Re: [patch 3/5] mm, memcg: introduce own oom handler to iterate only over its own threads

[Sha Zhengju <handai.szj@gmail.com>]

On 06/30/2012 05:06 AM, David Rientjes wrote:
> The global oom killer is serialized by the zonelist being used in the
> page allocation.  Concurrent oom kills are thus a rare event and only
> occur in systems using mempolicies and with a large number of nodes.
>
> Memory controller oom kills, however, can frequently be concurrent since
> there is no serialization once the oom killer is called for oom
> conditions in several different memcgs in parallel.
>
> This creates a massive contention on tasklist_lock since the oom killer
> requires the readside for the tasklist iteration.  If several memcgs are
> calling the oom killer, this lock can be held for a substantial amount of
> time, especially if threads continue to enter it as other threads are
> exiting.
>
> Since the exit path grabs the writeside of the lock with irqs disabled in
> a few different places, this can cause a soft lockup on cpus as a result
> of tasklist_lock starvation.
>
> The kernel lacks unfair writelocks, and successful calls to the oom
> killer usually result in at least one thread entering the exit path, so
> an alternative solution is needed.
>
> This patch introduces a seperate oom handler for memcgs so that they do
> not require tasklist_lock for as much time.  Instead, it iterates only
> over the threads attached to the oom memcg and grabs a reference to the
> selected thread before calling oom_kill_process() to ensure it doesn't
> prematurely exit.
>
> This still requires tasklist_lock for the tasklist dump, iterating
> children of the selected process, and killing all other threads on the
> system sharing the same memory as the selected victim.  So while this
> isn't a complete solution to tasklist_lock starvation, it significantly
> reduces the amount of time that it is held.
>

Looks good.
You can add Reviewed-by: Sha Zhengju <handai.szj@taobao.com>

Thanks,
Sha

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