Re: [PATCH v3 13/14] mm, hugetlb: retry if failed to allocate and there is concurrent user

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Thu, Dec 19, 2013 at 06:15:20PM -0800, Andrew Morton wrote:
> On Fri, 20 Dec 2013 10:58:10 +0900 Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:
> 
> > On Thu, Dec 19, 2013 at 05:02:02PM -0800, Andrew Morton wrote:
> > > On Wed, 18 Dec 2013 15:53:59 +0900 Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:
> > > 
> > > > If parallel fault occur, we can fail to allocate a hugepage,
> > > > because many threads dequeue a hugepage to handle a fault of same address.
> > > > This makes reserved pool shortage just for a little while and this cause
> > > > faulting thread who can get hugepages to get a SIGBUS signal.
> > > > 
> > > 
> > > So if I'm understanding this correctly...  if N threads all generate a
> > > fault against the same address, they will all dive in and allocate a
> > > hugepage, will then do an enormous memcpy into that page and will then
> > > attempt to instantiate the page in pagetables.  All threads except one
> > > will lose the race and will free the page again!  This sounds terribly
> > > inefficient; it would be useful to write a microbenchmark which
> > > triggers this scenario so we can explore the impact.
> > 
> > Yes, you understand correctly, I think.
> > 
> > I have an idea to prevent this overhead. It is that marking page when it
> > is zeroed and unmarking when it is mapped to page table. If page mapping
> > is failed due to current thread, the zeroed page will keep the marker and
> > later we can determine if it is zeroed or not.
> 
> Well OK, but the other threads will need to test that in-progress flag
> and then do <something>.  Where <something> will involve some form of
> open-coded sleep/wakeup thing.  To avoid all that wheel-reinventing we
> can avoid using an internal flag and use an external flag instead. 
> There's one in struct mutex!

My idea consider only hugetlb_no_page() and doesn't need a sleep.
It just set <some> page flag after zeroing and if some thread takes
the page with this flag when faulting, simply use it without zeroing.

> 
> I doubt if the additional complexity of the external flag is worth it,
> but convincing performance testing results would sway me ;) Please have
> a think about it all.
> 
> > If you want to include this functionality in this series, I can do it ;)
> > Please let me know your decision.
> > 
> > > I'm wondering if a better solution to all of this would be to make
> > > hugetlb_instantiation_mutex an array of, say, 1024 mutexes and index it
> > > with a hash of the faulting address.  That will 99.9% solve the
> > > performance issue which you believe exists without introducing this new
> > > performance issue?
> > 
> > Yes, that approach would solve the performance issue.
> > IIRC, you already suggested this idea roughly 6 months ago and it is
> > implemented by Davidlohr. I remembered that there is a race issue on
> > COW case with this approach. See following link for more information.
> > https://lkml.org/lkml/2013/8/7/142
> 
> That seems to be unrelated to hugetlb_instantiation_mutex?

Yes, it is related to hugetlb_instantiation_mutex. In the link, I mentioned
about race condition of table mutex patches which is for replacing
hugetlb_instantiation_mutex, although conversation isn't easy to follow-up.

> 
> > And we need 1-3 patches to prevent other theorectical race issue
> > regardless any approaches.
> 
> Yes, I'll be going through patches 1-12 very soon, thanks.

Okay. Thanks :)

> 
> 
> And to reiterate: I'm very uncomfortable mucking around with
> performance patches when we have run no tests to measure their
> magnitude, or even whether they are beneficial at all!

Okay. I will keep in mind it. :)

Thanks.