Re: [RFC 6/7] mm: extend process_madvise syscall to support vector arrary

[Minchan Kim]

On Thu, May 30, 2019 at 08:57:55AM +0200, Michal Hocko wrote:
> On Thu 30-05-19 11:17:48, Minchan Kim wrote:
> > On Wed, May 29, 2019 at 12:33:52PM +0200, Michal Hocko wrote:
> > > On Wed 29-05-19 03:08:32, Daniel Colascione wrote:
> > > > On Mon, May 27, 2019 at 12:49 AM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, May 21, 2019 at 12:37:26PM +0200, Michal Hocko wrote:
> > > > > > On Tue 21-05-19 19:26:13, Minchan Kim wrote:
> > > > > > > On Tue, May 21, 2019 at 08:24:21AM +0200, Michal Hocko wrote:
> > > > > > > > On Tue 21-05-19 11:48:20, Minchan Kim wrote:
> > > > > > > > > On Mon, May 20, 2019 at 11:22:58AM +0200, Michal Hocko wrote:
> > > > > > > > > > [Cc linux-api]
> > > > > > > > > >
> > > > > > > > > > On Mon 20-05-19 12:52:53, Minchan Kim wrote:
> > > > > > > > > > > Currently, process_madvise syscall works for only one address range
> > > > > > > > > > > so user should call the syscall several times to give hints to
> > > > > > > > > > > multiple address range.
> > > > > > > > > >
> > > > > > > > > > Is that a problem? How big of a problem? Any numbers?
> > > > > > > > >
> > > > > > > > > We easily have 2000+ vma so it's not trivial overhead. I will come up
> > > > > > > > > with number in the description at respin.
> > > > > > > >
> > > > > > > > Does this really have to be a fast operation? I would expect the monitor
> > > > > > > > is by no means a fast path. The system call overhead is not what it used
> > > > > > > > to be, sigh, but still for something that is not a hot path it should be
> > > > > > > > tolerable, especially when the whole operation is quite expensive on its
> > > > > > > > own (wrt. the syscall entry/exit).
> > > > > > >
> > > > > > > What's different with process_vm_[readv|writev] and vmsplice?
> > > > > > > If the range needed to be covered is a lot, vector operation makes senese
> > > > > > > to me.
> > > > > >
> > > > > > I am not saying that the vector API is wrong. All I am trying to say is
> > > > > > that the benefit is not really clear so far. If you want to push it
> > > > > > through then you should better get some supporting data.
> > > > >
> > > > > I measured 1000 madvise syscall vs. a vector range syscall with 1000
> > > > > ranges on ARM64 mordern device. Even though I saw 15% improvement but
> > > > > absoluate gain is just 1ms so I don't think it's worth to support.
> > > > > I will drop vector support at next revision.
> > > > 
> > > > Please do keep the vector support. Absolute timing is misleading,
> > > > since in a tight loop, you're not going to contend on mmap_sem. We've
> > > > seen tons of improvements in things like camera start come from
> > > > coalescing mprotect calls, with the gains coming from taking and
> > > > releasing various locks a lot less often and bouncing around less on
> > > > the contended lock paths. Raw throughput doesn't tell the whole story,
> > > > especially on mobile.
> > > 
> > > This will always be a double edge sword. Taking a lock for longer can
> > > improve a throughput of a single call but it would make a latency for
> > > anybody contending on the lock much worse.
> > > 
> > > Besides that, please do not overcomplicate the thing from the early
> > > beginning please. Let's start with a simple and well defined remote
> > > madvise alternative first and build a vector API on top with some
> > > numbers based on _real_ workloads.
> > 
> > First time, I didn't think about atomicity about address range race
> > because MADV_COLD/PAGEOUT is not critical for the race.
> > However you raised the atomicity issue because people would extend
> > hints to destructive ones easily. I agree with that and that's why
> > we discussed how to guarantee the race and Daniel comes up with good idea.
> 
> Just for the clarification, I didn't really mean atomicity but rather a
> _consistency_ (essentially time to check to time to use consistency).

What do you mean by *consistency*? Could you elaborate it more?

>  
> >   - vma configuration seq number via process_getinfo(2).
> > 
> > We discussed the race issue without _read_ workloads/requests because
> > it's common sense that people might extend the syscall later.
> > 
> > Here is same. For current workload, we don't need to support vector
> > for perfomance point of view based on my experiment. However, it's
> > rather limited experiment. Some configuration might have 10000+ vmas
> > or really slow CPU. 
> > 
> > Furthermore, I want to have vector support due to atomicity issue
> > if it's really the one we should consider.
> > With vector support of the API and vma configuration sequence number
> > from Daniel, we could support address ranges operations's atomicity.
> 
> I am not sure what do you mean here. Perform all ranges atomicaly wrt.
> other address space modifications? If yes I am not sure we want that

Yub, I think it's *necessary* if we want to support destructive hints
via process_madvise.

> semantic because it can cause really long stalls for other operations

It could be or it couldn't be.

For example, if we could multiplex several syscalls which we should
enumerate all of page table lookup, it could be more effective rather
than doing each page table on each syscall.

> but that is a discussion on its own and I would rather focus on a simple
> interface first.

It seems it's time to send RFCv2 since we discussed a lot although we
don't have clear conclution yet. But still want to understand what you
meant _consistency_.

Thanks for the review, Michal! It's very helpful.

> 
> > However, since we don't introduce vector at this moment, we need to
> > introduce *another syscall* later to be able to handle multile ranges
> > all at once atomically if it's okay.
> 
> Agreed.
> 
> > Other thought:
> > Maybe we could extend address range batch syscall covers other MM
> > syscall like mmap/munmap/madvise/mprotect and so on because there
> > are multiple users that would benefit from this general batching
> > mechanism.
> 
> Again a discussion on its own ;)
> 
> -- 
> Michal Hocko
> SUSE Labs