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Date:   Fri, 30 Aug 2019 08:23:40 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Yang Shi <shy828301@gmail.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable
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On Thu 29-08-19 10:03:21, Yang Shi wrote:
> On Wed, Aug 28, 2019 at 9:02 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Wed 28-08-19 17:46:59, Kirill A. Shutemov wrote:
> > > On Wed, Aug 28, 2019 at 02:12:53PM +0000, Michal Hocko wrote:
> > > > On Wed 28-08-19 17:03:29, Kirill A. Shutemov wrote:
> > > > > On Wed, Aug 28, 2019 at 09:57:08AM +0200, Michal Hocko wrote:
> > > > > > On Tue 27-08-19 10:06:20, Yang Shi wrote:
> > > > > > >
> > > > > > >
> > > > > > > On 8/27/19 5:59 AM, Kirill A. Shutemov wrote:
> > > > > > > > On Tue, Aug 27, 2019 at 03:17:39PM +0300, Kirill A. Shutemov wrote:
> > > > > > > > > On Tue, Aug 27, 2019 at 02:09:23PM +0200, Michal Hocko wrote:
> > > > > > > > > > On Tue 27-08-19 14:01:56, Vlastimil Babka wrote:
> > > > > > > > > > > On 8/27/19 1:02 PM, Kirill A. Shutemov wrote:
> > > > > > > > > > > > On Tue, Aug 27, 2019 at 08:01:39AM +0200, Michal Hocko wrote:
> > > > > > > > > > > > > On Mon 26-08-19 16:15:38, Kirill A. Shutemov wrote:
> > > > > > > > > > > > > > Unmapped completely pages will be freed with current code. Deferred split
> > > > > > > > > > > > > > only applies to partly mapped THPs: at least on 4k of the THP is still
> > > > > > > > > > > > > > mapped somewhere.
> > > > > > > > > > > > > Hmm, I am probably misreading the code but at least current Linus' tree
> > > > > > > > > > > > > reads page_remove_rmap -> [page_remove_anon_compound_rmap ->\ deferred_split_huge_page even
> > > > > > > > > > > > > for fully mapped THP.
> > > > > > > > > > > > Well, you read correctly, but it was not intended. I screwed it up at some
> > > > > > > > > > > > point.
> > > > > > > > > > > >
> > > > > > > > > > > > See the patch below. It should make it work as intened.
> > > > > > > > > > > >
> > > > > > > > > > > > It's not bug as such, but inefficientcy. We add page to the queue where
> > > > > > > > > > > > it's not needed.
> > > > > > > > > > > But that adding to queue doesn't affect whether the page will be freed
> > > > > > > > > > > immediately if there are no more partial mappings, right? I don't see
> > > > > > > > > > > deferred_split_huge_page() pinning the page.
> > > > > > > > > > > So your patch wouldn't make THPs freed immediately in cases where they
> > > > > > > > > > > haven't been freed before immediately, it just fixes a minor
> > > > > > > > > > > inefficiency with queue manipulation?
> > > > > > > > > > Ohh, right. I can see that in free_transhuge_page now. So fully mapped
> > > > > > > > > > THPs really do not matter and what I have considered an odd case is
> > > > > > > > > > really happening more often.
> > > > > > > > > >
> > > > > > > > > > That being said this will not help at all for what Yang Shi is seeing
> > > > > > > > > > and we need a more proactive deferred splitting as I've mentioned
> > > > > > > > > > earlier.
> > > > > > > > > It was not intended to fix the issue. It's fix for current logic. I'm
> > > > > > > > > playing with the work approach now.
> > > > > > > > Below is what I've come up with. It appears to be functional.
> > > > > > > >
> > > > > > > > Any comments?
> > > > > > >
> > > > > > > Thanks, Kirill and Michal. Doing split more proactive is definitely a choice
> > > > > > > to eliminate huge accumulated deferred split THPs, I did think about this
> > > > > > > approach before I came up with memcg aware approach. But, I thought this
> > > > > > > approach has some problems:
> > > > > > >
> > > > > > > First of all, we can't prove if this is a universal win for the most
> > > > > > > workloads or not. For some workloads (as I mentioned about our usecase), we
> > > > > > > do see a lot THPs accumulated for a while, but they are very short-lived for
> > > > > > > other workloads, i.e. kernel build.
> > > > > > >
> > > > > > > Secondly, it may be not fair for some workloads which don't generate too
> > > > > > > many deferred split THPs or those THPs are short-lived. Actually, the cpu
> > > > > > > time is abused by the excessive deferred split THPs generators, isn't it?
> > > > > >
> > > > > > Yes this is indeed true. Do we have any idea on how much time that
> > > > > > actually is?
> > > > >
> > > > > For uncontented case, splitting 1G worth of pages (2MiB x 512) takes a bit
> > > > > more than 50 ms in my setup. But it's best-case scenario: pages not shared
> > > > > across multiple processes, no contention on ptl, page lock, etc.
> > > >
> > > > Any idea about a bad case?
> > >
> > > Not really.
> > >
> > > How bad you want it to get? How many processes share the page? Access
> > > pattern? Locking situation?
> >
> > Let's say how hard a regular user can make this?
> >
> > > Worst case scenarion: no progress on splitting due to pins or locking
> > > conflicts (trylock failure).
> > >
> > > > > > > With memcg awareness, the deferred split THPs actually are isolated and
> > > > > > > capped by memcg. The long-lived deferred split THPs can't be accumulated too
> > > > > > > many due to the limit of memcg. And, cpu time spent in splitting them would
> > > > > > > just account to the memcgs who generate that many deferred split THPs, who
> > > > > > > generate them who pay for it. This sounds more fair and we could achieve
> > > > > > > much better isolation.
> > > > > >
> > > > > > On the other hand, deferring the split and free up a non trivial amount
> > > > > > of memory is a problem I consider quite serious because it affects not
> > > > > > only the memcg workload which has to do the reclaim but also other
> > > > > > consumers of memory beucase large memory blocks could be used for higher
> > > > > > order allocations.
> > > > >
> > > > > Maybe instead of drive the split from number of pages on queue we can take
> > > > > a hint from compaction that is struggles to get high order pages?
> > > >
> > > > This is still unbounded in time.
> > >
> > > I'm not sure we should focus on time.
> > >
> > > We need to make sure that we don't overal system health worse. Who cares
> > > if we have pages on deferred split list as long as we don't have other
> > > user for the memory?
> >
> > We do care for all those users which do not want to get stalled when
> > requesting that memory. And you cannot really predict that, right? So
> > the sooner the better. Modulo time wasted for the pointless splitting of
> > course. I am afraid defining the best timing here is going to be hard
> > but let's focus on workloads that are known to generate partial THPs and
> > see how that behaves.
> 
> I'm supposed we are just concerned by the global memory pressure
> incurred by the excessive deferred split THPs. As long as no other
> users for that memory we don't have to waste time to care about it.
> So, I'm wondering why not we do harder in kswapd?

kswapd is already late. There shouldn't be any need for the reclaim as
long as there is a lot of memory that can be directly freed.

> Currently, deferred split THPs get shrunk like slab. The number of
> objects scanned is determined by some factors, i.e. scan priority,
> shrinker->seeks, etc, to avoid over reclaim for filesystem caches to
> avoid extra I/O. But, we don't have to worry about over reclaim for
> deferred split THPs, right? We definitely could shrink them more
> aggressively in kswapd context.

This is certainly possible. I am just wondering why should we cram this
into the reclaim when we have a reasonable trigger to do that.

> For example, we could simply set shrinker->seeks to 0, now it is
> DEFAULT_SEEKS.
> 
> And, we also could consider boost water mark to wake up kswapd earlier
> once we see excessive deferred split THPs accumulated.

This has other side effect, right?

-- 
Michal Hocko
SUSE Labs