Re: [PATCH 00/14] per memcg lru_lock

[Alex Shi]

å 2019/8/24 äå9:59, Hugh Dickins åé:
> On Wed, 21 Aug 2019, Alex Shi wrote:
>> å 2019/8/21 äå2:24, Hugh Dickins åé:
>>> 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