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SubjectRe: [PATCH v12 00/31] Speculative page faults
On Tue 23-04-19 05:41:48, Matthew Wilcox wrote:
> 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

Thank you, will have a look.

> 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?

I thought we had something for the mmap_sem scaling but I do not see
this in a list of proposed topics. But we can certainly add it there.

> > 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.

Well, we are talking about
45 files changed, 1277 insertions(+), 196 deletions(-)

which is a _major_ surgery in my book. Having a real life workloads numbers
is nothing unfair to ask for IMHO.

And let me remind you that I am not really opposing SPF in general. I
would just like to see a simpler approach before we go such a large
change. If the range locking is not really a scalable approach then all
right but from why I've see it should help a lot of most bottle-necks I
have seen.
--
Michal Hocko
SUSE Labs

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