Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

[Kirill A. Shutemov]

On Mon, Aug 26, 2019 at 09:40:35AM +0200, Michal Hocko wrote:
> On Thu 22-08-19 18:29:34, Kirill A. Shutemov wrote:
> > On Thu, Aug 22, 2019 at 02:56:56PM +0200, Vlastimil Babka wrote:
> > > On 8/22/19 10:04 AM, Michal Hocko wrote:
> > > > On Thu 22-08-19 01:55:25, Yang Shi wrote:
> > > >> Available memory is one of the most important metrics for memory
> > > >> pressure.
> > > > 
> > > > I would disagree with this statement. It is a rough estimate that tells
> > > > how much memory you can allocate before going into a more expensive
> > > > reclaim (mostly swapping). Allocating that amount still might result in
> > > > direct reclaim induced stalls. I do realize that this is simple metric
> > > > that is attractive to use and works in many cases though.
> > > > 
> > > >> Currently, the deferred split THPs are not accounted into
> > > >> available memory, but they are reclaimable actually, like reclaimable
> > > >> slabs.
> > > >> 
> > > >> And, they seems very common with the common workloads when THP is
> > > >> enabled.  A simple run with MariaDB test of mmtest with THP enabled as
> > > >> always shows it could generate over fifteen thousand deferred split THPs
> > > >> (accumulated around 30G in one hour run, 75% of 40G memory for my VM).
> > > >> It looks worth accounting in MemAvailable.
> > > > 
> > > > OK, this makes sense. But your above numbers are really worrying.
> > > > Accumulating such a large amount of pages that are likely not going to
> > > > be used is really bad. They are essentially blocking any higher order
> > > > allocations and also push the system towards more memory pressure.
> > > > 
> > > > IIUC deferred splitting is mostly a workaround for nasty locking issues
> > > > during splitting, right? This is not really an optimization to cache
> > > > THPs for reuse or something like that. What is the reason this is not
> > > > done from a worker context? At least THPs which would be freed
> > > > completely sound like a good candidate for kworker tear down, no?
> > > 
> > > Agreed that it's a good question. For Kirill :) Maybe with kworker approach we
> > > also wouldn't need the cgroup awareness?
> > 
> > I don't remember a particular locking issue, but I cannot say there's
> > none :P
> > 
> > It's artifact from decoupling PMD split from compound page split: the same
> > page can be mapped multiple times with combination of PMDs and PTEs. Split
> > of one PMD doesn't need to trigger split of all PMDs and underlying
> > compound page.
> > 
> > Other consideration is the fact that page split can fail and we need to
> > have fallback for this case.
> > 
> > Also in most cases THP split would be just waste of time if we would do
> > them at the spot. If you don't have memory pressure it's better to wait
> > until process termination: less pages on LRU is still beneficial.
> 
> This might be true but the reality shows that a lot of THPs might be
> waiting for the memory pressure that is essentially freeable on the
> spot. So I am not really convinced that "less pages on LRUs" is really a
> plausible justification. Can we free at least those THPs which are
> unmapped completely without any pte mappings?

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.

> > Main source of partly mapped THPs comes from exit path. When PMD mapping
> > of THP got split across multiple VMAs (for instance due to mprotect()),
> > in exit path we unmap PTEs belonging to one VMA just before unmapping the
> > rest of the page. It would be total waste of time to split the page in
> > this scenario.
> > 
> > The whole deferred split thing still looks as a reasonable compromise
> > to me.
> 
> Even when it leads to all other problems mentioned in this and memcg
> deferred reclaim series?

Yes.

You would still need deferred split even if you *try* to split the page on
the spot. split_huge_page() can fail (due to pin on the page) and you will
need to have a way to try again later.

You'll not win anything in complexity by trying split_huge_page()
immediately. I would ague you'll create much more complexity.

> > We may have some kind of watermark and try to keep the number of deferred
> > split THP under it. But it comes with own set of problems: what if all
> > these pages are pinned for really long time and effectively not available
> > for split.
> 
> Again, why cannot we simply push the freeing where there are no other
> mappings? This should be pretty common case, right?

Partly mapped THP is not common case at all.

To get to this point you will need to create a mapping, fault in THP and
then unmap part of it. It requires very active memory management on
application side. This kind of applications usually knows if THP is a fit
for them.

> I am still not sure that waiting for the memory reclaim is a general
> win.

It wins CPU cycles by not doing the work that is likely unneeded.
split_huge_page() is not particularly lightweight operation from locking
and atomic ops POV.

> Do you have any examples of workloads that measurably benefit from
> this lazy approach without any other downsides? In other words how
> exactly do we measure cost/benefit model of this heuristic?

Example? Sure.

Compiling mm/memory.c in my setup generates 8 deferred split. 4 of them
triggered from exit path. The rest 4 comes from MADV_DONTNEED. It doesn't
make sense to convert any of them to in-place split: for short-lived
process any split if waste of time without any benefit.

-- 
 Kirill A. Shutemov