netdev - Re: [PATCH net-next] net: try to avoid unneeded backlog flush

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Date:   Thu, 10 Sep 2020 16:36:21 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH net-next] net: try to avoid unneeded backlog flush

On Thu, Sep 10, 2020 at 4:21 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> flush_all_backlogs() may cause deadlock on systems
> running processes with FIFO scheduling policy.
>
> The above is critical in -RT scenarios, where user-space
> specifically ensure no network activity is scheduled on
> the CPU running the mentioned FIFO process, but still get
> stuck.
>
> This commit tries to address the problem checking the
> backlog status on the remote CPUs before scheduling the
> flush operation. If the backlog is empty, we can skip it.

If it is not empty, the problem you want to fix is still there ?

>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/core/dev.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 46 insertions(+), 4 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 152ad3b578de..fdef40bf4b88 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5621,17 +5621,59 @@ static void flush_backlog(struct work_struct *work)
>         local_bh_enable();
>  }
>
> +static bool flush_required(int cpu)
> +{
> +#if IS_ENABLED(CONFIG_RPS)
> +       struct softnet_data *sd = &per_cpu(softnet_data, cpu);
> +       bool do_flush;
> +
> +       local_irq_disable();
> +       rps_lock(sd);
> +
> +       /* as insertion into process_queue happens with the rps lock held,
> +        * process_queue access may race only with dequeue
> +        */
> +       do_flush = !skb_queue_empty(&sd->input_pkt_queue) ||
> +                  !skb_queue_empty_lockless(&sd->process_queue);
> +       rps_unlock(sd);
> +       local_irq_enable();
> +
> +       return do_flush;
> +#endif
> +       /* without RPS we can't safely check input_pkt_queue: during a
> +        * concurrent remote skb_queue_splice() we can detect as empty both
> +        * input_pkt_queue and process_queue even if the latter could end-up
> +        * containing a lot of packets.
> +        */
> +       return true;
> +}
> +
>  static void flush_all_backlogs(void)
>  {
> +       static cpumask_t flush_cpus  = { CPU_BITS_NONE };
>         unsigned int cpu;
>
> +       /* since we are under rtnl lock protection we can use static data
> +        * for the cpumask and avoid allocating on stack the possibly
> +        * large mask
> +        */
> +       ASSERT_RTNL();
> +

OK, but you only set bits in this bitmask.

You probably want to clear it here, not rely on one time CPU_BITS_NONE

>         get_online_cpus();
>
> -       for_each_online_cpu(cpu)
> -               queue_work_on(cpu, system_highpri_wq,
> -                             per_cpu_ptr(&flush_works, cpu));
> +       for_each_online_cpu(cpu) {
> +               if (flush_required(cpu)) {
> +                       queue_work_on(cpu, system_highpri_wq,
> +                                     per_cpu_ptr(&flush_works, cpu));
> +                       cpumask_set_cpu(cpu, &flush_cpus);
> +               }
> +       }
>
> -       for_each_online_cpu(cpu)
> +       /* we can have in flight packet[s] on the cpus we are not flushing,
> +        * synchronize_net() in rollback_registered_many() will take care of
> +        * them
> +        */
> +       for_each_cpu(cpu, &flush_cpus)
>                 flush_work(per_cpu_ptr(&flush_works, cpu));
>



>         put_online_cpus();
> --
> 2.26.2
>