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

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Message-ID: <20230119192002.GX2948950@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 19 Jan 2023 11:20:02 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com, luto@...nel.org,
        songliubraving@...com, peterx@...hat.com, david@...hat.com,
        dhowells@...hat.com, hughd@...gle.com, bigeasy@...utronix.de,
        kent.overstreet@...ux.dev, punit.agrawal@...edance.com,
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        leewalsh@...gle.com, posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

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@...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. 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?

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