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

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Date:   Wed, 29 May 2019 12:33:52 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Brian Geffon <bgeffon@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [RFC 6/7] mm: extend process_madvise syscall to support vector
 arrary

On Wed 29-05-19 03:08:32, Daniel Colascione wrote:
> On Mon, May 27, 2019 at 12:49 AM Minchan Kim <minchan@...nel.org> 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.
-- 
Michal Hocko
SUSE Labs