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Message-ID: <alpine.LSU.2.00.1201031802170.1254@eggly.anvils>
Date: Tue, 3 Jan 2012 18:56:41 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: kosaki.motohiro@...il.com
cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
David Rientjes <rientjes@...gle.com>,
Minchan Kim <minchan.kim@...il.com>,
Mel Gorman <mel@....ul.ie>,
Johannes Weiner <jweiner@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH 1/2] mm,mlock: drain pagevecs asynchronously
On Sun, 1 Jan 2012, kosaki.motohiro@...il.com wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@...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
That's very nice; but it is an artificial test of the codepath you
have clearly speeded up, without any results for the codepaths you
have probably slowed down - I see Minchan is worried about that too.
>
> 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@...bao.com>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Minchan Kim <minchan.kim@...il.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Cc: Mel Gorman <mel@....ul.ie>
> Cc: Johannes Weiner <jweiner@...hat.com>
> Cc: Andrew Morton <akpm@...ux-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(¤t->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(¤t->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(¤t->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)
I couldn't make sense of that name! Though, to be fair, I never made
sense of "lru_add_drain" either: I always imagine a plumber adding a
drain.
pages_are_pending_lru_add(cpu)?
> +{
> + struct pagevec *pvecs = per_cpu(lru_add_pvecs, cpu);
> + struct pagevec *pvec;
> + int lru;
> +
> + for_each_lru(lru) {
> + pvec = &pvecs[lru - LRU_BASE];
Hmm, yes, I see other such loops say "- LRU_BASE" too.
Good thing LRU_BASE is 0!
> + 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);
> + }
> +}
You'll be peeking at a lot of remote cpus here, rather than sending them
all an IPI. My guess is that it is more efficient your way, but that's
not as obvious as I'd like.
I haven't added Gilad Ben-Yossef to the Cc list, but be aware that he's
particularly interested in reducing IPIs, and has some patches on lkml.
I have added Michel Lespinasse to the Cc list, he's good to Cc on
mlock matters. He thought that if the pages_are_pending check is
a win, then you should do it in lru_add_drain_all() too. But more,
he wondered if we couldn't do a better job without all this, by
doing just lru_add_drain() first, and then doing the extra work
(of prodding other cpus) only if we come across a page which needs it.
> +
> /*
> * 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;
> + }
> + }
> }
Right, this is where the overhead comes, in everybody's pagevec_lru_add_fn.
Now, I do like that you're resolving the un/evictable issue here, and
if the overhead really doesn't hurt, then this is a nice way to go.
But I think you should go further to reduce your overhead. For one
thing, certainly take that silly almost-always-NULL vma arg out of
page_unevictable, and test vma directly in the one or two places that
want it; maybe make page_unevictable(page) an mm_inline function.
And update those various places (many or all in vmscan.c) which are
asking if page_unevictable(), to avoid using pagevec for unevictable
page because they know it doesn't work: you are now making it work.
Hah, I see my own putback_lru(or inactive)_pages patch is open to the
same criticism: if I'm now using a page_list there, it could handle the
unevictable pages directly instead of having to avoid them. I think.
(I've never noticed before, it worries me now that page_unevictable()
is referring to page mapping flags at a point when we generally have
no hold on the mapping at all; but I've never heard of that being a
problem, and I guess if the mapping is stale, the page will very soon
be freed, so no matter if it went on the Unevictable list for a moment.)
Hugh
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