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

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Jan 19, 2023 at 11:47:36AM -0800, Suren Baghdasaryan wrote:
> 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.

Thank you for the reminder, real problem needs solution.  ;-)

							Thanx, Paul

> > 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.