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    SubjectRe: [PATCH 00/14] per memcg lru_lock
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    在 2019/8/24 上午9:59, Hugh Dickins 写道:
    > On Wed, 21 Aug 2019, Alex Shi wrote:
    >> 在 2019/8/21 上午2:24, Hugh Dickins 写道:
    >>> I'll set aside what I'm doing, and switch to rebasing ours to v5.3-rc
    >>> and/or mmotm. Then compare with what Alex has, to see if there's any
    >>> good reason to prefer one to the other: if no good reason to prefer ours,
    >>> I doubt we shall bother to repost, but just use it as basis for helping
    >>> to review or improve Alex's.
    >>
    >> For your review, my patchset are pretty straight and simple.
    >> It just use per lruvec lru_lock to replace necessary pgdat lru_lock.
    >> just this. We could talk more after I back to work. :)
    >
    > Sorry to be bearer of bad news, Alex, but when you said "straight and
    > simple", I feared that your patchset would turn out to be fundamentally
    > too simple.
    >
    > And that is so. I have only to see the
    > lruvec = mem_cgroup_page_lruvec(page, pgdat);
    > line in isolate_migratepages_block() in mm/compaction.c, and check
    > that mem_cgroup_page_lruvec() is little changed in mm/mempolicy.c.
    >
    > The central problem with per-memcg lru_lock is that you do not know
    > for sure what lock to take (which memcg a page belongs to) until you
    > have taken the expected lock, and then checked whether page->memcg
    > is still the same - backing out and trying again if not.
    >
    > Fix that central problem, and you end up with a more complicated
    > patchset, much like ours. It's true that when ours was first developed,
    > the memcg situation was more complicated in several ways, and perhaps
    > some aspects of our patchset could be simplified now (though I've not
    > identified any). Johannes in particular has done a great deal of
    > simplifying work in memcg over the last few years, but there are still
    > situations in which a page's memcg can change (move_charge_at_immigrate
    > and swapin readahead spring to mind - or perhaps the latter is only an
    > issue when MEMCG_SWAP is not enabled, I forget; and I often wonder if
    > reparenting will be brought back one day).
    >
    > I did not review your patchset in detail, and wasn't able to get very
    > far in testing it. At first I was put off by set_task_reclaim_state
    > warnings from mm/vmscan.c, but those turned out to be in v5.3-rc5
    > itself, not from your patchset or mine (but I've not yet investigated
    > what's responsible for them). Within a minute of starting swapping
    > load, kcompactd compact_lock_irqsave() in isolate_migratepages_block()
    > would deadlock, and I did not get further. (Though I did also notice
    > that booting the CONFIG_MEMCG=y kernel with "cgroup_disable=memory"
    > froze in booting - tiresomely, one has to keep both the memcg and
    > no-memcg locking to cope with that case, and I guess you had not.)
    >
    > Rather than duplicating effort, I would advise you to give our patchset
    > a try, and if it works for you, help towards getting that one merged:
    > but of course, it's up to you.

    Thanks a lot for all infos and reminders! Yes, the page->memcg change would be a problem. I will studying your patchset and try to merge them.

    >
    > I've attached a tarfile of it rebased to v5.3-rc5: I do not want to
    > spam the list with patches yet, because I do not have any stats or
    > argument in support of the series, as Andrew asked for years ago and
    > Michal asks again now. But aside from that I consider it ready, and
    > will let Shakeel take it over from here, while I get back to what I
    > diverted from (but of course I'll try to answer questions on it).
    >
    I will trying to look into them. Thanks for your kindly offer. :)

    Thanks!
    Alex

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