[PATCH next 1/3] slub defrag: unpin writeback pages

[PATCH next 1/3] slub defrag: unpin writeback pages

[Hugh Dickins]

A repetitive swapping load on powerpc G5 went progressively slower after
nine hours: Inactive(file) was rising, as if inactive file pages pinned.
Yes, slub defrag's kick_buffers() was forgetting to put_page() whenever
it met a page already under writeback.

That PageWriteback test should be made while PageLocked in trigger_write(),
just as it is in try_to_free_buffers() - if there are complex reasons why
that's not actually necessary, I'd rather not have to think through them.
A preliminary check before taking the lock?  No, it's not that important.

And trigger_write() must remember to unlock_page() in each of the cases
where it doesn't reach the writepage().

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
Andrew, I mentioned at KS that I'd just completed a slow -mm bisection,
which had mysteriously converged on one of your patches: that was your
vm-dont-run-touch_buffer-during-buffercache-lookups.patch
In fact that turned out just to speed up the rate at which writebacks
were pinning these pages: I've nothing against (nor for!) your patch.

I'm wondering whether kick_buffers() ought to check PageLRU: I've no
evidence it's needed, but coming to writepage() or try_to_free_buffers()
from this direction (by virtue of having a buffer_head in a partial slab
page) is new, and I worry it might cause some ordering problem; whereas
if the page is PageLRU, then it is already fair game for such reclaim.

 fs/buffer.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- 2.6.27-rc7-mmotm/fs/buffer.c	2008-09-26 13:18:50.000000000 +0100
+++ linux/fs/buffer.c	2008-10-03 19:43:44.000000000 +0100
@@ -3354,13 +3354,16 @@ static void trigger_write(struct page *p
 		.for_reclaim = 0
 	};
 
+	if (PageWriteback(page))
+		goto unlock;
+
 	if (!mapping->a_ops->writepage)
 		/* No write method for the address space */
-		return;
+		goto unlock;
 
 	if (!clear_page_dirty_for_io(page))
 		/* Someone else already triggered a write */
-		return;
+		goto unlock;
 
 	rc = mapping->a_ops->writepage(page, &wbc);
 	if (rc < 0)
@@ -3368,7 +3371,7 @@ static void trigger_write(struct page *p
 		return;
 
 	if (rc == AOP_WRITEPAGE_ACTIVATE)
-		unlock_page(page);
+unlock:		unlock_page(page);
 }
 
 /*
@@ -3420,7 +3423,7 @@ static void kick_buffers(struct kmem_cac
 	for (i = 0; i < nr; i++) {
 		page = v[i];
 
-		if (!page || PageWriteback(page))
+		if (!page)
 			continue;
 
 		if (trylock_page(page)) {

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[PATCH next 2/3] slub defrag: dma_kmalloc_cache add_tail

[Hugh Dickins]

Why did that slowdown from mispinned pages manifest only on the G5?

Because something in my x86_32 and x86_64 configs (CONFIG_BLK_DEV_SR
I believe) is giving me a kmalloc_dma-512 cache, and dma_kmalloc_cache()
had not been updated to satisfy the assumption in kmem_cache_defrag(),
that defragmentable caches come first in the list.

So, any DMAable cache was preventing all slub defragmentation: which
looks like it's not been getting the testing exposure it deserves.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---

 mm/slub.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.27-rc7-mmotm/mm/slub.c	2008-09-26 13:18:53.000000000 +0100
+++ linux/mm/slub.c	2008-10-04 20:10:46.000000000 +0100
@@ -2636,7 +2636,7 @@ static noinline struct kmem_cache *dma_k
 		goto unlock_out;
 	}
 
-	list_add(&s->list, &slab_caches);
+	list_add_tail(&s->list, &slab_caches);
 	kmalloc_caches_dma[index] = s;
 
 	schedule_work(&sysfs_add_work);

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[PATCH next 3/3] slub defrag: slabinfo help trivia

[Hugh Dickins]

Fix typo on --display-defrag line of slabinfo's help message;
fix misaligning tabs to spaces on two other lines of that message.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---

 Documentation/vm/slabinfo.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- 2.6.27-rc7-mmotm/Documentation/vm/slabinfo.c	2008-09-26 13:18:44.000000000 +0100
+++ linux/Documentation/vm/slabinfo.c	2008-10-04 19:47:40.000000000 +0100
@@ -117,14 +117,14 @@ void usage(void)
 		"-e|--empty             Show empty slabs\n"
 		"-f|--first-alias       Show first alias\n"
 		"-F|--defrag            Show defragmentable caches\n"
-		"-G:--display-defrag    Display defrag counters\n"
+		"-G|--display-defrag    Display defrag counters\n"
 		"-h|--help              Show usage information\n"
 		"-i|--inverted          Inverted list\n"
 		"-l|--slabs             Show slabs\n"
 		"-n|--numa              Show NUMA information\n"
-		"-o|--ops		Show kmem_cache_ops\n"
+		"-o|--ops               Show kmem_cache_ops\n"
 		"-s|--shrink            Shrink slabs\n"
-		"-r|--report		Detailed report on single slabs\n"
+		"-r|--report            Detailed report on single slabs\n"
 		"-S|--Size              Sort by size\n"
 		"-t|--tracking          Show alloc/free information\n"
 		"-T|--Totals            Show summary information\n"

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Re: [PATCH next 3/3] slub defrag: slabinfo help trivia

[Pekka Enberg]

On Sun, 2008-10-05 at 03:28 +0100, Hugh Dickins wrote:
> Fix typo on --display-defrag line of slabinfo's help message;
> fix misaligning tabs to spaces on two other lines of that message.
> 
> Signed-off-by: Hugh Dickins <hugh@veritas.com>

Applied, thanks!

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Re: [PATCH next 2/3] slub defrag: dma_kmalloc_cache add_tail

[Pekka Enberg]

On Sun, 2008-10-05 at 03:27 +0100, Hugh Dickins wrote:
> Why did that slowdown from mispinned pages manifest only on the G5?
> 
> Because something in my x86_32 and x86_64 configs (CONFIG_BLK_DEV_SR
> I believe) is giving me a kmalloc_dma-512 cache, and dma_kmalloc_cache()
> had not been updated to satisfy the assumption in kmem_cache_defrag(),
> that defragmentable caches come first in the list.
> 
> So, any DMAable cache was preventing all slub defragmentation: which
> looks like it's not been getting the testing exposure it deserves.
> 
> Signed-off-by: Hugh Dickins <hugh@veritas.com>

Applied, thanks!

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Re: [PATCH next 1/3] slub defrag: unpin writeback pages

[Pekka Enberg]

On Sun, 2008-10-05 at 03:25 +0100, Hugh Dickins wrote:
> A repetitive swapping load on powerpc G5 went progressively slower after
> nine hours: Inactive(file) was rising, as if inactive file pages pinned.
> Yes, slub defrag's kick_buffers() was forgetting to put_page() whenever
> it met a page already under writeback.
> 
> That PageWriteback test should be made while PageLocked in trigger_write(),
> just as it is in try_to_free_buffers() - if there are complex reasons why
> that's not actually necessary, I'd rather not have to think through them.
> A preliminary check before taking the lock?  No, it's not that important.
> 
> And trigger_write() must remember to unlock_page() in each of the cases
> where it doesn't reach the writepage().
> 
> Signed-off-by: Hugh Dickins <hugh@veritas.com>

Looks good to me. I've applied the patch now but would really like an
ACK from Christoph/Andrew as well. Thanks again, Hugh!

		Pekka

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Re: [PATCH next 1/3] slub defrag: unpin writeback pages

[Christoph Lameter]

Hugh Dickins wrote:
> A repetitive swapping load on powerpc G5 went progressively slower after
> nine hours: Inactive(file) was rising, as if inactive file pages pinned.
> Yes, slub defrag's kick_buffers() was forgetting to put_page() whenever
> it met a page already under writeback.

Thanks for finding that.

> That PageWriteback test should be made while PageLocked in trigger_write(),
> just as it is in try_to_free_buffers() - if there are complex reasons why
> that's not actually necessary, I'd rather not have to think through them.
> A preliminary check before taking the lock?  No, it's not that important.

The writeback check in kick_buffers() is a performance optimization. If the
page is under writeback then there is no point in trying to kick out the page.
That will only succeed after writeback is complete.

If a page is under writeback then try_to_free_buffers() will fail immediately.
So no need to check under pagelock.


> And trigger_write() must remember to unlock_page() in each of the cases
> where it doesn't reach the writepage().

Ack.


> --- 2.6.27-rc7-mmotm/fs/buffer.c	2008-09-26 13:18:50.000000000 +0100
> +++ linux/fs/buffer.c	2008-10-03 19:43:44.000000000 +0100
> @@ -3354,13 +3354,16 @@ static void trigger_write(struct page *p
>  		.for_reclaim = 0
>  	};
>  
> +	if (PageWriteback(page))
> +		goto unlock;
> +

Is that necessary? Wont writepage do the appropriate thing?


  >  /*
> @@ -3420,7 +3423,7 @@ static void kick_buffers(struct kmem_cac
>  	for (i = 0; i < nr; i++) {
>  		page = v[i];
>  
> -		if (!page || PageWriteback(page))
> +		if (!page)
>  			continue;

Thats just an optimization. No need to lock a page if its under writeback
which would make try_to_free_buffers() fail.


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Re: [PATCH next 2/3] slub defrag: dma_kmalloc_cache add_tail

[Christoph Lameter]

Thanks for finding the DMA cache issues. DMA caches are rarely used since Andi
tried to get rid of all of them for x86.


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Re: [PATCH next 1/3] slub defrag: unpin writeback pages

[Hugh Dickins]

On Tue, 7 Oct 2008, Christoph Lameter wrote:
> Hugh Dickins wrote:
> 
> > That PageWriteback test should be made while PageLocked in trigger_write(),
> > just as it is in try_to_free_buffers() - if there are complex reasons why
> > that's not actually necessary, I'd rather not have to think through them.
> > A preliminary check before taking the lock?  No, it's not that important.
> 
> The writeback check in kick_buffers() is a performance optimization. If the
> page is under writeback then there is no point in trying to kick out the page.
> That will only succeed after writeback is complete.

I think it's more than a performance optimization: I may have missed
somewhere, but the only place I can find which calls ->writepage without
checking PageWriteback first, while holding PageLock across the calls,
is mm/page-writeback.c write_one_page() - which seems to be for internal
use by filesystems, which could make the check themselves - but every
existing caller passes wait arg 1 to wait for PageWriteback to clear.

I wrote that while running a test, my version of fs/buffer.c with
the PageWriteback test removed from trigger_write(): that's now hit
kernel BUG at fs/buffer.c:1757! which is BUG_ON(PageWriteback(page))
in __block_write_full_page().  Fair enough, though what I'd been
looking forward to was the 
	if (!test_clear_page_writeback(page))
		BUG();
in mm/filemap.c end_page_writeback().

> 
> If a page is under writeback then try_to_free_buffers() will fail immediately.
> So no need to check under pagelock.

What I meant was that try_to_free_buffers() is already checking it
under pagelock, so let's be symmetrical and check it under pagelock
in trigger_write(), rather than moving the test within kick_buffers().

> > --- 2.6.27-rc7-mmotm/fs/buffer.c	2008-09-26 13:18:50.000000000 +0100
> > +++ linux/fs/buffer.c	2008-10-03 19:43:44.000000000 +0100
> > @@ -3354,13 +3354,16 @@ static void trigger_write(struct page *p
> >  		.for_reclaim = 0
> >  	};
> >  
> > +	if (PageWriteback(page))
> > +		goto unlock;
> > +
> 
> Is that necessary? Wont writepage do the appropriate thing?

Some might, but in general no: my BUG came below blkdev_writepage().

> >  /*
> > @@ -3420,7 +3423,7 @@ static void kick_buffers(struct kmem_cac
> >  	for (i = 0; i < nr; i++) {
> >  		page = v[i];
> >  
> > -		if (!page || PageWriteback(page))
> > +		if (!page)
> >  			continue;
> 
> Thats just an optimization. No need to lock a page if its under writeback
> which would make try_to_free_buffers() fail.

My first version of the patch did go on to say
-		if (trylock_page(page)) {
+		if (!PageWriteback(page) && trylock_page(page)) {

But since the cacheline is already dirty (from get_page_unless_zero),
and it's only a trylock, and we (almost certainly) need to repeat the
PageWriteback test once we've got the lock, and it does not hit this
case often enough for you to have noticed the missing put_page() bug,
I decided to save icache instead by just removing your optimization.

Hugh

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Re: [PATCH next 1/3] slub defrag: unpin writeback pages

[Christoph Lameter]

Hugh Dickins wrote:

> But since the cacheline is already dirty (from get_page_unless_zero),
> and it's only a trylock, and we (almost certainly) need to repeat the
> PageWriteback test once we've got the lock, and it does not hit this
> case often enough for you to have noticed the missing put_page() bug,
> I decided to save icache instead by just removing your optimization.

Ok. Acked-by: Christoph Lameter <cl@linux-foundation.org>


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