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

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Date:   Thu, 19 Jan 2023 13:59:31 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in
 vm_area_free

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.

-- 
Michal Hocko
SUSE Labs