Re: [dpdk-dev] [PATCH 2/4] hash: add memory ordering to avoid race conditions

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> 
> Some general comments for  the various __atomic_store/load added,
> 
> 1. Although it passes the compiler check, but I just want to confirm that if we
> should use GCC/clang builtins, or if There are higher level APIs in DPDK to do
> atomic operations?
> 
I have used gcc builtins (just like rte_ring does)

> 2. We believe compiler will translate the atomic_store/load to regular MOV
> instruction on Total Store Order architecture (e.g. X86_64). But we run the
> perf test on x86 and here is the relative slowdown on lookup comparing to
> master head. I am not sure if the performance drop comes from the atomic
> buitins.
> 
C11 atomics also block compiler reordering. Other than this, the retry loop is an addition to lookup.
The patch also has the alignment corrected. I am not sure how is that affecting the perf numbers.

> Keysize | single lookup | bulk lookup
> 4 | 0.93 | 0.95
> 8 | 0.95 | 0.96
> 16 | 0.97 | 0.96
> 32 | 0.97 | 1.00
> 48 | 1.03 | 0.99
> 64 | 1.04 | 0.98
> 9 | 0.91 | 0.96
> 13 | 0.97 | 0.98
> 37 | 1.04 | 1.03
> 40 | 1.02 | 0.98
> 
I assume this is the data from the test cases in test_hash_perf.c file. I tried to reproduce this data, but my data is worse. Can you specify the actual test from test_hash_perf.c you are using (With locks/Pre-computed hash/With data/Elements in primary)?
IMO, the differences you have provided are not high.

> Other comments inlined.
> 
> >-----Original Message-----
> >Only race condition that can occur is -  using the key store element
> >before the key write is completed. Hence, while inserting the element
> >the release memory order is used. Any other race condition is caught by
> >the key comparison. Memory orderings are added only where needed.
> >For ex: reads in the writer's context do not need memory ordering as
> >there is a single writer.
> >
> [Wang, Yipeng] I assume this commit works together with the next one to
> enable read-write concurrency on relaxed memory ordering machine. This
> commit by itself does not do that, right?
> If this is the case, should we merge the two commits, or maybe just explain it
> a little bit more in the commit message?
This commit works only with the next commit. I kept it separated to make it easy for review.
I will merge it in the final patch.

> 
> >key_idx in the bucket entry and pdata in the key store element are used
> >for synchronisation. key_idx is used to release an inserted entry in
> >the bucket to the reader. Use of pdata for synchronisation is required
> >due to updation of an existing entry where-in only the pdata is updated
> >without updating key_idx.
> >
> >
> >-/* Search a key from bucket and update its data */
> >+/* Search a key from bucket and update its data.
> >+ * Writer holds the lock before calling this.
> >+ */
> > static inline int32_t
> > search_and_update(const struct rte_hash *h, void *data, const void *key,
> > 	struct rte_hash_bucket *bkt, hash_sig_t sig, hash_sig_t alt_hash) @@
> >-499,8 +501,13 @@ search_and_update(const struct rte_hash *h, void *data,
> const void *key,
> > 			k = (struct rte_hash_key *) ((char *)keys +
> > 					bkt->key_idx[i] * h->key_entry_size);
> > 			if (rte_hash_cmp_eq(key, k->key, h) == 0) {
> >-				/* Update data */
> >-				k->pdata = data;
> >+				/* 'pdata' acts as the synchronization point
> >+				 * when an existing hash entry is updated.
> >+				 * Key is not updated in this case.
> >+				 */
> [Wang, Yipeng] I did not quite understand why do we need synchronization
> for hash data update.
> Since pdata write is already atomic, the lookup will either read out the stale
> data or the new data, which should be fine without synchronization.
> Is it to ensure the order of multiple reads in lookup threads?
Yes, it is to ensure the order of reads in the lookup threads. There might be reads in the application and we want to make sure they do not get reordered with this.

> 
> >@@ -1243,11 +1289,19 @@ __rte_hash_lookup_bulk(const struct rte_hash
> *h, const void **keys,
> > 		while (sec_hitmask[i]) {
> > 			uint32_t hit_index = __builtin_ctzl(sec_hitmask[i]);
> >
> >-			uint32_t key_idx = secondary_bkt[i]-
> >key_idx[hit_index];
> >+			uint32_t key_idx =
> >+			__atomic_load_n(
> >+				&secondary_bkt[i]->key_idx[hit_index],
> >+				__ATOMIC_ACQUIRE);
> > 			const struct rte_hash_key *key_slot =
> > 				(const struct rte_hash_key *)(
> > 				(const char *)h->key_store +
> > 				key_idx * h->key_entry_size);
> >+
> >+			if (key_idx != EMPTY_SLOT)
> [Wang, Yipeng] I think even for current code, we need to check empty_slot.
> Could you export this as a bug fix commit?
> 
In the existing code, there is check 'if (!!key_idx & !rte_hash....)'. Are you referring to '!!key_idx'? I think this should be changed to '(key_idx != EMPTY_SLOT)'.