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

[Suren Baghdasaryan]

On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mhocko at 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?


>
> --
> Michal Hocko
> SUSE Labs