Re: [PATCH v2 2/3] mempool: include non-DPDK threads in statistics

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

On 2022-11-02 10:09, Morten Brørup wrote:
>> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
>> Sent: Wednesday, 2 November 2022 08.53
>>
>> On 2022-10-31 12:26, Morten Brørup wrote:
>>> Offset the stats array index by one, and count non-DPDK threads at
>> index
>>> zero.
>>>
>>> This patch provides two benefits:
>>> * Non-DPDK threads are also included in the statistics.
>>> * A conditional in the fast path is removed. Static branch prediction
>> was
>>>     correct, so the performance improvement is negligible.
>>>
>>> v2:
>>> * New. No v1 of this patch in the series.
>>>
>>> Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
>>> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
>>> ---
>>>    lib/mempool/rte_mempool.c |  2 +-
>>>    lib/mempool/rte_mempool.h | 12 ++++++------
>>>    2 files changed, 7 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/lib/mempool/rte_mempool.c b/lib/mempool/rte_mempool.c
>>> index 62d1ce764e..e6208125e0 100644
>>> --- a/lib/mempool/rte_mempool.c
>>> +++ b/lib/mempool/rte_mempool.c
>>> @@ -1272,7 +1272,7 @@ rte_mempool_dump(FILE *f, struct rte_mempool
>> *mp)
>>>    #ifdef RTE_LIBRTE_MEMPOOL_STATS
>>>    	rte_mempool_ops_get_info(mp, &info);
>>>    	memset(&sum, 0, sizeof(sum));
>>> -	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++) {
>>> +	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE + 1; lcore_id++) {
>>>    		sum.put_bulk += mp->stats[lcore_id].put_bulk;
>>>    		sum.put_objs += mp->stats[lcore_id].put_objs;
>>>    		sum.put_common_pool_bulk += mp-
>>> stats[lcore_id].put_common_pool_bulk;
>>> diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
>>> index 9c4bf5549f..16e7e62e3c 100644
>>> --- a/lib/mempool/rte_mempool.h
>>> +++ b/lib/mempool/rte_mempool.h
>>> @@ -238,8 +238,11 @@ struct rte_mempool {
>>>    	struct rte_mempool_memhdr_list mem_list; /**< List of memory
>> chunks */
>>>
>>>    #ifdef RTE_LIBRTE_MEMPOOL_STATS
>>> -	/** Per-lcore statistics. */
>>> -	struct rte_mempool_debug_stats stats[RTE_MAX_LCORE];
>>> +	/** Per-lcore statistics.
>>> +	 *
>>> +	 * Offset by one, to include non-DPDK threads.
>>> +	 */
>>> +	struct rte_mempool_debug_stats stats[RTE_MAX_LCORE + 1];
>>>    #endif
>>>    }  __rte_cache_aligned;
>>>
>>> @@ -304,10 +307,7 @@ struct rte_mempool {
>>>     */
>>>    #ifdef RTE_LIBRTE_MEMPOOL_STATS
>>>    #define RTE_MEMPOOL_STAT_ADD(mp, name, n) do {                  \
>>> -		unsigned __lcore_id = rte_lcore_id();           \
>>> -		if (__lcore_id < RTE_MAX_LCORE) {               \
>>> -			mp->stats[__lcore_id].name += n;        \
>>> -		}                                               \
>>> +		(mp)->stats[rte_lcore_id() + 1].name += n;      \
>>
>> This relies on LCORE_ID_ANY being UINT32_MAX, and a wrap to 0, for an
>> unregistered non-EAL thread? Might be worth a comment, or better a
>> rewrite with an explicit LCORE_ID_ANY comparison.
> 
> The purpose of this patch is to avoid the comparison here.
> 
> Yes, it relies on the wrap to zero, and these conditions:
> 1. LCORE_ID_ANY being UINT32_MAX, and
> 2. the return type of rte_lcore_id() being unsigned int, and
> 3. unsigned int being uint32_t.
> 
> When I wrote this, I considered it safe to assume that LCORE_ID_ANY will remain the unsigned equivalent of -1 using the return type of rte_lcore_id(). In other words: If the return type of rte_lcore_id() should change from 32 to 64 bit, LCORE_ID_ANY would be updated accordingly to UINT64_MAX.
> 
> Because of this assumption, I didn't use [(rte_lcore_id() + 1) & UINT32_MAX], but just [rte_lcore_id() + 1].
> 
> I struggled writing an appropriate comment without making it unacceptably long, but eventually gave up, and settled for the one-line comment in the structure only.
> 
>>
>> You anyways need a conditional. An atomic add must be used in the
>> unregistered EAL thread case.
> 
> Good point: The "+= n" must be atomic for non-isolated threads.
> 

If the various unregistered non-EAL threads are run on isolated cores or 
not does not matter.

> I just took a look at how software maintained stats are handled elsewhere, and the first thing I found, is the IOAT DMA driver, which also seems to be using non-atomic increment [1] regardless if used by a DPDK thread or not.
> 
> [1]: https://elixir.bootlin.com/dpdk/v22.11-rc2/source/drivers/dma/ioat/ioat_dmadev.c#L228
> 
> However, doing it wrong elsewhere doesn't make it correct doing it wrong here too. :-)
> 
> Atomic increments are costly, so I would rather follow your suggestion and reintroduce the comparison. How about:
> 
> #define RTE_MEMPOOL_STAT_ADD(mp, name, n) do { \
>      unsigned int __lcore_id = rte_lcore_id(); \
>      if (likely(__lcore_id < RTE_MAX_LCORE)) { \
>          (mp)->stats[__lcore_id].name += n; \
>      } else {
>          rte_atomic64_add( \
>                  (rte_atomic64_t*)&((mp)->stats[RTE_MAX_LCORE].name), n);\
>      } \
> }
You are supposed to use GCC C11 intrinsics (e.g., __atomic_fetch_add()).

In addition: technically, you must use an atomic store for the EAL 
thread case (and an atomic load on the reader side), although there are 
tons of examples in DPDK where tearing is ignored. (The generated code 
will likely look the same.)

DPDK coding conventions require there be no braces for a single statement.

Other than that, it looks good.

> 
> And the structure comment could be:
>   * Plus one, for non-DPDK threads.
> 

"Unregistered non-EAL threads". This is the term the EAL documentation uses.