在 2020/8/13 上午10:17, Alexander Duyck 写道: >> zone lock is probability better. you can try and test. > So I spent a good chunk of today looking the code over and what I > realized is that we probably don't even really need to have this code > protected by the zone lock since the LRU bit in the pageblock should > do most of the work for us. In addition we can get rid of the test > portion of this and just make it a set only operation if I am not > mistaken. > >>>>> 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. >>> My concern is that what you can end up with is multiple threads all >>> working over the same pageblock for isolation. With the old code the >>> LRU lock was used to make certain that test_and_set_skip was being >>> synchronized on the first page in the pageblock so you would only have >>> one thread going through and working a single pageblock. However after >>> your changes it doesn't seem like the test_and_set_skip has that >>> protection since only one thread will ever be able to successfully >>> call it for the first page in the pageblock assuming that the LRU flag >>> is set on the first page in the pageblock block. >>> >>>>> 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? >>> So only one thread really getting to fully use test_and_set_skip is >>> good, however the issue is that there is nothing to synchronize the >>> testing from the other threads. As a result the other threads could >>> have isolated other pages within the pageblock before the thread that >>> is calling test_and_set_skip will get to complete the setting of the >>> skip bit. This will result in isolation failures for the thread that >>> set the skip bit which may be undesirable behavior. >>> >>> With the old code the threads were all synchronized on testing the >>> first PFN in the pageblock while holding the LRU lock and that is what >>> we lost. My concern is the cases where skip_on_failure == true are >>> going to fail much more often now as the threads can easily interfere >>> with each other. >> I have a patch to fix this, which is on >> https://github.com/alexshi/linux.git lrunext > I don't think that patch helps to address anything. You are now > failing to set the bit in the case that something modifies the > pageblock flags while you are attempting to do so. I think it would be > better to just leave the cmpxchg loop as it is. It do increae the case-lru-file-mmap-read in vm-scalibity about 3% performance. Yes, I am glad to see it can be make better. > >>>>> 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? >>> I plan to do that. I have already been working on a few things to >>> clean up and optimize your patch set further. I will try to submit an >>> RFC this evening so we can discuss. >>> >> Glad to see your new code soon. Would you like do it base on >> https://github.com/alexshi/linux.git lrunext > I can rebase off of that tree. It may add another half hour or so. I > have barely had any time to test my code. When I enabled some of the > debugging features in the kernel related to using the vm-scalability > tests the boot time became incredibly slow so I may just make certain > I can boot and not mess the system up before submitting my patches as > an RFC. I can probably try testing them more tomorrow. > >>>>>>> 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. >>> Sorry I had a typo in my comment as well as it is the LRU bit that >>> will be flickering, not the compound. The goal here is to avoid >>> clearing the LRU bit unless we are sure we are going to take the >>> lruvec lock and pull the page from the list. >>> >>>>> 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(); >>>> >>> So actually there is more we could do than just this. Specifically a >>> few lines below the rcu_read_lock there is yet another PageCompound >>> check that sets low_pfn yet again. So in theory we could combine both >>> of those and modify the code so you end up with something more like: >>> @@ -968,6 +974,16 @@ isolate_migratepages_block(struct compact_control >>> *cc, unsigned long low_pfn, >>> if (unlikely(!get_page_unless_zero(page))) >>> goto isolate_fail; >>> >>> + if (PageCompound(page)) { >>> + const unsigned int order = compound_order(page); >>> + >>> + if (likely(order < MAX_ORDER)) >>> + low_pfn += (1UL << order) - 1; >>> + >>> + if (unlikely(!cc->alloc_contig)) >>> + goto isolate_fail_put; >>> >> The current don't check this unless locked changed. But anyway check it >> every page may have no performance impact. > Yes and no. The same code is also ran outside the lock and that is why > I suggested merging the two and creating this block of logic. It will > be clearer once I have done some initial smoke testing and submitted > my patch. > >> + } >>> + >>> if (__isolate_lru_page_prepare(page, isolate_mode) != 0) >>> goto isolate_fail_put; >>> >>> Doing this you would be more likely to skip over the entire compound >>> page in a single jump should you not be able to either take the LRU >>> bit or encounter a busy page in __isolate_Lru_page_prepare. I had >>> copied this bit from an earlier check and modified it as I was not >>> sure I can guarantee that this is a THP since we haven't taken the LRU >>> lock yet. However I believe the page cannot be split up while we are >>> holding the extra reference so the PageCompound flag and order should >>> not change until we call put_page. >>> >> It looks like the lock_page protect this instead of get_page that just works >> after split func called. > So I thought that the call to page_ref_freeze that is used in > functions like split_huge_page_to_list is meant to address this case. > What it is essentially doing is setting the reference count to zero if > the count is at the expected value. So with the get_page_unless_zero > it would either fail because the value is already zero, or the > page_ref_freeze would fail because the count would be one higher than > the expected value. Either that or I am still missing another piece in > the understanding of this. Uh, the front xa_lock or anon_vma lock guard the -refcount, so long locking path... Thanks Alex
