Re: [PATCH v5] eal: add seqlock

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2022-05-06 03:26, fengchengwen wrote:
> On 2022/5/1 22:03, Mattias Rönnblom wrote:
>> A sequence lock (seqlock) is synchronization primitive which allows
>> for data-race free, low-overhead, high-frequency reads, especially for
>> data structures shared across many cores and which are updated
>> relatively infrequently.
>>
> 
> ...
> 
>> +}
>> +
>> +static void
>> +reader_stop(struct reader *reader)
>> +{
>> +	__atomic_store_n(&reader->stop, 1, __ATOMIC_RELAXED);
>> +}
>> +
>> +#define NUM_WRITERS (2) /* main lcore + one worker */
>> +#define MIN_NUM_READERS (2)
>> +#define MAX_READERS (RTE_MAX_LCORE - NUM_WRITERS - 1)
> 
> Why minus 1 ?
> Suggest define MAX_READERS RTE_MAX_LCORE to avoid underflow with small size VM.
> 

OK.

>> +#define MIN_LCORE_COUNT (NUM_WRITERS + MIN_NUM_READERS)
>> +
>> +/* Only a compile-time test */
>> +static rte_seqlock_t __rte_unused static_init_lock = RTE_SEQLOCK_INITIALIZER;
>> +
>> +static int
>> +test_seqlock(void)
>> +{
>> +	struct reader readers[MAX_READERS];
>> +	unsigned int num_readers;
>> +	unsigned int num_lcores;
>> +	unsigned int i;
>> +	unsigned int lcore_id;
>> +	unsigned int reader_lcore_ids[MAX_READERS];
>> +	unsigned int worker_writer_lcore_id = 0;
>> +	int rc = TEST_SUCCESS;
>> +
>> +	num_lcores = rte_lcore_count();
>> +
>> +	if (num_lcores < MIN_LCORE_COUNT) {
>> +		printf("Too few cores to run test. Skipping.\n");
>> +		return TEST_SKIPPED;
>> +	}
>> +
>> +	num_readers = num_lcores - NUM_WRITERS;
>> +
>> +	struct data *data = rte_zmalloc(NULL, sizeof(struct data), 0);
> 
> Please check whether the value of data is NULL.
> 

OK.

>> +
>> +	i = 0;
>> +	RTE_LCORE_FOREACH_WORKER(lcore_id) {
>> +		if (i == 0) {
>> +			rte_eal_remote_launch(writer_run, data, lcore_id);
>> +			worker_writer_lcore_id = lcore_id;
>> +		} else {
>> +			unsigned int reader_idx = i - 1;
>> +			struct reader *reader = &readers[reader_idx];
>> +
>> +			reader->data = data;
>> +			reader->stop = 0;
>> +
>> +			rte_eal_remote_launch(reader_run, reader, lcore_id);
>> +			reader_lcore_ids[reader_idx] = lcore_id;
>> +		}
>> +		i++;
>> +	}
>> +
>> +	if (writer_run(data) != 0 ||
>> +	    rte_eal_wait_lcore(worker_writer_lcore_id) != 0)
>> +		rc = TEST_FAILED;
>> +
>> +	for (i = 0; i < num_readers; i++) {
>> +		reader_stop(&readers[i]);
>> +		if (rte_eal_wait_lcore(reader_lcore_ids[i]) != 0)
>> +			rc = TEST_FAILED;
>> +	}
>> +
> 
> Please free data memory.
> 

OK.

>> +	return rc;
>> +}
>> +
>> +REGISTER_TEST_COMMAND(seqlock_autotest, test_seqlock);
>> diff --git a/doc/api/doxy-api-index.md b/doc/api/doxy-api-index.md
>> index 4245b9635c..f23e33ae30 100644
>> --- a/doc/api/doxy-api-index.md
>> +++ b/doc/api/doxy-api-index.md
>> @@ -77,6 +77,7 @@ The public API headers are grouped by topics:
>>     [rwlock]             (@ref rte_rwlock.h),
>>     [spinlock]           (@ref rte_spinlock.h),
>>     [ticketlock]         (@ref rte_ticketlock.h),
>> +  [seqlock]            (@ref rte_seqlock.h),
>>     [RCU]                (@ref rte_rcu_qsbr.h)
>>   
> 
> ...
> 
>> + */
>> +__rte_experimental
>> +static inline bool
>> +rte_seqlock_read_retry(const rte_seqlock_t *seqlock, uint32_t begin_sn)
>> +{
>> +	uint32_t end_sn;
>> +
>> +	/* An odd sequence number means the protected data was being
>> +	 * modified already at the point of the rte_seqlock_read_begin()
>> +	 * call.
>> +	 */
>> +	if (unlikely(begin_sn & 1))
>> +		return true;
>> +
>> +	/* make sure the data loads happens before the sn load */
>> +	rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
> 
> In ARMv8, the rte_atomic_thread_fence(__ATOMIC_ACQUIRE) and rte_smp_rmb() both output 'dma ishld'
> Suggest use rte_smp_rmb(), please see below comment.
> 
>> +
>> +	end_sn = __atomic_load_n(&seqlock->sn, __ATOMIC_RELAXED);
>> +
>> +	/* A writer incremented the sequence number during this read
>> +	 * critical section.
>> +	 */
>> +	if (unlikely(begin_sn != end_sn))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>> +/**
>> + * @warning
>> + * @b EXPERIMENTAL: this API may change without prior notice.
>> + *
>> + * Begin a write-side critical section.
>> + *
>> + * A call to this function acquires the write lock associated @p
>> + * seqlock, and marks the beginning of a write-side critical section.
>> + *
>> + * After having called this function, the caller may go on to modify
>> + * (both read and write) the protected data, in an atomic or
>> + * non-atomic manner.
>> + *
>> + * After the necessary updates have been performed, the application
>> + * calls rte_seqlock_write_unlock().
>> + *
>> + * This function is not preemption-safe in the sense that preemption
>> + * of the calling thread may block reader progress until the writer
>> + * thread is rescheduled.
>> + *
>> + * Unlike rte_seqlock_read_begin(), each call made to
>> + * rte_seqlock_write_lock() must be matched with an unlock call.
>> + *
>> + * @param seqlock
>> + *   A pointer to the seqlock.
>> + *
>> + * @see rte_seqlock_write_unlock()
>> + */
>> +__rte_experimental
>> +static inline void
>> +rte_seqlock_write_lock(rte_seqlock_t *seqlock)
>> +{
>> +	uint32_t sn;
>> +
>> +	/* to synchronize with other writers */
>> +	rte_spinlock_lock(&seqlock->lock);
>> +
>> +	sn = seqlock->sn + 1;
>> +
>> +	__atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELAXED);
>> +
>> +	/* __ATOMIC_RELEASE to prevent stores after (in program order)
>> +	 * from happening before the sn store.
>> +	 */
>> +	rte_atomic_thread_fence(__ATOMIC_RELEASE);
> 
> In ARMv8, rte_atomic_thread_fence(__ATOMIC_RELEASE) will output 'dmb ish', and
> rte_smp_wmb() will output 'dma ishst'.
> Suggest use rte_smp_wmb(). I think here only need to use store mb here.
> 

(This has already been discussed further down in the mail thread, and I 
have nothing to add.)

>> +}
>> +
>> +/**
>> + * @warning
>> + * @b EXPERIMENTAL: this API may change without prior notice.
>> + *
>> + * End a write-side critical section.
>> + *
>> + * A call to this function marks the end of the write-side critical
>> + * section, for @p seqlock. After this call has been made, the protected
>> + * data may no longer be modified.
>> + *
>> + * @param seqlock
>> + *   A pointer to the seqlock.
>> + *
>> + * @see rte_seqlock_write_lock()
>> + */
>> +__rte_experimental
>> +static inline void
>> +rte_seqlock_write_unlock(rte_seqlock_t *seqlock)
>> +{
>> +	uint32_t sn;
>> +
>> +	sn = seqlock->sn + 1;
>> +
>> +	/* synchronizes-with the load acquire in rte_seqlock_read_begin() */
>> +	__atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELEASE);
>> +
>> +	rte_spinlock_unlock(&seqlock->lock);
>> +}
>> +
>> +#ifdef __cplusplus
>> +}
>> +#endif
>> +
>> +#endif  /* _RTE_SEQLOCK_H_ */
>> diff --git a/lib/eal/version.map b/lib/eal/version.map
>> index b53eeb30d7..4a9d0ed899 100644
>> --- a/lib/eal/version.map
>> +++ b/lib/eal/version.map
>> @@ -420,6 +420,9 @@ EXPERIMENTAL {
>>   	rte_intr_instance_free;
>>   	rte_intr_type_get;
>>   	rte_intr_type_set;
>> +
>> +	# added in 22.07
>> +	rte_seqlock_init;
>>   };
>>   
>>   INTERNAL {
>>
> 
> Reviewed-by: Chengwen Feng <fengchengwen@huawei.com>
> 
>
Thanks a lot for the review!