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

[Michal Hocko]

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).
 
>   - 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
semantic because it can cause really long stalls for other operations
but that is a discussion on its own and I would rather focus on a simple
interface first.

> 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