linux-kernel - Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area

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Message-ID: <Y85acuBFkGr03Qd7@dhcp22.suse.cz>
Date:   Mon, 23 Jan 2023 10:59:14 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

On Fri 20-01-23 08:20:43, Suren Baghdasaryan wrote:
> On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote:
> > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mhocko@...e.com> wrote:
> > > >
> > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote:
> > > > > call_rcu() can take a long time when callback offloading is enabled.
> > > > > Its use in the vm_area_free can cause regressions in the exit path when
> > > > > multiple VMAs are being freed. To minimize that impact, place VMAs into
> > > > > a list and free them in groups using one call_rcu() call per group.
> > > >
> > > > After some more clarification I can understand how call_rcu might not be
> > > > super happy about thousands of callbacks to be invoked and I do agree
> > > > that this is not really optimal.
> > > >
> > > > On the other hand I do not like this solution much either.
> > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that
> > > > much with processes with a huge number of vmas either. It would still be
> > > > in housands of callbacks to be scheduled without a good reason.
> > > >
> > > > Instead, are there any other cases than remove_vma that need this
> > > > batching? We could easily just link all the vmas into linked list and
> > > > use a single call_rcu instead, no? This would both simplify the
> > > > implementation, remove the scaling issue as well and we do not have to
> > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1.
> > >
> > > Yes, I agree the solution is not stellar. I wanted something simple
> > > but this is probably too simple. OTOH keeping all dead vm_area_structs
> > > on the list without hooking up a shrinker (additional complexity) does
> > > not sound too appealing either.
> >
> > I suspect you have missed my idea. I do not really want to keep the list
> > around or any shrinker. It is dead simple. Collect all vmas in
> > remove_vma and then call_rcu the whole list at once after the whole list
> > (be it from exit_mmap or remove_mt). See?
> 
> Yes, I understood your idea but keeping dead objects until the process
> exits even when the system is low on memory (no shrinkers attached)
> seems too wasteful. If we do this I would advocate for attaching a
> shrinker.

I am still not sure we are on the same page here. No, vmas shouldn't lay
around un ntil the process exit. I am really suggesting queuing only for
remove_vma paths. You can have a different rcu callback than the one
used for trivial single vma removal paths.

-- 
Michal Hocko
SUSE Labs