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

Eelco Chaudron echaudro at redhat.com
Tue May 7 13:46:34 UTC 2019



On 30 Apr 2019, at 14:17, 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 an atomic
> count.
>
> Signed-off-by: David Marchand <david.marchand at redhat.com>
> ---
>  lib/dpif-netdev.c | 43 ++++++++++++++++++++++++++++++-------------
>  1 file changed, 30 insertions(+), 13 deletions(-)
>
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 30774ed..b2b21fd 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -335,6 +335,9 @@ struct dp_netdev {
>      /* The time that a packet can wait in output batch for sending. 
> */
>      atomic_uint32_t tx_flush_interval;
>
> +    /* Count of pmds currently reloading */
> +    atomic_count reloading_pmds;
> +
>      /* Meters. */
>      struct ovs_mutex meter_locks[N_METER_LOCKS];
>      struct dp_meter *meters[MAX_METERS]; /* Meter bands. */
> @@ -647,9 +650,6 @@ 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
> @@ -1525,6 +1525,8 @@ create_dp_netdev(const char *name, const struct 
> dpif_class *class,
>      atomic_init(&dp->emc_insert_min, DEFAULT_EM_FLOW_INSERT_MIN);
>      atomic_init(&dp->tx_flush_interval, DEFAULT_TX_FLUSH_INTERVAL);
>
> +    atomic_count_init(&dp->reloading_pmds, 0);
> +
>      cmap_init(&dp->poll_threads);
>      dp->pmd_rxq_assign_cyc = true;
>
> @@ -1754,11 +1756,8 @@ dp_netdev_reload_pmd__(struct 
> dp_netdev_pmd_thread *pmd)
>          return;
>      }
>
> -    ovs_mutex_lock(&pmd->cond_mutex);
>      seq_change(pmd->reload_seq);
>      atomic_store_relaxed(&pmd->reload, true);
> -    ovs_mutex_cond_wait(&pmd->cond, &pmd->cond_mutex);
> -    ovs_mutex_unlock(&pmd->cond_mutex);
>  }
>
>  static uint32_t
> @@ -4641,9 +4640,27 @@ rxq_scheduling(struct dp_netdev *dp, bool 
> pinned) OVS_REQUIRES(dp->port_mutex)
>  }
>
>  static void
> +wait_reloading_pmds(struct dp_netdev *dp)
> +{
> +    while (atomic_count_get(&dp->reloading_pmds) != 0) {
> +    }
> +}
> +
> +static void
>  reload_affected_pmds(struct dp_netdev *dp)
>  {
>      struct dp_netdev_pmd_thread *pmd;
> +    unsigned int pmd_count = 0;
> +
> +    CMAP_FOR_EACH (pmd, node, &dp->poll_threads) {
> +        if (pmd->core_id == NON_PMD_CORE_ID) {
> +            continue;
> +        }
> +        if (pmd->need_reload) {
> +            pmd_count++;
> +        }
> +    }
> +    atomic_count_set(&dp->reloading_pmds, pmd_count);
>      CMAP_FOR_EACH (pmd, node, &dp->poll_threads) {
>          if (pmd->need_reload) {
> @@ -4652,6 +4669,10 @@ reload_affected_pmds(struct dp_netdev *dp)

The above atomic_count_set() is a relaxed set, so is the 
atomic_store_relaxed(&pmd->reload, true) in dp_netdev_reload_pmd__(pmd).
Which could lead to the PMDs decreasing the dp->reloading_pmd before 
it’s set. Guess if the correct memory_order is selected for 
pmd->reload as suggested by Ilya in patch 1/5 it should work.

>              pmd->need_reload = false;
>          }
>      }
> +
> +    if (pmd_count != 0) {
> +        wait_reloading_pmds(dp);
> +    }
>  }
>
>  static void
> @@ -5813,11 +5834,9 @@ 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_count_dec(&pmd->dp->reloading_pmds);
>  }
>
>  /* Finds and refs the dp_netdev_pmd_thread on core 'core_id'.  
> Returns
> @@ -5902,8 +5921,6 @@ 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);
> @@ -5946,8 +5963,6 @@ 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);
>  }
> @@ -5967,7 +5982,9 @@ dp_netdev_del_pmd(struct dp_netdev *dp, struct 
> dp_netdev_pmd_thread *pmd)
>          ovs_mutex_unlock(&dp->non_pmd_mutex);
>      } else {
>          atomic_store_relaxed(&pmd->exit, true);
> +        atomic_count_set(&dp->reloading_pmds, 1);
>          dp_netdev_reload_pmd__(pmd);
> +        wait_reloading_pmds(dp);
>          xpthread_join(pmd->thread, NULL);
>      }
>
> -- 
> 1.8.3.1


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