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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 19 Jan 2023 11:47:36 -0800
Message-ID: <CAJuCfpEoGCs6JgjiWL1ACS8S8TmwM1x5EF7x8D=M9zqnkyqxBA@mail.gmail.com>
Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free
To:     paulmck@kernel.org
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On Thu, Jan 19, 2023 at 11:20 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Thu, Jan 19, 2023 at 10:52:03AM -0800, Suren Baghdasaryan wrote:
> > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mhocko@suse.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. WDYT about time domain throttling to
> > limit draining the list to say once per second like this:
> >
> > void vm_area_free(struct vm_area_struct *vma)
> > {
> >        struct mm_struct *mm = vma->vm_mm;
> >        bool drain;
> >
> >        free_anon_vma_name(vma);
> >
> >        spin_lock(&mm->vma_free_list.lock);
> >        list_add(&vma->vm_free_list, &mm->vma_free_list.head);
> >        mm->vma_free_list.size++;
> > -       drain = mm->vma_free_list.size > VM_AREA_FREE_LIST_MAX;
> > +       drain = jiffies > mm->last_drain_tm + HZ;
> >
> >        spin_unlock(&mm->vma_free_list.lock);
> >
> > -       if (drain)
> > +       if (drain) {
> >               drain_free_vmas(mm);
> > +             mm->last_drain_tm = jiffies;
> > +       }
> > }
> >
> > Ultimately we want to prevent very frequent call_rcu() calls, so
> > throttling in the time domain seems appropriate. That's the simplest
> > way I can think of to address your concern about a quick spike in VMA
> > freeing. It does not place any restriction on the list size and we
> > might have excessive dead vm_area_structs if after a large spike there
> > are no vm_area_free() calls but I don't know if that's a real problem,
> > so not sure we should be addressing it at this time. WDYT?
>
> Just to double-check, we really did try the very frequent call_rcu()
> invocations and we really did see a problem, correct?

Correct. More specifically with CONFIG_RCU_NOCB_CPU=y we saw
regressions when a process exits and all its VMAs get destroyed,
causing a flood of call_rcu()'s.

>
> Although it is not perfect, call_rcu() is designed to take a fair amount
> of abuse.  So if we didn't see a real problem, the frequent call_rcu()
> invocations might be a bit simpler.
>
>                                                         Thanx, Paul