclaim/compact as necessary
+ * If set to defer then do only background reclaim/compact and defer to khugepaged
* If set to madvise and the VMA is flagged then directly reclaim/compact
+ * When direct reclaim/compact is allowed, don't retry except for flagged VMA's
*/
static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
{
- gfp_t reclaim_flags = 0;
+ bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);

- if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags) &&
- (vma->vm_flags & VM_HUGEPAGE))
- reclaim_flags = __GFP_DIRECT_RECLAIM;
- else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags))
- reclaim_flags = __GFP_KSWAPD_RECLAIM;
- else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags))
- reclaim_flags = __GFP_DIRECT_RECLAIM;
+ if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG,
+ &transparent_hugepage_flags) && vma_madvised)
+ return GFP_TRANSHUGE;
+ else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
+ &transparent_hugepage_flags))
+ return GFP_TRANSHUGE_LIGHT | __GFP_KSWAPD_RECLAIM;
+ else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
+ &transparent_hugepage_flags))
+ return GFP_TRANSHUGE | (vma_madvised ? 0 : __GFP_NORETRY);

- return GFP_TRANSHUGE | reclaim_flags;
+ return GFP_TRANSHUGE_LIGHT;
}

/* Defrag for khugepaged will enter direct reclaim/compaction if necessary */
static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
{
- return GFP_TRANSHUGE | (khugepaged_defrag() ? __GFP_DIRECT_RECLAIM : 0);
+ return khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT;
}

/* Caller must hold page table lock. */
diff --git a/mm/migrate.c b/mm/migrate.c
index 53ab6398e7a2..bc82c56fa3af 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1771,7 +1771,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
goto out_dropref;

new_page = alloc_pages_node(node,
- (GFP_TRANSHUGE | __GFP_THISNODE) & ~__GFP_RECLAIM,
+ (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
HPAGE_PMD_ORDER);
if (!new_page)
goto out_fail;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0cee863397e4..4a34187827ca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3619,11 +3619,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
/*
* Looks like reclaim/compaction is worth trying, but
* sync compaction could be very expensive, so keep
- * using async compaction, unless it's khugepaged
- * trying to collapse.
+ * using async compaction.
*/
- if (!(current->flags & PF_KTHREAD))
- migration_mode = MIGRATE_ASYNC;
+ migration_mode = MIGRATE_ASYNC;
}
}

diff --git a/tools/perf/builtin-kmem.c b/tools/perf/builtin-kmem.c
index 5da5a9511cef..7fde754b344d 100644
--- a/tools/perf/builtin-kmem.c
+++ b/tools/perf/builtin-kmem.c
@@ -608,6 +608,7 @@ static const struct {
const char *compact;
} gfp_compact_table[] = {
{ "GFP_TRANSHUGE", "THP" },
+ { "GFP_TRANSHUGE_LIGHT", "THL" },
{ "GFP_HIGHUSER_MOVABLE", "HUM" },
{ "GFP_HIGHUSER", "HU" },
{ "GFP_USER", "U" },
--
2.8.2