[ovs-dev] [v13 12/12] dpcls-avx512: Enable avx512 vector popcount instruction.

Cian Ferriter cian.ferriter at intel.com
Thu Jun 17 16:18:25 UTC 2021


From: Harry van Haaren <harry.van.haaren at intel.com>

This commit enables the AVX512-VPOPCNTDQ Vector Popcount
instruction. This instruction is not available on every CPU
that supports the AVX512-F Foundation ISA, hence it is enabled
only when the additional VPOPCNTDQ ISA check is passed.

The vector popcount instruction is used instead of the AVX512
popcount emulation code present in the avx512 optimized DPCLS today.
It provides higher performance in the SIMD miniflow processing
as that requires the popcount to calculate the miniflow block indexes.

Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>

---

v13:
- Rebased and Improved comment on use_vpop variable (Ian)
---
 NEWS                                   |  3 +
 lib/dpdk.c                             |  1 +
 lib/dpif-netdev-lookup-avx512-gather.c | 85 ++++++++++++++++++++------
 3 files changed, 71 insertions(+), 18 deletions(-)

diff --git a/NEWS b/NEWS
index f03bfeb5d..bc1db7948 100644
--- a/NEWS
+++ b/NEWS
@@ -18,6 +18,9 @@ Post-v2.15.0
      * Enable AVX512 optimized DPCLS to search subtables with larger miniflows.
      * Add more specialized DPCLS subtables to cover common rules, enhancing
        the lookup performance.
+     * Enable the AVX512 DPCLS implementation to use VPOPCNT instruction if the
+       CPU supports it. This enhances performance by using the native vpopcount
+       instructions, instead of the emulated version of vpopcount.
    - ovs-ctl:
      * New option '--no-record-hostname' to disable hostname configuration
        in ovsdb on startup.
diff --git a/lib/dpdk.c b/lib/dpdk.c
index c883a4b8b..a9494a40f 100644
--- a/lib/dpdk.c
+++ b/lib/dpdk.c
@@ -655,6 +655,7 @@ dpdk_get_cpu_has_isa(const char *arch, const char *feature)
 #if __x86_64__
     /* CPU flags only defined for the architecture that support it. */
     CHECK_CPU_FEATURE(feature, "avx512f", RTE_CPUFLAG_AVX512F);
+    CHECK_CPU_FEATURE(feature, "avx512vpopcntdq", RTE_CPUFLAG_AVX512VPOPCNTDQ);
     CHECK_CPU_FEATURE(feature, "bmi2", RTE_CPUFLAG_BMI2);
 #endif
 
diff --git a/lib/dpif-netdev-lookup-avx512-gather.c b/lib/dpif-netdev-lookup-avx512-gather.c
index 0b51ef9dc..bc359dc4a 100644
--- a/lib/dpif-netdev-lookup-avx512-gather.c
+++ b/lib/dpif-netdev-lookup-avx512-gather.c
@@ -53,6 +53,15 @@
 
 VLOG_DEFINE_THIS_MODULE(dpif_lookup_avx512_gather);
 
+
+/* Wrapper function required to enable ISA. */
+static inline __m512i
+__attribute__((__target__("avx512vpopcntdq")))
+_mm512_popcnt_epi64_wrapper(__m512i v_in)
+{
+    return _mm512_popcnt_epi64(v_in);
+}
+
 static inline __m512i
 _mm512_popcnt_epi64_manual(__m512i v_in)
 {
@@ -131,6 +140,7 @@ netdev_rule_matches_key(const struct dpcls_rule *rule,
  *   pkt_mf_u0_pop: population count of bits in u0 of the packet.
  *   zero_mask: bitmask of lanes to zero as packet doesn't have mf bits set.
  *   u64_lanes_mask: bitmask of lanes to process.
+ *   use_vpop: compile-time constant indicating if VPOPCNT instruction allowed.
  */
 static inline ALWAYS_INLINE __m512i
 avx512_blocks_gather(__m512i v_u0,
@@ -141,7 +151,8 @@ avx512_blocks_gather(__m512i v_u0,
                      __mmask64 u1_bcast_msk,
                      const uint64_t pkt_mf_u0_pop,
                      __mmask64 zero_mask,
-                     __mmask64 u64_lanes_mask)
+                     __mmask64 u64_lanes_mask,
+                     const uint32_t use_vpop)
 {
         /* Suggest to compiler to load tbl blocks ahead of gather(). */
         __m512i v_tbl_blocks = _mm512_maskz_loadu_epi64(u64_lanes_mask,
@@ -155,8 +166,15 @@ avx512_blocks_gather(__m512i v_u0,
                                                       tbl_mf_masks);
         __m512i v_masks = _mm512_and_si512(v_pkt_bits, v_tbl_masks);
 
-        /* Manual AVX512 popcount for u64 lanes. */
-        __m512i v_popcnts = _mm512_popcnt_epi64_manual(v_masks);
+        /* Calculate AVX512 popcount for u64 lanes using the native instruction
+         * if available, or using emulation if not available.
+         */
+        __m512i v_popcnts;
+        if (use_vpop) {
+            v_popcnts = _mm512_popcnt_epi64_wrapper(v_masks);
+        } else {
+            v_popcnts = _mm512_popcnt_epi64_manual(v_masks);
+        }
 
         /* Add popcounts and offset for u1 bits. */
         __m512i v_idx_u0_offset = _mm512_maskz_set1_epi64(u1_bcast_msk,
@@ -181,7 +199,8 @@ avx512_lookup_impl(struct dpcls_subtable *subtable,
                    const struct netdev_flow_key *keys[],
                    struct dpcls_rule **rules,
                    const uint32_t bit_count_u0,
-                   const uint32_t bit_count_u1)
+                   const uint32_t bit_count_u1,
+                   const uint32_t use_vpop)
 {
     OVS_ALIGNED_VAR(CACHE_LINE_SIZE)uint64_t block_cache[BLOCKS_CACHE_SIZE];
     uint32_t hashes[NETDEV_MAX_BURST];
@@ -233,7 +252,8 @@ avx512_lookup_impl(struct dpcls_subtable *subtable,
                                                 u1_bcast_mask,
                                                 pkt_mf_u0_pop,
                                                 zero_mask,
-                                                bit_count_total_mask);
+                                                bit_count_total_mask,
+                                                use_vpop);
         _mm512_storeu_si512(&block_cache[i * MF_BLOCKS_PER_PACKET], v_blocks);
 
         if (bit_count_total > 8) {
@@ -254,7 +274,8 @@ avx512_lookup_impl(struct dpcls_subtable *subtable,
                                                     u1_bcast_mask_gt8,
                                                     pkt_mf_u0_pop,
                                                     zero_mask_gt8,
-                                                    bit_count_gt8_mask);
+                                                    bit_count_gt8_mask,
+                                                    use_vpop);
             _mm512_storeu_si512(&block_cache[(i * MF_BLOCKS_PER_PACKET) + 8],
                                 v_blocks_gt8);
         }
@@ -303,7 +324,11 @@ avx512_lookup_impl(struct dpcls_subtable *subtable,
     return found_map;
 }
 
-/* Expand out specialized functions with U0 and U1 bit attributes. */
+/* Expand out specialized functions with U0 and U1 bit attributes. As the
+ * AVX512 vpopcnt instruction is not supported on all AVX512 capable CPUs,
+ * create two functions for each miniflow signature. This allows the runtime
+ * CPU detection in probe() to select the ideal implementation.
+ */
 #define DECLARE_OPTIMIZED_LOOKUP_FUNCTION(U0, U1)                             \
     static uint32_t                                                           \
     dpcls_avx512_gather_mf_##U0##_##U1(struct dpcls_subtable *subtable,       \
@@ -311,7 +336,20 @@ avx512_lookup_impl(struct dpcls_subtable *subtable,
                                        const struct netdev_flow_key *keys[],  \
                                        struct dpcls_rule **rules)             \
     {                                                                         \
-        return avx512_lookup_impl(subtable, keys_map, keys, rules, U0, U1);   \
+        const uint32_t use_vpop = 0;                                          \
+        return avx512_lookup_impl(subtable, keys_map, keys, rules,            \
+                                  U0, U1, use_vpop);                          \
+    }                                                                         \
+                                                                              \
+    static uint32_t __attribute__((__target__("avx512vpopcntdq")))            \
+    dpcls_avx512_gather_mf_##U0##_##U1##_vpop(struct dpcls_subtable *subtable,\
+                                       uint32_t keys_map,                     \
+                                       const struct netdev_flow_key *keys[],  \
+                                       struct dpcls_rule **rules)             \
+    {                                                                         \
+        const uint32_t use_vpop = 1;                                          \
+        return avx512_lookup_impl(subtable, keys_map, keys, rules,            \
+                                  U0, U1, use_vpop);                          \
     }                                                                         \
 
 DECLARE_OPTIMIZED_LOOKUP_FUNCTION(9, 4)
@@ -321,11 +359,18 @@ DECLARE_OPTIMIZED_LOOKUP_FUNCTION(5, 1)
 DECLARE_OPTIMIZED_LOOKUP_FUNCTION(4, 1)
 DECLARE_OPTIMIZED_LOOKUP_FUNCTION(4, 0)
 
-/* Check if a specialized function is valid for the required subtable. */
-#define CHECK_LOOKUP_FUNCTION(U0, U1)                                         \
+/* Check if a specialized function is valid for the required subtable.
+ * The use_vpop variable is used to decide if the VPOPCNT instruction can be
+ * used or not.
+ */
+#define CHECK_LOOKUP_FUNCTION(U0, U1, use_vpop)                               \
     ovs_assert((U0 + U1) <= (NUM_U64_IN_ZMM_REG * 2));                        \
     if (!f && u0_bits == U0 && u1_bits == U1) {                               \
-        f = dpcls_avx512_gather_mf_##U0##_##U1;                               \
+        if (use_vpop) {                                                       \
+            f = dpcls_avx512_gather_mf_##U0##_##U1##_vpop;                    \
+        } else {                                                              \
+            f = dpcls_avx512_gather_mf_##U0##_##U1;                           \
+        }                                                                     \
     }
 
 static uint32_t
@@ -333,9 +378,11 @@ dpcls_avx512_gather_mf_any(struct dpcls_subtable *subtable, uint32_t keys_map,
                            const struct netdev_flow_key *keys[],
                            struct dpcls_rule **rules)
 {
+    const uint32_t use_vpop = 0;
     return avx512_lookup_impl(subtable, keys_map, keys, rules,
                               subtable->mf_bits_set_unit0,
-                              subtable->mf_bits_set_unit1);
+                              subtable->mf_bits_set_unit1,
+                              use_vpop);
 }
 
 dpcls_subtable_lookup_func
@@ -349,12 +396,14 @@ dpcls_subtable_avx512_gather_probe(uint32_t u0_bits, uint32_t u1_bits)
         return NULL;
     }
 
-    CHECK_LOOKUP_FUNCTION(9, 4);
-    CHECK_LOOKUP_FUNCTION(9, 1);
-    CHECK_LOOKUP_FUNCTION(5, 3);
-    CHECK_LOOKUP_FUNCTION(5, 1);
-    CHECK_LOOKUP_FUNCTION(4, 1);
-    CHECK_LOOKUP_FUNCTION(4, 0);
+    int use_vpop = dpdk_get_cpu_has_isa("x86_64", "avx512vpopcntdq");
+
+    CHECK_LOOKUP_FUNCTION(9, 4, use_vpop);
+    CHECK_LOOKUP_FUNCTION(9, 1, use_vpop);
+    CHECK_LOOKUP_FUNCTION(5, 3, use_vpop);
+    CHECK_LOOKUP_FUNCTION(5, 1, use_vpop);
+    CHECK_LOOKUP_FUNCTION(4, 1, use_vpop);
+    CHECK_LOOKUP_FUNCTION(4, 0, use_vpop);
 
     /* Check if the _any looping version of the code can perform this miniflow
      * lookup. Performance gain may be less pronounced due to non-specialized
-- 
2.32.0



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