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Subject[PATCH 1/2] mm,mlock: drain pagevecs asynchronously
Date
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Tao Ma reported current mlock is much slower than old 2.6.18 kernel. Because
lru_add_drain_all() spent much time. The problem are two. 1) lru_add_drain_all()
broadcast a worker thread to all cpus unconditionally. then, the performance
penalty is increased in proportion to number of cpus. 2) lru_add_drain_all()
wait the worker finished unnecessary. It makes bigger penalty.

This patch makes lru_add_drain_all_async() and changes mlock/mlockall use it.

Technical side note:
- has_pages_lru_pvecs() checks pagevecs locklessly. Of course, it's racy.
But it's no matter because asynchronous worker itself is also racy.
any lock can't close a race.
- Now, we drain pagevec at last of mlock instead of beginning. because
a page drain function (____pagevec_lru_add_fn) is PG_mlocked aware now.
Then it's safe and it close more race.
Without the patch:
% time ./test_mlock -c 100000

real 1m13.608s
user 0m0.204s
sys 0m40.115s

i.e. 200usec per mlock

With the patch:
% time ./test_mlock -c 100000
real 0m3.939s
user 0m0.060s
sys 0m3.868s

i.e. 13usec per mlock

test_mlock.c
==========================================
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
#include <sys/time.h>
#include <sys/mman.h>
#define MM_SZ1 24
#define MM_SZ2 56
#define MM_SZ3 4168

void mlock_test()
{
char ptr1[MM_SZ1];
char ptr2[MM_SZ2];
char ptr3[MM_SZ3];
if(0 != mlock(ptr1, MM_SZ1) )
perror("mlock MM_SZ1\n");
if(0 != mlock(ptr2, MM_SZ2) )
perror("mlock MM_SZ2\n");
if(0 != mlock(ptr3, MM_SZ3) )
perror("mlock MM_SZ3\n");
if(0 != munlock(ptr1, MM_SZ1) )
perror("munlock MM_SZ1\n");
if(0 != munlock(ptr2, MM_SZ2) )
perror("munlock MM_SZ2\n");
if(0 != munlock(ptr3, MM_SZ3) )
perror("munlock MM_SZ3\n");
}
int main(int argc, char *argv[])
{
int ret, opt;
int i,cnt;
while((opt = getopt(argc, argv, "c:")) != -1 )
{
switch(opt){
case 'c':
cnt = atoi(optarg);
break;
default:
printf("Usage: %s [-c count] arg...\n", argv[0]);
exit(EXIT_FAILURE);
}
}
for(i = 0; i < cnt; i++)
mlock_test();

return 0;
}
===========================================
Reported-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/swap.h | 1 +
mm/mlock.c | 7 +----
mm/swap.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 63 insertions(+), 9 deletions(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 1e22e12..11ad301 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -223,6 +223,7 @@ extern void activate_page(struct page *);
extern void mark_page_accessed(struct page *);
extern void lru_add_drain(void);
extern int lru_add_drain_all(void);
+extern void lru_add_drain_all_async(void);
extern void rotate_reclaimable_page(struct page *page);
extern void deactivate_page(struct page *page);
extern void swap_setup(void);
diff --git a/mm/mlock.c b/mm/mlock.c
index 4f4f53b..08f5b6b 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -487,8 +487,6 @@ SYSCALL_DEFINE2(mlock, unsigned long, start, size_t, len)
if (!can_do_mlock())
return -EPERM;

- lru_add_drain_all(); /* flush pagevec */
-
down_write(&current->mm->mmap_sem);
len = PAGE_ALIGN(len + (start & ~PAGE_MASK));
start &= PAGE_MASK;
@@ -505,6 +503,7 @@ SYSCALL_DEFINE2(mlock, unsigned long, start, size_t, len)
up_write(&current->mm->mmap_sem);
if (!error)
error = do_mlock_pages(start, len, 0);
+ lru_add_drain_all_async();
return error;
}

@@ -557,9 +556,6 @@ SYSCALL_DEFINE1(mlockall, int, flags)
if (!can_do_mlock())
goto out;

- if (flags & MCL_CURRENT)
- lru_add_drain_all(); /* flush pagevec */
-
down_write(&current->mm->mmap_sem);

lock_limit = rlimit(RLIMIT_MEMLOCK);
@@ -573,6 +569,7 @@ SYSCALL_DEFINE1(mlockall, int, flags)
if (!ret && (flags & MCL_CURRENT)) {
/* Ignore errors */
do_mlock_pages(0, TASK_SIZE, 1);
+ lru_add_drain_all_async();
}
out:
return ret;
diff --git a/mm/swap.c b/mm/swap.c
index a91caf7..2690f04 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -569,6 +569,49 @@ int lru_add_drain_all(void)
return schedule_on_each_cpu(lru_add_drain_per_cpu);
}

+static DEFINE_PER_CPU(struct work_struct, lru_drain_work);
+
+static int __init lru_drain_work_init(void)
+{
+ struct work_struct *work;
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ work = &per_cpu(lru_drain_work, cpu);
+ INIT_WORK(work, &lru_add_drain_per_cpu);
+ }
+
+ return 0;
+}
+core_initcall(lru_drain_work_init);
+
+static bool has_pages_lru_pvecs(int cpu)
+{
+ struct pagevec *pvecs = per_cpu(lru_add_pvecs, cpu);
+ struct pagevec *pvec;
+ int lru;
+
+ for_each_lru(lru) {
+ pvec = &pvecs[lru - LRU_BASE];
+ if (pagevec_count(pvec))
+ return true;
+ }
+
+ return false;
+}
+
+void lru_add_drain_all_async(void)
+{
+ int cpu;
+
+ for_each_online_cpu(cpu) {
+ struct work_struct *work = &per_cpu(lru_drain_work, cpu);
+
+ if (has_pages_lru_pvecs(cpu))
+ schedule_work_on(cpu, work);
+ }
+}
+
/*
* Batched page_cache_release(). Decrement the reference count on all the
* passed pages. If it fell to zero then remove the page from the LRU and
@@ -704,10 +747,23 @@ static void ____pagevec_lru_add_fn(struct page *page, void *arg)
VM_BUG_ON(PageLRU(page));

SetPageLRU(page);
- if (active)
- SetPageActive(page);
- update_page_reclaim_stat(zone, page, file, active);
- add_page_to_lru_list(zone, page, lru);
+ redo:
+ if (page_evictable(page, NULL)) {
+ if (active)
+ SetPageActive(page);
+ update_page_reclaim_stat(zone, page, file, active);
+ add_page_to_lru_list(zone, page, lru);
+ } else {
+ SetPageUnevictable(page);
+ add_page_to_lru_list(zone, page, LRU_UNEVICTABLE);
+ smp_mb();
+
+ if (page_evictable(page, NULL)) {
+ del_page_from_lru_list(zone, page, LRU_UNEVICTABLE);
+ ClearPageUnevictable(page);
+ goto redo;
+ }
+ }
}

/*
--
1.7.1


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