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

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Message-ID: <Y8fApgKJaTs9nrPO@dhcp22.suse.cz>
Date:   Wed, 18 Jan 2023 10:49:26 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
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Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

On Tue 17-01-23 17:19:46, Suren Baghdasaryan wrote:
> On Tue, Jan 17, 2023 at 7:57 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.
> >
> > What kind of regressions.
> >
> > > To minimize that impact, place VMAs into
> > > a list and free them in groups using one call_rcu() call per group.
> >
> > Please add some data to justify this additional complexity.
> 
> Sorry, should have done that in the first place. A 4.3% regression was
> noticed when running execl test from unixbench suite. spawn test also
> showed 1.6% regression. Profiling revealed that vma freeing was taking
> longer due to call_rcu() which is slow when RCU callback offloading is
> enabled.

Could you be more specific? vma freeing is async with the RCU so how
come this has resulted in a regression? Is there any heavy
rcu_synchronize in the exec path? That would be an interesting
information.
-- 
Michal Hocko
SUSE Labs