Re: [RFC PATCH 0/6] Configurable fair allocation zone policy v3

From: Johannes Weiner
Date: Tue Dec 17 2013 - 15:02:27 EST

Hi Mel,

On Tue, Dec 17, 2013 at 04:48:18PM +0000, Mel Gorman wrote:
> This series is currently untested and is being posted to sync up discussions
> on the treatment of page cache pages, particularly the sysv part. I have
> not thought it through in detail but postings patches is the easiest way
> to highlight where I think a problem might be.
>
> Changelog since v2
> o Drop an accounting patch, behaviour is deliberate
> o Special case tmpfs and shmem pages for discussion
>
> Changelog since v1
> o Fix lot of brain damage in the configurable policy patch
> o Yoink a page cache annotation patch
> o Only account batch pages against allocations eligible for the fair policy
> o Add patch that default distributes file pages on remote nodes
>
> Commit 81c0a2bb ("mm: page_alloc: fair zone allocator policy") solved a
> bug whereby new pages could be reclaimed before old pages because of how
> the page allocator and kswapd interacted on the per-zone LRU lists.

Not just that, it was about ensuring predictable cache replacement and
maximizing the cache's effectiveness. This implicitely fixed the
kswapd interaction bug, but that was not the sole reason (I realize
that the original changelog is incomplete and I apologize for that).

I have had offline discussions with Andrea back then and his first
suggestion was too to make this a zone fairness placement that is
exclusive to the local node, but eventually he agreed that the problem
applies just as much on the global level and that we should apply
fairness throughout the system as long as we honor zone_reclaim_mode
and hard bindings. During our discussions now, it turned out that
zone_reclaim_mode is a terrible predictor for preferred locality, but
we also more or less agreed that the locality issues in the first
place are not really applicable to cache loads dominated by IO cost.

So I think the main discrepancy between the original patch and what we
truly want is that aging fairness is really only relevant for actual
cache backed by secondary storage, because cache replacement is an
ongoing operation that involves IO. As opposed to memory types that
involve IO only in extreme cases (anon, tmpfs, shmem) or no IO at all
(slab, kernel allocations), in which case we prefer NUMA locality.

> Unfortunately a side-effect missed during review was that it's now very
> easy to allocate remote memory on NUMA machines. The problem is that
> it is not a simple case of just restoring local allocation policies as
> there are genuine reasons why global page aging may be prefereable. It's
> still a major change to default behaviour so this patch makes the policy
> configurable and sets what I think is a sensible default.
>
> The patches are on top of some NUMA balancing patches currently in -mm.
> It's untested and posted to discuss patches 4 and 6.

It might be easier in dealing with -stable if we start with the
critical fix(es) to restore sane functionality as much and as compact
as possible and then place the cleanups on top?

In my local tree, I have the following as the first patch:

---
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Subject: [patch] mm: page_alloc: restrict fair allocator policy to page cache

81c0a2bb515f ("mm: page_alloc: fair zone allocator policy") was merged
in order to ensure predictable page cache replacement and to maximize
the cache's effectiveness of reducing IO regardless of zone or node
topology.

However, it was overzealous in round-robin placing every type of
allocation over all allowable nodes, instead of preferring locality,
which resulted in severe regressions on certain NUMA workloads that
have nothing to do with page cache.

This patch drastically reduces the impact of the original change by
having the round-robin placement policy only apply to page cache
backed by secondary storage, and no longer to anonymous memory, shmem,
tmpfs, slab allocations.

This still changes the long-standing behavior of page cache adhering
to the configured memory policy and preferring local allocations per
default, so make it configurable in case somebody relies on it.
However, we also expect the majority of users to prefer maximium cache
effectiveness and a predictable replacement behavior over memory
locality, so reflect this in the default setting of the sysctl.
---
Documentation/sysctl/vm.txt | 21 +++++++++++++++++
Documentation/vm/numa_memory_policy.txt | 8 +++++++
include/linux/gfp.h | 4 +++-
include/linux/pagemap.h | 2 +-
include/linux/swap.h | 2 ++
kernel/sysctl.c | 8 +++++++
mm/filemap.c | 2 ++
mm/page_alloc.c | 41 +++++++++++++++++++++++++--------
8 files changed, 76 insertions(+), 12 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 1fbd4eb7b64a..50d250f7470f 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -38,6 +38,7 @@ Currently, these files are in /proc/sys/vm:
- memory_failure_early_kill
- memory_failure_recovery
- min_free_kbytes
+- pagecache_mempolicy_mode
- min_slab_ratio
- min_unmapped_ratio
- mmap_min_addr
@@ -404,6 +405,26 @@ Setting this too high will OOM your machine instantly.

=============================================================

+pagecache_mempolicy_mode:
+
+This is available only on NUMA kernels.
+
+Per default, the configured memory policy is applicable to anonymous
+memory, shmem, tmpfs, etc., whereas pagecache is allocated in an
+interleaving fashion over all allowed nodes (hardbindings and
+zone_reclaim_mode excluded).
+
+The assumption is that, when it comes to pagecache, users generally
+prefer predictable replacement behavior regardless of NUMA topology
+and maximizing the cache's effectiveness in reducing IO over memory
+locality.
+
+This behavior can be changed by enabling pagecache_mempolicy_mode, in
+which case page cache allocations will be placed according to the
+configured memory policy (Documentation/vm/numa_memory_policy.txt).
+
+=============================================================
+
min_slab_ratio:

This is available only on NUMA kernels.
diff --git a/Documentation/vm/numa_memory_policy.txt b/Documentation/vm/numa_memory_policy.txt
index 4e7da6543424..64d48b6378db 100644
--- a/Documentation/vm/numa_memory_policy.txt
+++ b/Documentation/vm/numa_memory_policy.txt
@@ -16,6 +16,14 @@ programming interface that a NUMA-aware application can take advantage of. When
both cpusets and policies are applied to a task, the restrictions of the cpuset
takes priority. See "MEMORY POLICIES AND CPUSETS" below for more details.

+Note that, per default, the memory policies as described below apply to process
+memory and shmem/tmpfs/ramfs only. Pagecache backed by secondary storage will
+be interleaved fairly over all allowable nodes (respecting hardbindings and
+zone_reclaim_mode) in order to maximize the cache's effectiveness in reducing IO
+and to ensure predictable cache replacement. Special setups that require
+pagecache to adhere to the configured memory policy can change this behavior by
+enabling pagecache_mempolicy_mode (see Documentation/sysctl/vm.txt).
+
MEMORY POLICY CONCEPTS

Scope of Memory Policies
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 9b4dd491f7e8..f69e4cb78ccf 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -35,6 +35,7 @@ struct vm_area_struct;
#define ___GFP_NO_KSWAPD 0x400000u
#define ___GFP_OTHER_NODE 0x800000u
#define ___GFP_WRITE 0x1000000u
+#define ___GFP_PAGECACHE 0x2000000u
/* If the above are modified, __GFP_BITS_SHIFT may need updating */

/*
@@ -92,6 +93,7 @@ struct vm_area_struct;
#define __GFP_OTHER_NODE ((__force gfp_t)___GFP_OTHER_NODE) /* On behalf of other node */
#define __GFP_KMEMCG ((__force gfp_t)___GFP_KMEMCG) /* Allocation comes from a memcg-accounted resource */
#define __GFP_WRITE ((__force gfp_t)___GFP_WRITE) /* Allocator intends to dirty page */
+#define __GFP_PAGECACHE ((__force gfp_t)___GFP_PAGECACHE) /* Page cache allocation */

/*
* This may seem redundant, but it's a way of annotating false positives vs.
@@ -99,7 +101,7 @@ struct vm_area_struct;
*/
#define __GFP_NOTRACK_FALSE_POSITIVE (__GFP_NOTRACK)

-#define __GFP_BITS_SHIFT 25 /* Room for N __GFP_FOO bits */
+#define __GFP_BITS_SHIFT 26 /* Room for N __GFP_FOO bits */
#define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))

/* This equals 0, but use constants in case they ever change */
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e3dea75a078b..bda48453af8e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -221,7 +221,7 @@ extern struct page *__page_cache_alloc(gfp_t gfp);
#else
static inline struct page *__page_cache_alloc(gfp_t gfp)
{
- return alloc_pages(gfp, 0);
+ return alloc_pages(gfp | __GFP_PAGECACHE, 0);
}
#endif

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 46ba0c6c219f..3458994b0881 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -320,11 +320,13 @@ extern unsigned long vm_total_pages;

#ifdef CONFIG_NUMA
extern int zone_reclaim_mode;
+extern int pagecache_mempolicy_mode;
extern int sysctl_min_unmapped_ratio;
extern int sysctl_min_slab_ratio;
extern int zone_reclaim(struct zone *, gfp_t, unsigned int);
#else
#define zone_reclaim_mode 0
+#define pagecache_mempolicy_mode 0
static inline int zone_reclaim(struct zone *z, gfp_t mask, unsigned int order)
{
return 0;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 34a604726d0b..a8c56c1dc98e 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1359,6 +1359,14 @@ static struct ctl_table vm_table[] = {
.extra1 = &zero,
},
{
+ .procname = "pagecache_mempolicy_mode",
+ .data = &pagecache_mempolicy_mode,
+ .maxlen = sizeof(pagecache_mempolicy_mode),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ .extra1 = &zero,
+ },
+ {
.procname = "min_unmapped_ratio",
.data = &sysctl_min_unmapped_ratio,
.maxlen = sizeof(sysctl_min_unmapped_ratio),
diff --git a/mm/filemap.c b/mm/filemap.c
index b7749a92021c..5bb922506906 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -517,6 +517,8 @@ struct page *__page_cache_alloc(gfp_t gfp)
int n;
struct page *page;

+ gfp |= __GFP_PAGECACHE;
+
if (cpuset_do_page_mem_spread()) {
unsigned int cpuset_mems_cookie;
do {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 580a5f075ed0..b28370932950 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1547,7 +1547,15 @@ again:
get_pageblock_migratetype(page));
}

+ /*
+ * All allocations eat into the round-robin batch, even
+ * allocations that are not subject to round-robin placement
+ * themselves. This makes sure that allocations that ARE
+ * subject to round-robin placement compensate for the
+ * allocations that aren't, to have equal placement overall.
+ */
__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
+
__count_zone_vm_events(PGALLOC, zone, 1 << order);
zone_statistics(preferred_zone, zone, gfp_flags);
local_irq_restore(flags);
@@ -1699,6 +1707,15 @@ bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,

#ifdef CONFIG_NUMA
/*
+ * pagecache_mempolicy_mode - whether page cache should honor the
+ * configured memory policy and allocate from the zonelist in order of
+ * preference, or whether it should be interleaved fairly over all
+ * allowed zones in the given zonelist to maximize cache effects and
+ * ensure predictable cache replacement.
+ */
+int pagecache_mempolicy_mode __read_mostly;
+
+/*
* zlc_setup - Setup for "zonelist cache". Uses cached zone data to
* skip over zones that are not allowed by the cpuset, or that have
* been recently (in last second) found to be nearly full. See further
@@ -1816,7 +1833,7 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)

static bool zone_local(struct zone *local_zone, struct zone *zone)
{
- return node_distance(local_zone->node, zone->node) == LOCAL_DISTANCE;
+ return local_zone->node == zone->node;
}

static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
@@ -1908,22 +1925,25 @@ zonelist_scan:
if (unlikely(alloc_flags & ALLOC_NO_WATERMARKS))
goto try_this_zone;
/*
- * Distribute pages in proportion to the individual
- * zone size to ensure fair page aging. The zone a
- * page was allocated in should have no effect on the
- * time the page has in memory before being reclaimed.
+ * Distribute page cache pages in proportion to the
+ * individual zone size to ensure fair page aging.
+ * The zone a page was allocated in should have no
+ * effect on the time the page has in memory before
+ * being reclaimed.
*
- * When zone_reclaim_mode is enabled, try to stay in
- * local zones in the fastpath. If that fails, the
+ * When pagecache_mempolicy_mode or zone_reclaim_mode
+ * is enabled, try to allocate from zones within the
+ * preferred node in the fastpath. If that fails, the
* slowpath is entered, which will do another pass
* starting with the local zones, but ultimately fall
* back to remote zones that do not partake in the
* fairness round-robin cycle of this zonelist.
*/
- if (alloc_flags & ALLOC_WMARK_LOW) {
+ if ((alloc_flags & ALLOC_WMARK_LOW) &&
+ (gfp_mask & __GFP_PAGECACHE)) {
if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0)
continue;
- if (zone_reclaim_mode &&
+ if ((zone_reclaim_mode || pagecache_mempolicy_mode) &&
!zone_local(preferred_zone, zone))
continue;
}
@@ -2390,7 +2410,8 @@ static void prepare_slowpath(gfp_t gfp_mask, unsigned int order,
* thrash fairness information for zones that are not
* actually part of this zonelist's round-robin cycle.
*/
- if (zone_reclaim_mode && !zone_local(preferred_zone, zone))
+ if ((zone_reclaim_mode || pagecache_mempolicy_mode) &&
+ !zone_local(preferred_zone, zone))
continue;
mod_zone_page_state(zone, NR_ALLOC_BATCH,
high_wmark_pages(zone) -
--
1.8.4.2

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