Re: [dpdk-dev] [PATCH v2 2/2] qos: rearrange enqueue procedure

[Thomas Monjalon <thomas@monjalon.net>]

Hi all,
What is the conclusion for this patch and the number 1/2?

04/04/2021 01:53, Ananyev, Konstantin:
> 
> Hi guys,
> 
> > > > In many usage scenarios input mbufs for rte_sched_port_enqueue() are
> > > not
> > > > yet in the CPU cache(s). That causes quite significant stalls due to memory
> > > > latency. Current implementation tries to migitate it using SW pipeline and
> > > SW
> > > > prefetch techniques, but stalls are still present.
> > > > Rework rte_sched_port_enqueue() to do actual fetch of all mbufs
> > > metadata
> > > > as a first stage of that function.
> > > > That helps to minimise load stalls at further stages of enqueue() and
> > > > improves overall enqueue performance.
> > > > With examples/qos_sched I observed:
> > > > on ICX box: up to 30% cycles reduction
> > > > on CSX AND BDX: 20-15% cycles reduction
> > > > I also run tests with mbufs already in the cache (one core doing RX, QOS
> > > and
> > > > TX).
> > > > With such scenario, on all mentioned above IA boxes no performance drop
> > > > was observed.
> > > >
> > > > Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> > > > ---
> > > > v2: fix clang and checkpatch complains
> > > > ---
> > > >  lib/librte_sched/rte_sched.c | 219 +++++------------------------------
> > > >  1 file changed, 31 insertions(+), 188 deletions(-)
> > > >
> > > > diff --git a/lib/librte_sched/rte_sched.c b/lib/librte_sched/rte_sched.c
> > > index
> > > > 7c5688068..41ef147e0 100644
> > > > --- a/lib/librte_sched/rte_sched.c
> > > > +++ b/lib/librte_sched/rte_sched.c
> > > > @@ -1861,24 +1861,23 @@ debug_check_queue_slab(struct
> > > > rte_sched_subport *subport, uint32_t bmp_pos,  #endif /*
> > > > RTE_SCHED_DEBUG */
> > > >
> > > >  static inline struct rte_sched_subport * -rte_sched_port_subport(struct
> > > > rte_sched_port *port,
> > > > -	struct rte_mbuf *pkt)
> > > > +sched_port_subport(const struct rte_sched_port *port, struct
> > > > +rte_mbuf_sched sch)
> > > >  {
> > > > -	uint32_t queue_id = rte_mbuf_sched_queue_get(pkt);
> > > > +	uint32_t queue_id = sch.queue_id;
> > > >  	uint32_t subport_id = queue_id >> (port-
> > > > >n_pipes_per_subport_log2 + 4);
> > > >
> > > >  	return port->subports[subport_id];
> > > >  }
> > > >
> > > >  static inline uint32_t
> > > > -rte_sched_port_enqueue_qptrs_prefetch0(struct rte_sched_subport
> > > > *subport,
> > > > -	struct rte_mbuf *pkt, uint32_t subport_qmask)
> > > > +sched_port_enqueue_qptrs_prefetch0(const struct rte_sched_subport
> > > > *subport,
> > > > +	struct rte_mbuf_sched sch, uint32_t subport_qmask)
> > > >  {
> > > >  	struct rte_sched_queue *q;
> > > >  #ifdef RTE_SCHED_COLLECT_STATS
> > > >  	struct rte_sched_queue_extra *qe;
> > > >  #endif
> > > > -	uint32_t qindex = rte_mbuf_sched_queue_get(pkt);
> > > > +	uint32_t qindex = sch.queue_id;
> > > >  	uint32_t subport_queue_id = subport_qmask & qindex;
> > > >
> > > >  	q = subport->queue + subport_queue_id; @@ -1971,197 +1970,41
> > > > @@ int  rte_sched_port_enqueue(struct rte_sched_port *port, struct
> > > > rte_mbuf **pkts,
> > > >  		       uint32_t n_pkts)
> > > >  {
> > > > -	struct rte_mbuf *pkt00, *pkt01, *pkt10, *pkt11, *pkt20, *pkt21,
> > > > -		*pkt30, *pkt31, *pkt_last;
> > > > -	struct rte_mbuf **q00_base, **q01_base, **q10_base,
> > > > **q11_base,
> > > > -		**q20_base, **q21_base, **q30_base, **q31_base,
> > > > **q_last_base;
> > > > -	struct rte_sched_subport *subport00, *subport01, *subport10,
> > > > *subport11,
> > > > -		*subport20, *subport21, *subport30, *subport31,
> > > > *subport_last;
> > > > -	uint32_t q00, q01, q10, q11, q20, q21, q30, q31, q_last;
> > > > -	uint32_t r00, r01, r10, r11, r20, r21, r30, r31, r_last;
> > > > -	uint32_t subport_qmask;
> > > >  	uint32_t result, i;
> > > > +	struct rte_mbuf_sched sch[n_pkts];
> > > > +	struct rte_sched_subport *subports[n_pkts];
> > > > +	struct rte_mbuf **q_base[n_pkts];
> > > > +	uint32_t q[n_pkts];
> > > > +
> > > > +	const uint32_t subport_qmask =
> > > > +		(1 << (port->n_pipes_per_subport_log2 + 4)) - 1;
> > > >
> > > >  	result = 0;
> > > > -	subport_qmask = (1 << (port->n_pipes_per_subport_log2 + 4)) - 1;
> > > >
> > > > -	/*
> > > > -	 * Less then 6 input packets available, which is not enough to
> > > > -	 * feed the pipeline
> > > > -	 */
> > > > -	if (unlikely(n_pkts < 6)) {
> > > > -		struct rte_sched_subport *subports[5];
> > > > -		struct rte_mbuf **q_base[5];
> > > > -		uint32_t q[5];
> > > > -
> > > > -		/* Prefetch the mbuf structure of each packet */
> > > > -		for (i = 0; i < n_pkts; i++)
> > > > -			rte_prefetch0(pkts[i]);
> > > > -
> > > > -		/* Prefetch the subport structure for each packet */
> > > > -		for (i = 0; i < n_pkts; i++)
> > > > -			subports[i] = rte_sched_port_subport(port, pkts[i]);
> > > > -
> > > > -		/* Prefetch the queue structure for each queue */
> > > > -		for (i = 0; i < n_pkts; i++)
> > > > -			q[i] =
> > > > rte_sched_port_enqueue_qptrs_prefetch0(subports[i],
> > > > -					pkts[i], subport_qmask);
> > > > -
> > > > -		/* Prefetch the write pointer location of each queue */
> > > > -		for (i = 0; i < n_pkts; i++) {
> > > > -			q_base[i] =
> > > > rte_sched_subport_pipe_qbase(subports[i], q[i]);
> > > > -			rte_sched_port_enqueue_qwa_prefetch0(port,
> > > > subports[i],
> > > > -				q[i], q_base[i]);
> > > > -		}
> > > > +	/* Prefetch the mbuf structure of each packet */
> > > > +	for (i = 0; i < n_pkts; i++)
> > > > +		sch[i] = pkts[i]->hash.sched;
> > > >
> > >
> > > Hi Konstantin,  thanks for the patch. In above case, all packets are touched
> > > straight with any prefetch. If we consider the input burst size of 64 pkts, it
> > > means 512 bytes of packet addresses  (8 cache-lines) which is likely to be
> > > available in cache. For larger size burst, e.g. 128 or 256, there might be
> > > instances when some addresses are not available the cache, may stall core.
> > > How about adding explicit prefetch before starting to iterate through the
> > > packets if that helps?
> 
> I don't think we need any prefetch here.
> pkts[] is a sequential array, HW prefetcher should be able to do good job here.
> Again in majority of use-cases pkts[] contents will already present in the cache.
> Though there is a valid concern here: n_pkts can be big, in that case we probably
> don't want to store too much on the stack and read too much from pkts[].
> It is better to work in some fixed chunks (64 or so).
> I can prepare v2 with these changes, if there still is an interest in this patch.   
> 
> > Exactly. Konstantin, you might not be a fan of prefetches, but the current enqueue implementation (as well as the dequeue) uses a prefetch
> > state machine. Please keep the prefetch state machine in the scalar code.
> 
> It is not about our own preferences.
> From my measurements new version is faster and it is definitely simpler.
> 
> > Even if the examples/qos_sched might not show an advantage,
> > this is just a sample app and there are some more relevant use-cases as well.
> 
> Well, I hope that examples/qos_sched reflects at least some real-world use-cases for QOS library.
> Otherwise why do we have it inside DPDK codebase? 
> About 'more relevant use-cases': if you do know such, can you try them with the patch?
> I would really appreciate that.
> In fact, it is an ask not only to Cristian, but to all other interested parties:
> if your app does use librte_sched - please try this patch and provide the feedback.
> If some tests would flag a regression - I am absolutely ok to drop the patch.
> Konstantin