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    SubjectRe: [PATCH v18 00/32] per memcg lru_lock: reviews
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    在 2020/9/12 上午10:13, Hugh Dickins 写道:
    > On Fri, 11 Sep 2020, Alex Shi wrote:
    >> 在 2020/9/10 上午7:16, Hugh Dickins 写道:
    >>> On Wed, 9 Sep 2020, Alex Shi wrote:
    >>>> 在 2020/9/9 上午7:41, Hugh Dickins 写道:
    >>>>>
    >>>>> [PATCH v18 05/32] mm/thp: remove code path which never got into
    >>>>> This is a good simplification, but I see no sign that you understand
    >>>>> why it's valid: it relies on lru_add_page_tail() being called while
    >>>>> head refcount is frozen to 0: we would not get this far if someone
    >>>>> else holds a reference to the THP - which they must hold if they have
    >>>>> isolated the page from its lru (and that's true before or after your
    >>>>> per-memcg changes - but even truer after those changes, since PageLRU
    >>>>> can then be flipped without lru_lock at any instant): please explain
    >>>>> something of this in the commit message.
    >>>>
    >>>> Is the following commit log better?
    >>>>
    >>>> split_huge_page() will never call on a page which isn't on lru list, so
    >>>> this code never got a chance to run, and should not be run, to add tail
    >>>> pages on a lru list which head page isn't there.
    >>>>
    >>>> Hugh Dickins' mentioned:
    >>>> The path should never be called since lru_add_page_tail() being called
    >>>> while head refcount is frozen to 0: we would not get this far if someone
    >>>> else holds a reference to the THP - which they must hold if they have
    >>>> isolated the page from its lru.
    >>>>
    >>>> Although the bug was never triggered, it'better be removed for code
    >>>> correctness, and add a warn for unexpected calling.
    >>>
    >>> Not much better, no. split_huge_page() can easily be called for a page
    >>> which is not on the lru list at the time,
    >>
    >> Hi Hugh,
    >>
    >> Thanks for comments!
    >>
    >> There are some discussion on this point a couple of weeks ago,
    >> https://lkml.org/lkml/2020/7/9/760
    >>
    >> Matthew Wilcox and Kirill have the following comments,
    >>> I don't understand how we get to split_huge_page() with a page that's
    >>> not on an LRU list. Both anonymous and page cache pages should be on
    >>> an LRU list. What am I missing?
    >>
    >> Right, and it's never got removed from LRU during the split. The tail
    >> pages have to be added to LRU because they now separate from the tail
    >> page.
    >>
    >> --
    >> Kirill A. Shutemov
    >
    > Yes, those were among the mails that I read through before getting
    > down to review. I was surprised by their not understanding, but
    > it was a bit late to reply to that thread.
    >
    > Perhaps everybody had been focused on pages which have been and
    > naturally belong on an LRU list, rather than pages which are on
    > the LRU list at the instant that split_huge_page() is called.
    >
    > There are a number of places where PageLRU gets cleared, and a
    > number of places where we del_page_from_lru_list(), I think you'll
    > agree: your patches touch all or most of them. Let's think of a
    > common one, isolate_lru_pages() used by page reclaim, but the same
    > would apply to most of the others.
    >
    > Then there a number of places where split_huge_page() is called:
    > I am having difficulty finding any of those which cannot race with
    > page reclaim, but shall we choose anon THP's deferred_split_scan(),
    > or shmem THP's shmem_punch_compound()?
    >
    > What prevents either of those from calling split_huge_page() at
    > a time when isolate_lru_pages() has removed the page from LRU?
    >
    > But there's no problem in this race, because anyone isolating the
    > page from LRU must hold their own reference to the page (to prevent
    > it from being freed independently), and the can_split_huge_page() or
    > page_ref_freeze() in split_huge_page_to_list() will detect that and
    > fail the split with -EBUSY (or else succeed and prevent new references
    > from being acquired). So this case never reaches lru_add_page_tail().

    Hi Hugh,

    Thanks for comments!

    We are the same page here, we all know split_huge_page_to_list could block
    them go futher and the code is functionality right.
    If the comments 'Split start from PageLRU(head), and ...' doesn't make
    things clear as it's should be, I am glad to see you rewrite and improve
    them.

    >
    >>
    >>> and I don't know what was the
    >>> bug which was never triggered.
    >>
    >> So the only path to the removed part should be a bug, like sth here,
    >> https://lkml.org/lkml/2020/7/10/118
    >> or
    >> https://lkml.org/lkml/2020/7/10/972
    >
    > Oh, the use of split_huge_page() in __iommu_dma_alloc_pages() is just
    > nonsense, I thought it had already been removed - perhaps some debate
    > over __GFP_COMP held it up. Not something you need worry about in
    > this patchset.
    >
    >>
    >>> Stick with whatever text you end up with
    >>> for the combination of 05/32 and 18/32, and I'll rewrite it after.
    >>
    >> I am not object to merge them into one, I just don't know how to say
    >> clear about 2 patches in commit log. As patch 18, TestClearPageLRU
    >> add the incorrect posibility of remove lru bit during split, that's
    >> the reason of code path rewrite and a WARN there.
    >
    > I did not know that was why you were putting 18/32 in at that
    > point, it does not mention TestClearPageLRU at all. But the fact
    > remains that it's a nice cleanup, contains a reassuring WARN if we
    > got it wrong (and I've suggested a WARN on the other branch too),
    > it was valid before your changes, and it's valid after your changes.
    > Please merge it back into the uglier 05/32, and again I'll rewrite
    > whatever comment you come up with if necessary.

    I merge them together on the following git branch, and let the commit log
    to you. :)

    https://github.com/alexshi/linux.git lruv19
    >
    >>>
    >>>>> [PATCH v18 06/32] mm/thp: narrow lru locking
    >>>>> Why? What part does this play in the series? "narrow lru locking" can
    >>>>> also be described as "widen page cache locking":
    >>>>
    >>>> Uh, the page cache locking isn't widen, it's still on the old place.
    >>>
    >>> I'm not sure if you're joking there. Perhaps just a misunderstanding.
    >>>
    >>> Yes, patch 06/32 does not touch the xa_lock(&mapping->i_pages) and
    >>> xa_lock(&swap_cache->i_pages) lines (odd how we've arrived at two of
    >>> those, but please do not get into cleaning it up now); but it removes
    >>> the spin_lock_irqsave(&pgdata->lru_lock, flags) which used to come
    >>> before them, and inserts a spin_lock(&pgdat->lru_lock) after them.
    >>>
    >>> You call that narrowing the lru locking, okay, but I see it as also
    >>> pushing the page cache locking outwards: before this patch, page cache
    >>> lock was taken inside lru_lock; after this patch, page cache lock is
    >>> taken outside lru_lock. If you cannot see that, then I think you
    >>> should not have touched this code at all; but it's what we have
    >>> been testing, and I think we should go forward with it.
    >>>
    >>>>> But I wish you could give some reason for it in the commit message!
    >>>>
    >>>> It's a head scratch task. Would you like to tell me what's detailed info
    >>>> should be there? Thanks!
    >>>
    >>> So, you don't know why you did it either: then it will be hard to
    >>> justify. I guess I'll have to write something for it later. I'm
    >>> strongly tempted just to drop the patch, but expect it will become
    >>> useful later, for using lock_page_memcg() before getting lru_lock.
    >>>
    >>
    >> I thought the xa_lock and lru_lock relationship was described clear
    >> in the commit log,
    >
    > You say "lru_lock and page cache xa_lock have no reason with current
    > sequence", but you give no reason for inverting their sequence:
    > "let's" is not a reason.
    >
    >> and still no idea of the move_lock in the chain.
    >
    > memcg->move_lock is what's at the heart of lock_page_memcg(), but
    > as much as possible that tries to avoid the overhead of actually
    > taking it, since moving memcg is a rare operation. For lock ordering,
    > see the diagram in mm/rmap.c, which 23/32 updates to match this change.

    I see. thanks!

    >
    > Before this commit: lru_lock > move_lock > i_pages lock was the
    > expected lock ordering (but it looks as if the lru_lock > move_lock
    > requirement came from my per-memcg lru_lock patches).
    >
    > After this commit: move_lock > i_pages lock > lru_lock is the
    > required lock ordering, since there are strong reasons (in dirty
    > writeback) for move_lock > i_pages lock.
    >
    >> Please refill them for what I overlooked.
    >
    > Will do, but not before reviewing your remaining patches.

    IIRC, all of comments are accepted and push to
    https://github.com/alexshi/linux.git lruv19
    If you don't minder, could you change everything and send out a new version
    for further review?

    >
    >> Thanks!
    >>
    >>>>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
    >>>>> Is that correct? Or Wei Yang suggested some part of it perhaps?
    >>>>
    >>>> Yes, we talked a lot to confirm the locking change is safe.
    >>>
    >>> Okay, but the patch was written by you, and sent by you to Andrew:
    >>> that is not a case for "Signed-off-by: Someone Else".
    >>>
    >>
    >> Ok. let's remove his signed-off.
    >>
    >>>>> [PATCH v18 27/32] mm/swap.c: optimizing __pagevec_lru_add lru_lock
    >>>>> Could we please drop this one for the moment? And come back to it later
    >>>>> when the basic series is safely in. It's a good idea to try sorting
    >>>>> together those pages which come under the same lock (though my guess is
    >>>>> that they naturally gather themselves together quite well already); but
    >>>>> I'm not happy adding 360 bytes to the kernel stack here (and that in
    >>>>> addition to 192 bytes of horrid pseudo-vma in the shmem swapin case),
    >>>>> though that could be avoided by making it per-cpu. But I hope there's
    >>>>> a simpler way of doing it, as efficient, but also useful for the other
    >>>>> pagevec operations here: perhaps scanning the pagevec for same page->
    >>>>> mem_cgroup (and flags node bits), NULLing entries as they are done.
    >>>>> Another, easily fixed, minor defect in this patch: if I'm reading it
    >>>>> right, it reverses the order in which the pages are put on the lru?
    >>>>
    >>>> this patch could give about 10+% performance gain on my multiple memcg
    >>>> readtwice testing. fairness locking cost the performance much.
    >>>
    >>> Good to know, should have been mentioned. s/fairness/Repeated/
    >>>
    >>> But what was the gain or loss on your multiple memcg readtwice
    >>> testing without this patch, compared against node-only lru_lock?
    >>> The 80% gain mentioned before, I presume. So this further
    >>> optimization can wait until the rest is solid.
    >>
    >> the gain based on the patch 26.
    >
    > If I understand your brief comment there, you're saying that
    > in a fixed interval of time, the baseline 5.9-rc did 100 runs,
    > the patches up to and including 26/32 did 180 runs, then with
    > 27/32 on top, did 198 runs?

    Uh, I updated the testing with some new results here:
    https://lkml.org/lkml/2020/8/26/212

    >
    > That's a good improvement by 27/32, but not essential for getting
    > the patchset in: I don't think 27/32 is the right way to do it,
    > so I'd still prefer to hold it back from the "initial offering".

    I am ok to hold it back.
    >
    >>
    >>>
    >>>>
    >>>> I also tried per cpu solution but that cause much trouble of per cpu func
    >>>> things, and looks no benefit except a bit struct size of stack, so if
    >>>> stack size still fine. May we could use the solution and improve it better.
    >>>> like, functionlize, fix the reverse issue etc.
    >>>
    >>> I don't know how important the stack depth consideration is nowadays:
    >>> I still care, maybe others don't, since VMAP_STACK became an option.
    >>>
    >>> Yes, please fix the reversal (if I was right on that); and I expect
    >>> you could use a singly linked list instead of the double.
    >>
    >> single linked list is more saving, but do we have to reverse walking to seek
    >> the head or tail for correct sequence?
    >
    > I imagine all you need is to start off with a
    > for (i = pagevec_count(pvec) - 1; i >= 0; i--)

    a nice simple solution, thanks!

    Thanks
    alex

    > loop.
    >
    >>
    >>>
    >>> But I'll look for an alternative - later, once the urgent stuff
    >>> is completed - and leave the acks on this patch to others.
    >>
    >> Ok, looking forward for your new solution!
    >>
    >> Thanks
    >> Alex

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