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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 11 May 2022 15:57:09 -0700 Minchan Kim <minchan@kernel.org> 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"?

> > 
> > 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?

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