Re: [PATCH v17 14/21] mm/compaction: do page isolation first in compaction

[Alex Shi]

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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
---
 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