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SubjectRe: [PATCH] mm: Do not stall in synchronous compaction for THP allocations
On Mon, Nov 14, 2011 at 04:03:45PM -0800, Andrew Morton wrote:
> On Fri, 11 Nov 2011 10:01:56 +0000
> Mel Gorman <mgorman@suse.de> wrote:
> > A 1000-hour compute job will have its pages collapsed into hugepages by
> > khugepaged so they might not have the huge pages at the very beginning
> > but they get them. With khugepaged in place, there should be no need for
> > an additional tuneable.
>
> OK...

It's good idea to keep it monitored. But I guess the reduced rate will
only materialize at temporary VM stress times.

> Fair enough. One slight problem though:
>
> akpm:/usr/src/25> grep -r thp_collapse_alloc_failed Documentation
> akpm:/usr/src/25>

I didn't fill that gap but I was reading the code again and I don't
see why we keep retrying for -EAGAIN in the !sync case. Maybe the
below is good (untested). I doubt it's good to spend cpu to retry the
trylock or to retry the migrate on a pinned page by O_DIRECT. In fact
as far as THP success rate is concerned maybe we should "goto out"
instead of "goto fail" but I didn't change to that as compaction even
if it fails a subpage may still be successful at creating order
1/2/3/4...8 pages. I only avoid 9 loops to retry a trylock or a page
under O_DIRECT. Maybe that will save a bit of CPU, I doubt it can
decrease the success rate in any significant way. I'll test it at the
next build...

====
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: [PATCH] migrate: !sync don't retry

For !sync it's not worth retrying because we won't lock_page even
after the second pass. So make -EAGAIN behave like -EBUSY in the !sync
should be faster. The difference between -EAGAIN and -EBUSY remains as
usual for the sync case, where -EAGAIN will retry, while -EBUSY will
not.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
mm/migrate.c | 16 +++++++++-------
1 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 578e291..7d97a14 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -680,11 +680,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
* For !sync, there is no point retrying as the retry loop
* is expected to be too short for PageWriteback to be cleared
*/
- if (!sync) {
- rc = -EBUSY;
- goto uncharge;
- }
- if (!force)
+ if (!force || !sync)
goto uncharge;
wait_on_page_writeback(page);
}
@@ -794,7 +790,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,

rc = __unmap_and_move(page, newpage, force, offlining, sync);
out:
- if (rc != -EAGAIN) {
+ if (rc != -EAGAIN || !sync) {
/*
* A page that has been migrated has all references
* removed and will be freed. A page that has not been
@@ -874,7 +870,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
out:
unlock_page(hpage);

- if (rc != -EAGAIN) {
+ if (rc != -EAGAIN || !sync) {
list_del(&hpage->lru);
put_page(hpage);
}
@@ -934,11 +930,14 @@ int migrate_pages(struct list_head *from,
case -ENOMEM:
goto out;
case -EAGAIN:
+ if (!sync)
+ goto fail;
retry++;
break;
case 0:
break;
default:
+ fail:
/* Permanent failure */
nr_failed++;
break;
@@ -981,11 +980,14 @@ int migrate_huge_pages(struct list_head *from,
case -ENOMEM:
goto out;
case -EAGAIN:
+ if (!sync)
+ goto fail;
retry++;
break;
case 0:
break;
default:
+ fail:
/* Permanent failure */
nr_failed++;
break;

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