Re: [PATCH net-next 5/9] net: openvswitch: optimize flow-mask looking up

[Tonghao Zhang <xiangxia.m.yue@gmail.com>]

On Wed, Oct 2, 2019 at 10:03 AM Pravin Shelar <pshelar@ovn.org> wrote:
>
> On Sun, Sep 29, 2019 at 7:09 PM <xiangxia.m.yue@gmail.com> wrote:
> >
> > From: Tonghao Zhang <xiangxia.m.yue@gmail.com>
> >
> > The full looking up on flow table traverses all mask array.
> > If mask-array is too large, the number of invalid flow-mask
> > increase, performance will be drop.
> >
> > This patch optimizes mask-array operation:
> >
> > * Inserting, insert it [ma->count- 1] directly.
> > * Removing, only change last and current mask point, and free current mask.
> > * Looking up, full looking up will break if mask is NULL.
> >
> > The function which changes or gets *count* of struct mask_array,
> > is protected by ovs_lock, but flow_lookup (not protected) should use *max* of
> > struct mask_array.
> >
> > Functions protected by ovs_lock:
> > * tbl_mask_array_del_mask
> > * tbl_mask_array_add_mask
> > * flow_mask_find
> > * ovs_flow_tbl_lookup_exact
> > * ovs_flow_tbl_num_masks
> >
> > Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
> > ---
> >  net/openvswitch/flow_table.c | 101 +++++++++++++++++++++----------------------
> >  1 file changed, 49 insertions(+), 52 deletions(-)
> >
> > diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
> > index e59fac5..5257e4a 100644
> > --- a/net/openvswitch/flow_table.c
> > +++ b/net/openvswitch/flow_table.c
> > @@ -546,7 +546,7 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
> >
> >                 mask = rcu_dereference_ovsl(ma->masks[i]);
> >                 if (!mask)
> > -                       continue;
> > +                       break;
> >
> >                 flow = masked_flow_lookup(ti, key, mask, n_mask_hit);
> >                 if (flow) { /* Found */
> > @@ -640,15 +640,13 @@ struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
> >         int i;
> >
> >         /* Always called under ovs-mutex. */
> > -       for (i = 0; i < ma->max; i++) {
> > +       for (i = 0; i < ma->count; i++) {
> >                 struct table_instance *ti = rcu_dereference_ovsl(tbl->ti);
> >                 u32 __always_unused n_mask_hit;
> >                 struct sw_flow_mask *mask;
> >                 struct sw_flow *flow;
> >
> >                 mask = ovsl_dereference(ma->masks[i]);
> > -               if (!mask)
> > -                       continue;
> >
> >                 flow = masked_flow_lookup(ti, match->key, mask, &n_mask_hit);
> >                 if (flow && ovs_identifier_is_key(&flow->id) &&
> > @@ -712,21 +710,31 @@ static struct table_instance *table_instance_expand(struct table_instance *ti,
> >         return table_instance_rehash(ti, ti->n_buckets * 2, ufid);
> >  }
> >
> > -static void tbl_mask_array_delete_mask(struct mask_array *ma,
> > -                                      struct sw_flow_mask *mask)
> > +static void tbl_mask_array_del_mask(struct flow_table *tbl,
> > +                                   struct sw_flow_mask *mask)
> >  {
> > +       struct mask_array *ma = ovsl_dereference(tbl->mask_array);
> >         int i;
> >
> >         /* Remove the deleted mask pointers from the array */
> > -       for (i = 0; i < ma->max; i++) {
> > -               if (mask == ovsl_dereference(ma->masks[i])) {
> > -                       RCU_INIT_POINTER(ma->masks[i], NULL);
> > -                       ma->count--;
> > -                       kfree_rcu(mask, rcu);
> > -                       return;
> > -               }
> > +       for (i = 0; i < ma->count; i++) {
> > +               if (mask == ovsl_dereference(ma->masks[i]))
> > +                       goto found;
> >         }
> > +
> >         BUG();
> > +       return;
> > +
> > +found:
> > +       rcu_assign_pointer(ma->masks[i], ma->masks[ma->count -1]);
> > +       RCU_INIT_POINTER(ma->masks[ma->count -1], NULL);
> > +       ma->count--;
> > +       kfree_rcu(mask, rcu);
> > +
> > +       /* Shrink the mask array if necessary. */
> > +       if (ma->max >= (MASK_ARRAY_SIZE_MIN * 2) &&
> > +           ma->count <= (ma->max / 3))
> > +               tbl_mask_array_realloc(tbl, ma->max / 2);
> >  }
> >
> >  /* Remove 'mask' from the mask list, if it is not needed any more. */
> > @@ -740,17 +748,8 @@ static void flow_mask_remove(struct flow_table *tbl, struct sw_flow_mask *mask)
> >                 BUG_ON(!mask->ref_count);
> >                 mask->ref_count--;
> >
> > -               if (!mask->ref_count) {
> > -                       struct mask_array *ma;
> > -
> > -                       ma = ovsl_dereference(tbl->mask_array);
> > -                       tbl_mask_array_delete_mask(ma, mask);
> > -
> > -                       /* Shrink the mask array if necessary. */
> > -                       if (ma->max >= (MASK_ARRAY_SIZE_MIN * 2) &&
> > -                           ma->count <= (ma->max / 3))
> > -                               tbl_mask_array_realloc(tbl, ma->max / 2);
> > -               }
> > +               if (!mask->ref_count)
> > +                       tbl_mask_array_del_mask(tbl, mask);
> >         }
> >  }
> >
> > @@ -803,17 +802,38 @@ static struct sw_flow_mask *flow_mask_find(const struct flow_table *tbl,
> >         int i;
> >
> >         ma = ovsl_dereference(tbl->mask_array);
> > -       for (i = 0; i < ma->max; i++) {
> > +       for (i = 0; i < ma->count; i++) {
> >                 struct sw_flow_mask *t;
> >                 t = ovsl_dereference(ma->masks[i]);
> >
> > -               if (t && mask_equal(mask, t))
> > +               if (mask_equal(mask, t))
> >                         return t;
> >         }
> >
> >         return NULL;
> >  }
> >
> > +static int tbl_mask_array_add_mask(struct flow_table *tbl,
> > +                                  struct sw_flow_mask *new)
> > +{
> > +       struct mask_array *ma = ovsl_dereference(tbl->mask_array);
> > +       int err;
> > +
> > +       if (ma->count >= ma->max) {
> > +               err = tbl_mask_array_realloc(tbl, ma->max +
> > +                                             MASK_ARRAY_SIZE_MIN);
> > +               if (err)
> > +                       return err;
> > +       }
> > +
> > +       BUG_ON(ovsl_dereference(ma->masks[ma->count]));
> > +
> > +       rcu_assign_pointer(ma->masks[ma->count], new);
> > +       ma->count++;
> There is no barrier for ma->count. Fastpath is accessing ma->count
> without synchronization. Same issue with mask delete operation.
barrier will be added in v2,
In the fastpath, we use the ma->count and check ma->masks[*index] is
valid. if NULL we will break. so we can access ma->count without
synchronization in the fastpath.