[ovs-dev] [PATCH v3 2/5] dpif-netdev: Trigger parallel pmd reloads.

[Ilya Maximets]

On 04.07.2019 14:59, David Marchand wrote:
> pmd reloads are currently serialised in each steps calling
> reload_affected_pmds.
> Any pmd processing packets, waiting on a mutex etc... will make other
> pmd threads wait for a delay that can be undeterministic when syscalls
> adds up.
> 
> Switch to a little busy loop on the control thread using the existing
> per-pmd reload boolean.
> 
> The memory order on this atomic is rel-acq to have an explicit
> synchronisation between the pmd threads and the control thread.
> 
> Signed-off-by: David Marchand <david.marchand at redhat.com>
> ---
> Changelog since v2:
> - remove unneeded synchronisation on pmd thread join (Ilya)
> - "inlined" synchronisation loop in reload_affected_pmds
> 
> Changelog since v1:
> - removed the introduced reloading_pmds atomic and reuse the existing
>   pmd->reload boolean as a single synchronisation point (Ilya)
> 
> Changelog since RFC v1:
> - added memory ordering on 'reloading_pmds' atomic to serve as a
>   synchronisation point between pmd threads and control thread
> 
> ---
>  lib/dpif-netdev.c | 26 ++++++++++++--------------
>  1 file changed, 12 insertions(+), 14 deletions(-)
> 
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 8eeec63..415107b 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -649,9 +649,6 @@ struct dp_netdev_pmd_thread {
>      struct ovs_refcount ref_cnt;    /* Every reference must be refcount'ed. */
>      struct cmap_node node;          /* In 'dp->poll_threads'. */
>  
> -    pthread_cond_t cond;            /* For synchronizing pmd thread reload. */
> -    struct ovs_mutex cond_mutex;    /* Mutex for condition variable. */
> -
>      /* Per thread exact-match cache.  Note, the instance for cpu core
>       * NON_PMD_CORE_ID can be accessed by multiple threads, and thusly
>       * need to be protected by 'non_pmd_mutex'.  Every other instance
> @@ -1758,11 +1755,8 @@ dp_netdev_reload_pmd__(struct dp_netdev_pmd_thread *pmd)
>          return;
>      }
>  
> -    ovs_mutex_lock(&pmd->cond_mutex);
>      seq_change(pmd->reload_seq);
>      atomic_store_explicit(&pmd->reload, true, memory_order_release);
> -    ovs_mutex_cond_wait(&pmd->cond, &pmd->cond_mutex);
> -    ovs_mutex_unlock(&pmd->cond_mutex);
>  }
>  
>  static uint32_t
> @@ -4655,6 +4649,17 @@ reload_affected_pmds(struct dp_netdev *dp)
>          if (pmd->need_reload) {
>              flow_mark_flush(pmd);
>              dp_netdev_reload_pmd__(pmd);
> +        }
> +    }
> +
> +    CMAP_FOR_EACH (pmd, node, &dp->poll_threads) {
> +        if (pmd->need_reload) {
> +            bool reload;
> +
> +            do {
> +                atomic_read_explicit(&pmd->reload, &reload,
> +                                     memory_order_acquire);
> +            } while (reload);

If we'll ever set 'need_reload' for non-PMD thread there will be
kind of deadlock. We never waited for non-PMD thread previously,
because dp_netdev_reload_pmd__() has a special case for it.

>              pmd->need_reload = false;
>          }
>      }
> @@ -5842,11 +5847,8 @@ dpif_netdev_enable_upcall(struct dpif *dpif)
>  static void
>  dp_netdev_pmd_reload_done(struct dp_netdev_pmd_thread *pmd)
>  {
> -    ovs_mutex_lock(&pmd->cond_mutex);
> -    atomic_store_relaxed(&pmd->reload, false);
>      pmd->last_reload_seq = seq_read(pmd->reload_seq);
> -    xpthread_cond_signal(&pmd->cond);
> -    ovs_mutex_unlock(&pmd->cond_mutex);
> +    atomic_store_explicit(&pmd->reload, false, memory_order_release);
>  }
>  
>  /* Finds and refs the dp_netdev_pmd_thread on core 'core_id'.  Returns
> @@ -5931,8 +5933,6 @@ dp_netdev_configure_pmd(struct dp_netdev_pmd_thread *pmd, struct dp_netdev *dp,
>      pmd->reload_seq = seq_create();
>      pmd->last_reload_seq = seq_read(pmd->reload_seq);
>      atomic_init(&pmd->reload, false);
> -    xpthread_cond_init(&pmd->cond, NULL);
> -    ovs_mutex_init(&pmd->cond_mutex);
>      ovs_mutex_init(&pmd->flow_mutex);
>      ovs_mutex_init(&pmd->port_mutex);
>      cmap_init(&pmd->flow_table);
> @@ -5975,8 +5975,6 @@ dp_netdev_destroy_pmd(struct dp_netdev_pmd_thread *pmd)
>      cmap_destroy(&pmd->flow_table);
>      ovs_mutex_destroy(&pmd->flow_mutex);
>      seq_destroy(pmd->reload_seq);
> -    xpthread_cond_destroy(&pmd->cond);
> -    ovs_mutex_destroy(&pmd->cond_mutex);
>      ovs_mutex_destroy(&pmd->port_mutex);
>      free(pmd);
>  }
>