expr: Remove supersets from OR expressions.

While crushing OR expressions, OVN removes exact replicas of sub
expressions.  However, there could be many CMP expressions that are
supersets of each other.  These are most likely to be created as a
result of cross-product while expanding brackets in the AND expression
in crush_and_numeric(), i.e. while converting
"x && (a0 || a1) && (b0 || b1)" into "xa0b0 || xa0b1 || xa1b0 || xa1b1".

In addition to removal of exact duplicates introducing scan and removal
of supersets of other existing sub-expressions to reduce the amount of
generated flows.  This adds extra computations, but should save time
later, since less flows will be generated.

Example:

  "ip4.src == 172.168.0.0/16 && ip4.src!={172.168.13.0/24, 172.168.15.0/24}"

  Processing of this expression yields 42 flows:

  $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr"

  ip,nw_src=172.168.0.0/255.255.1.0
  ip,nw_src=172.168.0.0/255.255.10.0
  ip,nw_src=172.168.0.0/255.255.12.0
  ip,nw_src=172.168.0.0/255.255.3.0
  ip,nw_src=172.168.0.0/255.255.4.0
  ip,nw_src=172.168.0.0/255.255.5.0
  ip,nw_src=172.168.0.0/255.255.6.0
  ip,nw_src=172.168.0.0/255.255.8.0
  ip,nw_src=172.168.0.0/255.255.9.0
  ip,nw_src=172.168.128.0/17
  <... 32 more flows ...>

  We can see that many flows above do overlap, e.g. 255.255.3.0
  mask is a superset of 255.255.1.0.  Everything that matches
  255.255.3.0, will match 255.255.1.0 as well (the value is the same).

  By removing all the unnecessary supersets, the set of flows can
  be reduced from 42 down to 7:

  ip,nw_src=172.168.0.0/255.255.1.0
  ip,nw_src=172.168.0.0/255.255.4.0
  ip,nw_src=172.168.0.0/255.255.8.0
  ip,nw_src=172.168.128.0/17
  ip,nw_src=172.168.16.0/255.255.16.0
  ip,nw_src=172.168.32.0/255.255.32.0
  ip,nw_src=172.168.64.0/255.255.64.0

This change should be particularly useful for expressions with
inequality checks, like the one above.  Such expressions are
frequent among ACL rules.

  "ip4.src != {172.168.13.0/24, 172.168.14.0/24, 172.168.15.0/24}"

  Brefore:
  $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr" | wc -l
  2894

  After:
  $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr" | wc -l
  23

Superset lookups are performed only if there are expressions with
more bits in the mask than in the current one.  So, there is no extra
cost for equality checks on normal address sets, like port group sets,
where all the IPs are exect matches or have the same prefix length
otherwise.

Also, the superset optimization is not performed if expression is
tracking an address set.  This is done in order to preserve ability to
use address set I-P even if the user specified overlapping addresses
and CIDRs.  Note: having exact duplicates in user-specified sets is
highly unlikely, because of database constraints.

Use of bitmaps instead of subvalue functions significantly speeds up
processing since most of the subvalue space is an all-zero empty space.

Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2177197
Reported-by: Nadia Pinaeva <npinaeva@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Han Zhou <hzhou@ovn.org>

include/ovn/expr.h

@@ -384,6 +384,7 @@ struct expr {
                 struct {
                     union mf_subvalue value;
                     union mf_subvalue mask;
+                    size_t mask_n_bits;
                 };
             };
         } cmp;

lib/expr.c

@@ -15,21 +15,23 @@
  */
 
 #include <config.h>
+#include "bitmap.h"
 #include "byte-order.h"
-#include "openvswitch/json.h"
+#include "hmapx.h"
 #include "nx-match.h"
 #include "openvswitch/dynamic-string.h"
+#include "openvswitch/json.h"
 #include "openvswitch/match.h"
 #include "openvswitch/ofp-actions.h"
-#include "openvswitch/vlog.h"
 #include "openvswitch/shash.h"
+#include "openvswitch/vlog.h"
+#include "ovn-util.h"
 #include "ovn/expr.h"
 #include "ovn/lex.h"
 #include "ovn/logical-fields.h"
 #include "simap.h"
 #include "sset.h"
 #include "util.h"
-#include "ovn-util.h"
 
 VLOG_DEFINE_THIS_MODULE(expr);
 
@@ -2520,6 +2522,198 @@ crush_and_string(struct expr *expr, const struct expr_symbol *symbol)
     return expr_fix(expr);
 }
 
+static int
+compare_cmps_3way(const struct expr *a, const struct expr *b)
+{
+    ovs_assert(a->cmp.symbol == b->cmp.symbol);
+    if (!a->cmp.symbol->width) {
+        return strcmp(a->cmp.string, b->cmp.string);
+    } else if (a->cmp.mask_n_bits != b->cmp.mask_n_bits) {
+        return a->cmp.mask_n_bits < b->cmp.mask_n_bits ? -1 : 1;
+    } else {
+        int d = memcmp(&a->cmp.value, &b->cmp.value, sizeof a->cmp.value);
+        if (!d) {
+            d = memcmp(&a->cmp.mask, &b->cmp.mask, sizeof a->cmp.mask);
+        }
+        return d;
+    }
+}
+
+static int
+compare_cmps_cb(const void *a_, const void *b_)
+{
+    const struct expr *const *ap = a_;
+    const struct expr *const *bp = b_;
+    const struct expr *a = *ap;
+    const struct expr *b = *bp;
+    return compare_cmps_3way(a, b);
+}
+
+/* Similar to mf_subvalue_intersect(), but only checks the possibility of
+ * intersection without producing a result. */
+static bool
+expr_bitmap_intersect_check(const unsigned long *a_value,
+                            const unsigned long *a_mask,
+                            const unsigned long *b_value,
+                            const unsigned long *b_mask,
+                            size_t bit_width)
+{
+    for (size_t i = 0; i < bitmap_n_longs(bit_width); i++) {
+        if ((a_value[i] ^ b_value[i]) & (a_mask[i] & b_mask[i])) {
+            return false;
+        }
+    }
+    return true;
+}
+
+/* This function expects an OR expression with already crushed sub
+ * expressions, so they are plain comparisons.  Result is the same
+ * expression, but with unnecessary sub-expressions removed. */
+static struct expr *
+crush_or_supersets(struct expr *expr, const struct expr_symbol *symbol)
+{
+    ovs_assert(expr->type == EXPR_T_OR);
+
+    /* Calculate offset within subfield and a width that can be used
+     * in a bitmap. */
+    const size_t sz = CHAR_BIT * sizeof expr->cmp.value.be64[0];
+    const size_t bit_width = ROUND_UP(symbol->width, sz);
+    const size_t ofs = ARRAY_SIZE(expr->cmp.value.be64) - bit_width / sz;
+
+    /* Sort subexpressions by number of bits in the mask, value and the mask
+     * itself, to bring together duplicates and have expressions ordered by
+     * mask sizes. */
+    size_t n = ovs_list_size(&expr->andor);
+    struct expr **subs = xmalloc(n * sizeof *subs);
+    bool modified = false;
+    /* Linked list over the 'subs' array to quickly skip deleted elements,
+     * i.e. the index of the next potentially non-NULL element. */
+    size_t *next = xmalloc(n * sizeof *next);
+
+    size_t i = 0, j, max_n_bits = 0;
+    struct expr *sub;
+    LIST_FOR_EACH (sub, node, &expr->andor) {
+        if (symbol->width) {
+            const unsigned long *sub_mask;
+
+            sub_mask = (unsigned long *) &sub->cmp.mask.be64[ofs];
+            sub->cmp.mask_n_bits = bitmap_count1(sub_mask, bit_width);
+            max_n_bits = MAX(max_n_bits, sub->cmp.mask_n_bits);
+        }
+        next[i] = i + 1; /* Link 'i' -> 'i + 1'. */
+        subs[i++] = sub;
+    }
+    ovs_assert(i == n);
+
+    qsort(subs, n, sizeof *subs, compare_cmps_cb);
+
+    /* Eliminate duplicates. */
+    size_t last = 0;
+    for (i = 1; i < n; i++) {
+        if (compare_cmps_3way(subs[last], subs[i])) {
+            next[last] = i;
+            last = i;
+        } else {
+            expr_destroy(subs[i]);
+            subs[i] = NULL;
+            modified = true;
+        }
+    }
+
+    if (!symbol->width || symbol->level != EXPR_L_ORDINAL
+        || (!modified && expr->as_name)) {
+        /* Not a fully maskable field or this expression is tracking an
+         * address set.  Don't try to optimize to preserve address set I-P. */
+        goto done;
+    }
+
+    /* Build a mask size index.  'mask_index[n_bits]' is an index in 'subs',
+     * where expressions with 'n_bits' bits in mask start. */
+    size_t *mask_index, n_bits;
+    size_t index_size = (max_n_bits + 2) * sizeof *mask_index;
+
+    mask_index = xmalloc(index_size);
+    /* Initialize to maximum unsigned values. */
+    memset(mask_index, 0xff, index_size);
+
+    for (i = 0; i < n; i = next[i]) {
+        if (subs[i]) {
+            n_bits = subs[i]->cmp.mask_n_bits;
+            mask_index[n_bits] = MIN(mask_index[n_bits], i);
+        }
+    }
+
+    /* Fill the gaps, so they point to an index with more bits. */
+    for (i = max_n_bits; i > 0; i--) {
+        mask_index[i] = MIN(mask_index[i], mask_index[i + 1]);
+    }
+
+    /* Find and eliminate supersets. */
+    for (i = 0; i < n; i = next[i]) {
+        if (!subs[i]) {
+            continue;
+        }
+        /* 'subs' are sorted based on the number of bits in the mask.
+         * For an expression to be a subset, it has to have more bits. */
+        n_bits = subs[i]->cmp.mask_n_bits;
+        if (mask_index[n_bits + 1] > n) {
+            break;
+        }
+        for (last = 0, j = mask_index[n_bits + 1]; j < n; j = next[j]) {
+            struct expr *a = subs[i], *b = subs[j];
+
+            ovs_assert(i != j);
+            if (!b) {
+                continue;
+            }
+
+            const unsigned long *a_value, *a_mask, *b_value, *b_mask;
+            a_value = (unsigned long *) &a->cmp.value.be64[ofs];
+            b_value = (unsigned long *) &b->cmp.value.be64[ofs];
+            a_mask  = (unsigned long *) &a->cmp.mask.be64[ofs];
+            b_mask  = (unsigned long *) &b->cmp.mask.be64[ofs];
+
+            if (expr_bitmap_intersect_check(a_value, a_mask, b_value, b_mask,
+                                            bit_width)
+                && bitmap_is_superset(b_mask, a_mask, bit_width)) {
+                /* 'a' is the same expression with a smaller mask. */
+                expr_destroy(subs[j]);
+                subs[j] = NULL;
+                modified = true;
+
+                /* Shorten the path for the next round. */
+                if (last) {
+                    next[last] = next[j]; /* Skip the 'j'. */
+                } else {
+                    /* The first element with 'n_bits + 1' bits was removed. */
+                    mask_index[n_bits + 1] = next[j];
+                }
+            } else {
+                last = j; /* 'j' is the last non-NULL element seen. */
+            }
+        }
+    }
+    free(mask_index);
+
+done:
+    ovs_list_init(&expr->andor);
+    for (i = 0; i < n; i++) {
+        if (subs[i]) {
+            ovs_list_push_back(&expr->andor, &subs[i]->node);
+        }
+    }
+
+    if (modified) {
+        /* Members modified, so untrack address set. */
+        free(expr->as_name);
+        expr->as_name = NULL;
+    }
+
+    free(next);
+    free(subs);
+    return expr;
+}
+
 /* Implementation of crush_cmps() for expr->type == EXPR_T_AND and a
  * numeric-typed 'symbol'. */
 static struct expr *
@@ -2679,31 +2873,6 @@ crush_and_numeric(struct expr *expr, const struct expr_symbol *symbol)
     }
 }
 
-static int
-compare_cmps_3way(const struct expr *a, const struct expr *b)
-{
-    ovs_assert(a->cmp.symbol == b->cmp.symbol);
-    if (!a->cmp.symbol->width) {
-        return strcmp(a->cmp.string, b->cmp.string);
-    } else {
-        int d = memcmp(&a->cmp.value, &b->cmp.value, sizeof a->cmp.value);
-        if (!d) {
-            d = memcmp(&a->cmp.mask, &b->cmp.mask, sizeof a->cmp.mask);
-        }
-        return d;
-    }
-}
-
-static int
-compare_cmps_cb(const void *a_, const void *b_)
-{
-    const struct expr *const *ap = a_;
-    const struct expr *const *bp = b_;
-    const struct expr *a = *ap;
-    const struct expr *b = *bp;
-    return compare_cmps_3way(a, b);
-}
-
 /* Implementation of crush_cmps() for expr->type == EXPR_T_OR. */
 static struct expr *
 crush_or(struct expr *expr, const struct expr_symbol *symbol)
@@ -2723,34 +2892,8 @@ crush_or(struct expr *expr, const struct expr_symbol *symbol)
         return expr;
     }
 
-    /* Sort subexpressions by value and mask, to bring together duplicates. */
-    size_t n = ovs_list_size(&expr->andor);
-    struct expr **subs = xmalloc(n * sizeof *subs);
-
-    size_t i = 0;
-    LIST_FOR_EACH (sub, node, &expr->andor) {
-        subs[i++] = sub;
-    }
-    ovs_assert(i == n);
-
-    qsort(subs, n, sizeof *subs, compare_cmps_cb);
+    expr = crush_or_supersets(expr, symbol);
 
-    /* Eliminate duplicates. */
-    ovs_list_init(&expr->andor);
-    ovs_list_push_back(&expr->andor, &subs[0]->node);
-    for (i = 1; i < n; i++) {
-        struct expr *a = expr_from_node(ovs_list_back(&expr->andor));
-        struct expr *b = subs[i];
-        if (compare_cmps_3way(a, b)) {
-            ovs_list_push_back(&expr->andor, &b->node);
-        } else {
-            expr_destroy(b);
-            /* Member modified, so untrack address set. */
-            free(expr->as_name);
-            expr->as_name = NULL;
-        }
-    }
-    free(subs);
     return expr_fix(expr);
 }
 

tests/ovn.at

@@ -822,6 +822,18 @@ ip,nw_src=10.0.0.1,nw_dst=172.27.0.65
 ip,nw_src=10.0.0.2,nw_dst=172.27.0.65
 ip,nw_src=10.0.0.3,nw_dst=172.27.0.65
 ])
+AT_CHECK([expr_to_flow 'ip4.src == 172.168.13.0/16 && ip4.src != {172.168.13.0/24, 172.168.14.0/24}'], [0], [dnl
+ip,nw_src=172.168.0.0/255.255.3.0
+ip,nw_src=172.168.0.0/255.255.4.0
+ip,nw_src=172.168.0.0/255.255.8.0
+ip,nw_src=172.168.128.0/17
+ip,nw_src=172.168.16.0/255.255.16.0
+ip,nw_src=172.168.3.0/255.255.3.0
+ip,nw_src=172.168.32.0/255.255.32.0
+ip,nw_src=172.168.64.0/255.255.64.0
+])
+dnl Negative match flow explosion.
+AT_CHECK([test $(expr_to_flow 'ip4.src != {172.168.13.0/24, 172.168.14.0/24, 172.168.15.0/24}' | wc -l) -le 30])
 AT_CLEANUP
 
 AT_SETUP([converting expressions to flows -- port groups])