Re: [PATCH v4] mm: don't be stuck to rmap lock on reclaim path

[Minchan Kim]

On Wed, May 11, 2022 at 07:05:23PM -0700, Andrew Morton wrote:
> On Wed, 11 May 2022 15:57:09 -0700 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 
> > > 
> > > Could we burn much CPU time pointlessly churning though the LRU?  Could
> > > it mess up aging decisions enough to be performance-affecting in any
> > > workload?
> > 
> > Yes, correct. However, we are already churning LRUs by several
> > ways. For example, isolate and putback from LRU list for page
> > migration from several sources(typical example is compaction)
> > and trylock_page and sc->gfp_mask not allowing page to be
> > reclaimed in shrink_page_list.
> 
> Well.  "we're already doing a risky thing so it's OK to do more of that
> thing"?

I meant the aging is not rocket science.


> 
> > > 
> > > Something else?
> > 
> > One thing I am worry about was the granularity of the churning.
> > Example above was page granuarity churning so might be execuse
> > but this one is address space's churning, especically for file LRU
> > (i_mmap_rwsem) which might cause too many rotating and live-lock
> > in the end(keey rotating in small LRU with heavy memory pressure).
> > 
> > If it could be a problem, maybe we use sc->priority to stop
> > the skipping on a certain level of memory pressure.
> > 
> > Any thought? Do we really need it?
> 
> Are we able to think of a test which might demonstrate any worst case? 
> Whip that up and see what the numbers say?

Yeah, let me create a worst test case to see how it goes.

A thread keep reading a file-backed vma with 2xRAM file but other threads
keep changing other vmas mapped at the same file so heavy i_mmap_rwsem
contention in aging path.

> 
> It's a bit of a drag, but if we don't do it, our users surely will ;)