From: Alex Shi <alex.shi@linux.alibaba.com>
To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Mel Gorman <mgorman@techsingularity.net>,
Tejun Heo <tj@kernel.org>, Hugh Dickins <hughd@google.com>,
Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
Daniel Jordan <daniel.m.jordan@oracle.com>,
Yang Shi <yang.shi@linux.alibaba.com>,
Matthew Wilcox <willy@infradead.org>,
Johannes Weiner <hannes@cmpxchg.org>,
kbuild test robot <lkp@intel.com>, linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
cgroups@vger.kernel.org, Shakeel Butt <shakeelb@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Wei Yang <richard.weiyang@gmail.com>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Rong Chen <rong.a.chen@intel.com>
Subject: Re: [PATCH v17 14/21] mm/compaction: do page isolation first in compaction
Date: Wed, 12 Aug 2020 19:43:17 +0800 [thread overview]
Message-ID: <9581db48-cef3-788a-7f5a-8548fee56c13@linux.alibaba.com> (raw)
In-Reply-To: <CAKgT0Ues0ShkSbb1XtA7z7EYB8NCPgLGq8zZUjrXK_jcWn8mDQ@mail.gmail.com>
在 2020/8/11 下午10:47, Alexander Duyck 写道:
> On Tue, Aug 11, 2020 at 1:23 AM Alex Shi <alex.shi@linux.alibaba.com> wrote:
>>
>>
>>
>> 在 2020/8/10 下午10:41, Alexander Duyck 写道:
>>> On Mon, Aug 10, 2020 at 6:10 AM Alex Shi <alex.shi@linux.alibaba.com> wrote:
>>>>
>>>>
>>>>
>>>> 在 2020/8/7 下午10:51, Alexander Duyck 写道:
>>>>> I wonder if this entire section shouldn't be restructured. This is the
>>>>> only spot I can see where we are resetting the LRU flag instead of
>>>>> pulling the page from the LRU list with the lock held. Looking over
>>>>> the code it seems like something like that should be possible. I am
>>>>> not sure the LRU lock is really protecting us in either the
>>>>> PageCompound check nor the skip bits. It seems like holding a
>>>>> reference on the page should prevent it from switching between
>>>>> compound or not, and the skip bits are per pageblock with the LRU bits
>>>>> being per node/memcg which I would think implies that we could have
>>>>> multiple LRU locks that could apply to a single skip bit.
>>>>
>>>> Hi Alexander,
>>>>
>>>> I don't find problem yet on compound or skip bit usage. Would you clarify the
>>>> issue do you concerned?
>>>>
>>>> Thanks!
>>>
>>> The point I was getting at is that the LRU lock is being used to
>>> protect these and with your changes I don't think that makes sense
>>> anymore.
>>>
>>> The skip bits are per-pageblock bits. With your change the LRU lock is
>>> now per memcg first and then per node. As such I do not believe it
>>> really provides any sort of exclusive access to the skip bits. I still
>>> have to look into this more, but it seems like you need a lock per
>>> either section or zone that can be used to protect those bits and deal
>>> with this sooner rather than waiting until you have found an LRU page.
>>> The one part that is confusing though is that the definition of the
>>> skip bits seems to call out that they are a hint since they are not
>>> protected by a lock, but that is exactly what has been happening here.
>>>
>>
>> The skip bits are safe here, since even it race with other skip action,
>> It will still skip out. The skip action is try not to compaction too much,
>> not a exclusive action needs avoid race.
>
> That would be the case if it didn't have the impact that they
> currently do on the compaction process. What I am getting at is that a
> race was introduced when you placed this test between the clearing of
> the LRU flag and the actual pulling of the page from the LRU list. So
> if you tested the skip bits before clearing the LRU flag then I would
> be okay with the code, however because it is triggering an abort after
Hi Alexander,
Thanks a lot for comments and suggestions!
I have tried your suggestion:
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
---
mm/compaction.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index b99c96c4862d..6c881dee8c9a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -988,6 +988,13 @@ static bool too_many_isolated(pg_data_t *pgdat)
if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
goto isolate_fail_put;
+ /* Try get exclusive access under lock */
+ if (!skip_updated) {
+ skip_updated = true;
+ if (test_and_set_skip(cc, page, low_pfn))
+ goto isolate_fail_put;
+ }
+
/* Try isolate the page */
if (!TestClearPageLRU(page))
goto isolate_fail_put;
@@ -1006,13 +1013,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
lruvec_memcg_debug(lruvec, page);
- /* Try get exclusive access under lock */
- if (!skip_updated) {
- skip_updated = true;
- if (test_and_set_skip(cc, page, low_pfn))
- goto isolate_abort;
- }
-
/*
* Page become compound since the non-locked check,
* and it's on LRU. It can only be a THP so the order
--
Performance of case-lru-file-mmap-read in vm-scalibity is dropped a bit. not
helpful
> the LRU flag is cleared then you are creating a situation where
> multiple processes will be stomping all over each other as you can
> have each thread essentially take a page via the LRU flag, but only
> one thread will process a page and it could skip over all other pages
> that preemptively had their LRU flag cleared.
It increase a bit crowd here, but lru_lock do reduce some them, and skip_bit
could stop each other in a array check(bitmap). So compare to whole node
lru_lock, the net profit is clear in patch 17.
>
> If you take a look at the test_and_set_skip the function only acts on
> the pageblock aligned PFN for a given range. WIth the changes you have
> in place now that would mean that only one thread would ever actually
> call this function anyway since the first PFN would take the LRU flag
> so no other thread could follow through and test or set the bit as
Is this good for only one process could do test_and_set_skip? is that
the 'skip' meaning to be?
> well. The expectation before was that all threads would encounter this
> test and either proceed after setting the bit for the first PFN or
> abort after testing the first PFN. With you changes only the first
> thread actually runs this test and then it and the others will likely
> encounter multiple failures as they are all clearing LRU bits
> simultaneously and tripping each other up. That is why the skip bit
> must have a test and set done before you even get to the point of
> clearing the LRU flag.
It make the things warse in my machine, would you like to have a try by yourself?
>
>>> The point I was getting at with the PageCompound check is that instead
>>> of needing the LRU lock you should be able to look at PageCompound as
>>> soon as you call get_page_unless_zero() and preempt the need to set
>>> the LRU bit again. Instead of trying to rely on the LRU lock to
>>> guarantee that the page hasn't been merged you could just rely on the
>>> fact that you are holding a reference to it so it isn't going to
>>> switch between being compound or order 0 since it cannot be freed. It
>>> spoils the idea I originally had of combining the logic for
>>> get_page_unless_zero and TestClearPageLRU into a single function, but
>>> the advantage is you aren't clearing the LRU flag unless you are
>>> actually going to pull the page from the LRU list.
>>
>> Sorry, I still can not follow you here. Compound code part is unchanged
>> and follow the original logical. So would you like to pose a new code to
>> see if its works?
>
> No there are significant changes as you reordered all of the
> operations. Prior to your change the LRU bit was checked, but not
> cleared before testing for PageCompound. Now you are clearing it
> before you are testing if it is a compound page. So if compaction is
> running we will be seeing the pages in the LRU stay put, but the
> compound bit flickering off and on if the compound page is encountered
> with the wrong or NULL lruvec. What I was suggesting is that the
The lruvec could be wrong or NULL here, that is the base stone of whole
patchset.
> PageCompound test probably doesn't need to be concerned with the lock
> after your changes. You could test it after you call
> get_page_unless_zero() and before you call
> __isolate_lru_page_prepare(). Instead of relying on the LRU lock to
> protect us from the page switching between compound and not we would
> be relying on the fact that we are holding a reference to the page so
> it should not be freed and transition between compound or not.
>
I have tried the patch as your suggested, it has no clear help on performance
on above vm-scaliblity case. Maybe it's due to we checked the same thing
before lock already.
diff --git a/mm/compaction.c b/mm/compaction.c
index b99c96c4862d..cf2ac5148001 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -985,6 +985,16 @@ static bool too_many_isolated(pg_data_t *pgdat)
if (unlikely(!get_page_unless_zero(page)))
goto isolate_fail;
+ /*
+ * Page become compound since the non-locked check,
+ * and it's on LRU. It can only be a THP so the order
+ * is safe to read and it's 0 for tail pages.
+ */
+ if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
+ low_pfn += compound_nr(page) - 1;
+ goto isolate_fail_put;
+ }
+
if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
goto isolate_fail_put;
@@ -1013,16 +1023,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
goto isolate_abort;
}
- /*
- * Page become compound since the non-locked check,
- * and it's on LRU. It can only be a THP so the order
- * is safe to read and it's 0 for tail pages.
- */
- if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
- low_pfn += compound_nr(page) - 1;
- SetPageLRU(page);
- goto isolate_fail_put;
- }
} else
rcu_read_unlock();
Thanks
Alex
next prev parent reply other threads:[~2020-08-12 11:44 UTC|newest]
Thread overview: 102+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-25 12:59 [PATCH v17 00/21] per memcg lru lock Alex Shi
2020-07-25 12:59 ` [PATCH v17 01/21] mm/vmscan: remove unnecessary lruvec adding Alex Shi
2020-08-06 3:47 ` Alex Shi
2020-07-25 12:59 ` [PATCH v17 02/21] mm/page_idle: no unlikely double check for idle page counting Alex Shi
2020-07-25 12:59 ` [PATCH v17 03/21] mm/compaction: correct the comments of compact_defer_shift Alex Shi
2020-07-27 17:29 ` Alexander Duyck
2020-07-28 11:59 ` Alex Shi
2020-07-28 14:17 ` Alexander Duyck
2020-07-25 12:59 ` [PATCH v17 04/21] mm/compaction: rename compact_deferred as compact_should_defer Alex Shi
2020-07-25 12:59 ` [PATCH v17 05/21] mm/thp: move lru_add_page_tail func to huge_memory.c Alex Shi
2020-07-25 12:59 ` [PATCH v17 06/21] mm/thp: clean up lru_add_page_tail Alex Shi
2020-07-25 12:59 ` [PATCH v17 07/21] mm/thp: remove code path which never got into Alex Shi
2020-07-25 12:59 ` [PATCH v17 08/21] mm/thp: narrow lru locking Alex Shi
2020-07-25 12:59 ` [PATCH v17 09/21] mm/memcg: add debug checking in lock_page_memcg Alex Shi
2020-07-25 12:59 ` [PATCH v17 10/21] mm/swap: fold vm event PGROTATED into pagevec_move_tail_fn Alex Shi
2020-07-25 12:59 ` [PATCH v17 11/21] mm/lru: move lru_lock holding in func lru_note_cost_page Alex Shi
2020-08-05 21:18 ` Alexander Duyck
2020-07-25 12:59 ` [PATCH v17 12/21] mm/lru: move lock into lru_note_cost Alex Shi
2020-07-25 12:59 ` [PATCH v17 13/21] mm/lru: introduce TestClearPageLRU Alex Shi
2020-07-29 3:53 ` Alex Shi
2020-08-05 22:43 ` Alexander Duyck
2020-08-06 1:54 ` Alex Shi
2020-08-06 14:41 ` Alexander Duyck
2020-07-25 12:59 ` [PATCH v17 14/21] mm/compaction: do page isolation first in compaction Alex Shi
2020-08-04 21:35 ` Alexander Duyck
2020-08-06 18:38 ` Alexander Duyck
2020-08-07 3:24 ` Alex Shi
2020-08-07 14:51 ` Alexander Duyck
2020-08-10 13:10 ` Alex Shi
2020-08-10 14:41 ` Alexander Duyck
2020-08-11 8:22 ` Alex Shi
2020-08-11 14:47 ` Alexander Duyck
2020-08-12 11:43 ` Alex Shi [this message]
2020-08-12 12:16 ` Alex Shi
2020-08-12 16:51 ` Alexander Duyck
2020-08-13 1:46 ` Alex Shi
2020-08-13 2:17 ` Alexander Duyck
2020-08-13 3:52 ` Alex Shi
2020-08-13 4:02 ` [RFC PATCH 0/3] " Alexander Duyck
2020-08-13 4:02 ` [RFC PATCH 1/3] mm: Drop locked from isolate_migratepages_block Alexander Duyck
2020-08-13 6:56 ` Alex Shi
2020-08-13 14:32 ` Alexander Duyck
2020-08-14 7:25 ` Alex Shi
2020-08-13 7:44 ` Alex Shi
2020-08-13 14:26 ` Alexander Duyck
2020-08-13 4:02 ` [RFC PATCH 2/3] mm: Drop use of test_and_set_skip in favor of just setting skip Alexander Duyck
2020-08-14 7:19 ` Alex Shi
2020-08-14 14:24 ` Alexander Duyck
2020-08-14 21:15 ` Alexander Duyck
2020-08-15 9:49 ` Alex Shi
2020-08-17 15:38 ` Alexander Duyck
2020-08-18 6:50 ` Alex Shi
2020-08-13 4:02 ` [RFC PATCH 3/3] mm: Identify compound pages sooner in isolate_migratepages_block Alexander Duyck
2020-08-14 7:20 ` Alex Shi
2020-08-17 22:58 ` [PATCH v17 14/21] mm/compaction: do page isolation first in compaction Alexander Duyck
2020-07-25 12:59 ` [PATCH v17 15/21] mm/thp: add tail pages into lru anyway in split_huge_page() Alex Shi
2020-07-25 12:59 ` [PATCH v17 16/21] mm/swap: serialize memcg changes in pagevec_lru_move_fn Alex Shi
2020-07-25 12:59 ` [PATCH v17 17/21] mm/lru: replace pgdat lru_lock with lruvec lock Alex Shi
2020-07-27 23:34 ` Alexander Duyck
2020-07-28 7:15 ` Alex Shi
2020-07-28 11:19 ` Alex Shi
2020-07-28 14:54 ` Alexander Duyck
2020-07-29 1:00 ` Alex Shi
2020-07-29 1:27 ` Alexander Duyck
2020-07-29 2:27 ` Alex Shi
2020-07-28 15:39 ` Alex Shi
2020-07-28 15:55 ` Alexander Duyck
2020-07-29 0:48 ` Alex Shi
2020-07-29 3:54 ` Alex Shi
2020-08-06 7:41 ` Alex Shi
2020-07-25 12:59 ` [PATCH v17 18/21] mm/lru: introduce the relock_page_lruvec function Alex Shi
2020-07-29 17:52 ` Alexander Duyck
2020-07-30 6:08 ` Alex Shi
2020-07-31 14:20 ` Alexander Duyck
2020-07-31 21:14 ` [PATCH RFC] mm: Add function for testing if the current lruvec lock is valid alexander.h.duyck
2020-07-31 23:54 ` Alex Shi
2020-08-02 18:20 ` Alexander Duyck
2020-08-04 6:13 ` Alex Shi
2020-07-25 12:59 ` [PATCH v17 19/21] mm/vmscan: use relock for move_pages_to_lru Alex Shi
2020-08-03 22:49 ` Alexander Duyck
2020-08-04 6:23 ` Alex Shi
2020-07-25 12:59 ` [PATCH v17 20/21] mm/pgdat: remove pgdat lru_lock Alex Shi
2020-08-03 22:42 ` Alexander Duyck
2020-08-03 22:45 ` Alexander Duyck
2020-08-04 6:22 ` Alex Shi
2020-07-25 12:59 ` [PATCH v17 21/21] mm/lru: revise the comments of lru_lock Alex Shi
2020-08-03 22:37 ` Alexander Duyck
2020-08-04 10:04 ` Alex Shi
2020-08-04 14:29 ` Alexander Duyck
2020-08-06 1:39 ` Alex Shi
2020-08-06 16:27 ` Alexander Duyck
2020-07-27 5:40 ` [PATCH v17 00/21] per memcg lru lock Alex Shi
2020-07-29 14:49 ` Alex Shi
2020-07-29 18:06 ` Hugh Dickins
2020-07-30 2:16 ` Alex Shi
2020-08-03 15:07 ` Michal Hocko
2020-08-04 6:14 ` Alex Shi
2020-07-31 21:31 ` Alexander Duyck
2020-08-04 8:36 ` Alex Shi
2020-08-04 8:36 ` Alex Shi
2020-08-04 8:37 ` Alex Shi
2020-08-04 8:37 ` Alex Shi
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