[PATCH 1/2] memcg: coalescing uncharge at unmap/truncate (Oct/9)

From: KAMEZAWA Hiroyuki
Date: Fri Oct 09 2009 - 04:02:19 EST

Thank you for all review to previous ones (Oct/2).

This patch is against mmotm + softlimit fix patches.
(which are now in -rc git tree.)
==
In massive parallel enviroment, res_counter can be a performance bottleneck.
One strong techinque to reduce lock contention is reducing calls by
coalescing some amount of calls into one.

Considering charge/uncharge chatacteristic,
- charge is done one by one via demand-paging.
- uncharge is done by
- in chunk at munmap, truncate, exit, execve...
- one by one via vmscan/paging.

It seems we have a chance to coalesce uncharges for improving scalability
at unmap/truncation.

This patch is a for coalescing uncharge. For avoiding scattering memcg's
structure to functions under /mm, this patch adds memcg batch uncharge
information to the task. A reason for per-task batching is for making
use of caller's context information. We do batched uncharge (deleyed uncharge)
when truncation/unmap occurs but do direct uncharge when uncharge is
called by memory reclaim (vmscan.c).

The degree of coalescing depends on callers
- at invalidate/trucate... pagevec size
- at unmap ....ZAP_BLOCK_SIZE
(memory itself will be freed in this degree.)
Then, we'll not coalescing too much.

On x86-64 8cpu server, I tested overheads of memcg at page fault by
running a program which does map/fault/unmap in a loop. Running
a task per a cpu by taskset and see sum of the number of page faults
in 60secs.

[without memcg config]
40156968 page-faults # 0.085 M/sec ( +- 0.046% )
27.67 cache-miss/faults
[root cgroup]
36659599 page-faults # 0.077 M/sec ( +- 0.247% )
31.58 miss/faults
[in a child cgroup]
18444157 page-faults # 0.039 M/sec ( +- 0.133% )
69.96 miss/faults
[child with this patch]
27133719 page-faults # 0.057 M/sec ( +- 0.155% )
47.16 miss/faults

We can see some amounts of improvement.
(root cgroup doesn't affected by this patch)
Another patch for "charge" will follow this and above will be improved more.

Changelog(since 2009/10/02):
- renamed filed of memcg_batch (as pages to bytes, memsw to memsw_bytes)
- some clean up and commentary/description updates.
- added initialize code to copy_process(). (possible bug fix)

Changelog(old):
- fixed !CONFIG_MEM_CGROUP case.
- rebased onto the latest mmotm + softlimit fix patches.
- unified patch for callers
- added commetns.
- make ->do_batch as bool.
- removed css_get() at el. We don't need it.

Cc: Balbir Singh <balbir@xxxxxxxxxx>
Cc: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
include/linux/memcontrol.h | 13 ++++++
include/linux/sched.h | 8 +++
kernel/fork.c | 4 +
mm/memcontrol.c | 96 ++++++++++++++++++++++++++++++++++++++++++---
mm/memory.c | 2
mm/truncate.c | 6 ++
6 files changed, 123 insertions(+), 6 deletions(-)

Index: mmotm-2.6.31-Sep28/include/linux/memcontrol.h
===================================================================
--- mmotm-2.6.31-Sep28.orig/include/linux/memcontrol.h
+++ mmotm-2.6.31-Sep28/include/linux/memcontrol.h
@@ -54,6 +54,11 @@ extern void mem_cgroup_rotate_lru_list(s
extern void mem_cgroup_del_lru(struct page *page);
extern void mem_cgroup_move_lists(struct page *page,
enum lru_list from, enum lru_list to);
+
+/* For coalescing uncharge for reducing memcg' overhead*/
+extern void mem_cgroup_uncharge_start(void);
+extern void mem_cgroup_uncharge_end(void);
+
extern void mem_cgroup_uncharge_page(struct page *page);
extern void mem_cgroup_uncharge_cache_page(struct page *page);
extern int mem_cgroup_shmem_charge_fallback(struct page *page,
@@ -151,6 +156,14 @@ static inline void mem_cgroup_cancel_cha
{
}

+static inline void mem_cgroup_uncharge_start(void)
+{
+}
+
+static inline void mem_cgroup_uncharge_end(void)
+{
+}
+
static inline void mem_cgroup_uncharge_page(struct page *page)
{
}
Index: mmotm-2.6.31-Sep28/mm/memcontrol.c
===================================================================
--- mmotm-2.6.31-Sep28.orig/mm/memcontrol.c
+++ mmotm-2.6.31-Sep28/mm/memcontrol.c
@@ -1826,6 +1826,50 @@ void mem_cgroup_cancel_charge_swapin(str
css_put(&mem->css);
}

+static void
+__do_uncharge(struct mem_cgroup *mem, const enum charge_type ctype)
+{
+ struct memcg_batch_info *batch = NULL;
+ bool uncharge_memsw = true;
+ /* If swapout, usage of swap doesn't decrease */
+ if (!do_swap_account || ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
+ uncharge_memsw = false;
+ /*
+ * do_batch > 0 when unmapping pages or inode invalidate/truncate.
+ * In those cases, all pages freed continously can be expected to be in
+ * the same cgroup and we have chance to coalesce uncharges.
+ * But we do uncharge one by one if this is killed by OOM(TIF_MEMDIE)
+ * because we want to do uncharge as soon as possible.
+ */
+ if (!current->memcg_batch.do_batch || test_thread_flag(TIF_MEMDIE))
+ goto direct_uncharge;
+
+ batch = &current->memcg_batch;
+ /*
+ * In usual, we do css_get() when we remember memcg pointer.
+ * But in this case, we keep res->usage until end of a series of
+ * uncharges. Then, it's ok to ignore memcg's refcnt.
+ */
+ if (!batch->memcg)
+ batch->memcg = mem;
+ /*
+ * In typical case, batch->memcg == mem. This means we can
+ * merge a series of uncharges to an uncharge of res_counter.
+ * If not, we uncharge res_counter ony by one.
+ */
+ if (batch->memcg != mem)
+ goto direct_uncharge;
+ /* remember freed charge and uncharge it later */
+ batch->bytes += PAGE_SIZE;
+ if (uncharge_memsw)
+ batch->memsw_bytes += PAGE_SIZE;
+ return;
+direct_uncharge:
+ res_counter_uncharge(&mem->res, PAGE_SIZE);
+ if (uncharge_memsw)
+ res_counter_uncharge(&mem->memsw, PAGE_SIZE);
+ return;
+}

/*
* uncharge if !page_mapped(page)
@@ -1874,12 +1918,8 @@ __mem_cgroup_uncharge_common(struct page
break;
}

- if (!mem_cgroup_is_root(mem)) {
- res_counter_uncharge(&mem->res, PAGE_SIZE);
- if (do_swap_account &&
- (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
- res_counter_uncharge(&mem->memsw, PAGE_SIZE);
- }
+ if (!mem_cgroup_is_root(mem))
+ __do_uncharge(mem, ctype);
if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
mem_cgroup_swap_statistics(mem, true);
mem_cgroup_charge_statistics(mem, pc, false);
@@ -1925,6 +1965,50 @@ void mem_cgroup_uncharge_cache_page(stru
__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
}

+/*
+ * Batch_start/batch_end is called in unmap_page_range/invlidate/trucate.
+ * In that cases, pages are freed continuously and we can expect pages
+ * are in the same memcg. All these calls itself limits the number of
+ * pages freed at once, then uncharge_start/end() is called properly.
+ * This may be called prural(2) times in a context,
+ */
+
+void mem_cgroup_uncharge_start(void)
+{
+ current->memcg_batch.do_batch++;
+ /* We can do nest. */
+ if (current->memcg_batch.do_batch == 1) {
+ current->memcg_batch.memcg = NULL;
+ current->memcg_batch.bytes = 0;
+ current->memcg_batch.memsw_bytes = 0;
+ }
+}
+
+void mem_cgroup_uncharge_end(void)
+{
+ struct memcg_batch_info *batch = &current->memcg_batch;
+
+ if (!batch->do_batch)
+ return;
+
+ batch->do_batch--;
+ if (batch->do_batch) /* If stacked, do nothing. */
+ return;
+
+ if (!batch->memcg)
+ return;
+ /*
+ * This "batch->memcg" is valid without any css_get/put etc...
+ * bacause we hide charges behind us.
+ */
+ if (batch->bytes)
+ res_counter_uncharge(&batch->memcg->res, batch->bytes);
+ if (batch->memsw_bytes)
+ res_counter_uncharge(&batch->memcg->memsw, batch->memsw_bytes);
+ /* forget this pointer (for sanity check) */
+ batch->memcg = NULL;
+}
+
#ifdef CONFIG_SWAP
/*
* called after __delete_from_swap_cache() and drop "page" account.
Index: mmotm-2.6.31-Sep28/include/linux/sched.h
===================================================================
--- mmotm-2.6.31-Sep28.orig/include/linux/sched.h
+++ mmotm-2.6.31-Sep28/include/linux/sched.h
@@ -1549,6 +1549,14 @@ struct task_struct {
unsigned long trace_recursion;
#endif /* CONFIG_TRACING */
unsigned long stack_start;
+#ifdef CONFIG_CGROUP_MEM_RES_CTLR /* memcg uses this to do batch job */
+ struct memcg_batch_info {
+ int do_batch; /* incremented when batch uncharge started */
+ struct mem_cgroup *memcg; /* target memcg of uncharge */
+ unsigned long bytes; /* uncharged usage */
+ unsigned long memsw_bytes; /* uncharged mem+swap usage */
+ } memcg_batch;
+#endif
};

/* Future-safe accessor for struct task_struct's cpus_allowed. */
Index: mmotm-2.6.31-Sep28/mm/memory.c
===================================================================
--- mmotm-2.6.31-Sep28.orig/mm/memory.c
+++ mmotm-2.6.31-Sep28/mm/memory.c
@@ -940,6 +940,7 @@ static unsigned long unmap_page_range(st
details = NULL;

BUG_ON(addr >= end);
+ mem_cgroup_uncharge_start();
tlb_start_vma(tlb, vma);
pgd = pgd_offset(vma->vm_mm, addr);
do {
@@ -952,6 +953,7 @@ static unsigned long unmap_page_range(st
zap_work, details);
} while (pgd++, addr = next, (addr != end && *zap_work > 0));
tlb_end_vma(tlb, vma);
+ mem_cgroup_uncharge_end();

return addr;
}
Index: mmotm-2.6.31-Sep28/mm/truncate.c
===================================================================
--- mmotm-2.6.31-Sep28.orig/mm/truncate.c
+++ mmotm-2.6.31-Sep28/mm/truncate.c
@@ -272,6 +272,7 @@ void truncate_inode_pages_range(struct a
pagevec_release(&pvec);
break;
}
+ mem_cgroup_uncharge_start();
for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];

@@ -286,6 +287,7 @@ void truncate_inode_pages_range(struct a
unlock_page(page);
}
pagevec_release(&pvec);
+ mem_cgroup_uncharge_end();
}
}
EXPORT_SYMBOL(truncate_inode_pages_range);
@@ -327,6 +329,7 @@ unsigned long invalidate_mapping_pages(s
pagevec_init(&pvec, 0);
while (next <= end &&
pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {
+ mem_cgroup_uncharge_start();
for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];
pgoff_t index;
@@ -354,6 +357,7 @@ unsigned long invalidate_mapping_pages(s
break;
}
pagevec_release(&pvec);
+ mem_cgroup_uncharge_end();
cond_resched();
}
return ret;
@@ -428,6 +432,7 @@ int invalidate_inode_pages2_range(struct
while (next <= end && !wrapped &&
pagevec_lookup(&pvec, mapping, next,
min(end - next, (pgoff_t)PAGEVEC_SIZE - 1) + 1)) {
+ mem_cgroup_uncharge_start();
for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];
pgoff_t page_index;
@@ -477,6 +482,7 @@ int invalidate_inode_pages2_range(struct
unlock_page(page);
}
pagevec_release(&pvec);
+ mem_cgroup_uncharge_end();
cond_resched();
}
return ret;
Index: mmotm-2.6.31-Sep28/kernel/fork.c
===================================================================
--- mmotm-2.6.31-Sep28.orig/kernel/fork.c
+++ mmotm-2.6.31-Sep28/kernel/fork.c
@@ -1114,6 +1114,10 @@ static struct task_struct *copy_process(
#ifdef CONFIG_DEBUG_MUTEXES
p->blocked_on = NULL; /* not blocked yet */
#endif
+#ifdef CONFIG_CGROUP_MEM_RES_CTLR
+ p->memcg_batch.do_batch = 0;
+ p->memcg_batch.memcg = NULL;
+#endif

p->bts = NULL;


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