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Date: Tue, 23 Apr 2019 05:41:48 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v12 00/31] Speculative page faults
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On Tue, Apr 23, 2019 at 12:47:07PM +0200, Michal Hocko wrote:
> On Mon 22-04-19 14:29:16, Michel Lespinasse wrote:
> [...]
> > I want to add a note about mmap_sem. In the past there has been
> > discussions about replacing it with an interval lock, but these never
> > went anywhere because, mostly, of the fact that such mechanisms were
> > too expensive to use in the page fault path. I think adding the spf
> > mechanism would invite us to revisit this issue - interval locks may
> > be a great way to avoid blocking between unrelated mmap_sem writers
> > (for example, do not delay stack creation for new threads while a
> > large mmap or munmap may be going on), and probably also to handle
> > mmap_sem readers that can't easily use the spf mechanism (for example,
> > gup callers which make use of the returned vmas). But again that is a
> > separate topic to explore which doesn't have to get resolved before
> > spf goes in.
> 
> Well, I believe we should _really_ re-evaluate the range locking sooner
> rather than later. Why? Because it looks like the most straightforward
> approach to the mmap_sem contention for most usecases I have heard of
> (mostly a mm{unm}ap, mremap standing in the way of page faults).
> On a plus side it also makes us think about the current mmap (ab)users
> which should lead to an overall code improvements and maintainability.

Dave Chinner recently did evaluate the range lock for solving a problem
in XFS and didn't like what he saw:

https://lore.kernel.org/linux-fsdevel/20190418031013.GX29573@dread.disaster.area/T/#md981b32c12a2557a2dd0f79ad41d6c8df1f6f27c

I think scaling the lock needs to be tied to the actual data structure
and not have a second tree on-the-side to fake-scale the locking.  Anyway,
we're going to have a session on this at LSFMM, right?

> SPF sounds like a good idea but it is a really big and intrusive surgery
> to the #PF path. And more importantly without any real world usecase
> numbers which would justify this. That being said I am not opposed to
> this change I just think it is a large hammer while we haven't seen
> attempts to tackle problems in a simpler way.

I don't think the "no real world usecase numbers" is fair.  Laurent quoted:

> Ebizzy:
> -------
> The test is counting the number of records per second it can manage, the
> higher is the best. I run it like this 'ebizzy -mTt <nrcpus>'. To get
> consistent result I repeated the test 100 times and measure the average
> result. The number is the record processes per second, the higher is the best.
> 
>   		BASE		SPF		delta	
> 24 CPUs x86	5492.69		9383.07		70.83%
> 1024 CPUS P8 VM 8476.74		17144.38	102%

and cited 30% improvement for you-know-what product from an earlier
version of the patch.