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

["Morten Brørup" <mb@smartsharesystems.com>]

> 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.

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);\
    } \
}

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