Re: [PATCH v2 2/7] net/mlx4: inline more Tx functions

[Ophir Munk <ophirmu@mellanox.com>]

Hi,
Please see inside

On Thursday, October 26, 2017 10:49 AM Adrien Mazarguil wrote:
> To: Ophir Munk <ophirmu@mellanox.com>
> Cc: dev@dpdk.org; Thomas Monjalon <thomas@monjalon.net>; Olga Shern
> <olgas@mellanox.com>; Matan Azrad <matan@mellanox.com>
> Subject: Re: [PATCH v2 2/7] net/mlx4: inline more Tx functions
> 
> Hi Ophir,
> 
> Please see below.
> 
> On Wed, Oct 25, 2017 at 09:42:46PM +0000, Ophir Munk wrote:
> > Hi Adrien,
> >
> > On Wednesday, October 25, 2017 7:50 PM, Adrien Mazarguil wrote:
> > >
> > > Hi Ophir,
> > >
> > > On Mon, Oct 23, 2017 at 02:21:55PM +0000, Ophir Munk wrote:
> > > > Change functions to inline on Tx fast path to improve performance
> > > >
> > > > Inside the inline function call other functions to handle "unlikely"
> > > > cases such that the inline function code footprint is small.
> > > >
> > > > Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
> > >
> > > Reading this, it's like adding __rte_always_inline improves
> > > performance at all, which I doubt unless you can show proof through
> performance results.
> > >
> > > When in doubt, leave it to the compiler, the static keyword is
> > > usually enough of a hint. Too much forced inlining may actually be
> harmful.
> > >
> > > What this patch really does is splitting the heavy
> > > lookup/registration function in two halves with one small static
> > > inline function for the lookup part that calls the separate
> > > registration part in the unlikely event MR is not already registered.
> > >
> > > Thankfully the compiler doesn't inline the large registration
> > > function back, which results in the perceived performance
> > > improvement for the time being, however there is no guarantee it
> > > won't happen in the future (you didn't use the noinline keyword on the
> registration function for that).
> > >
> > > Therefore I have a bunch of comments and suggestions, see below.
> > >
> > > > ---
> > > >  drivers/net/mlx4/mlx4_rxtx.c | 43
> > > > ++++++------------------------------
> > > >  drivers/net/mlx4/mlx4_rxtx.h | 52
> > > > +++++++++++++++++++++++++++++++++++++++++++-
> > > >  2 files changed, 58 insertions(+), 37 deletions(-)
> > > >
> > > > diff --git a/drivers/net/mlx4/mlx4_rxtx.c
> > > > b/drivers/net/mlx4/mlx4_rxtx.c index 011ea79..ae37f9b 100644
> > > > --- a/drivers/net/mlx4/mlx4_rxtx.c
> > > > +++ b/drivers/net/mlx4/mlx4_rxtx.c
> > > > @@ -220,54 +220,25 @@ mlx4_txq_complete(struct txq *txq)
> > > >  	return 0;
> > > >  }
> > > >
> > > > -/**
> > > > - * Get memory pool (MP) from mbuf. If mbuf is indirect, the pool
> > > > from which
> > > > - * the cloned mbuf is allocated is returned instead.
> > > > - *
> > > > - * @param buf
> > > > - *   Pointer to mbuf.
> > > > - *
> > > > - * @return
> > > > - *   Memory pool where data is located for given mbuf.
> > > > - */
> > > > -static struct rte_mempool *
> > > > -mlx4_txq_mb2mp(struct rte_mbuf *buf) -{
> > > > -	if (unlikely(RTE_MBUF_INDIRECT(buf)))
> > > > -		return rte_mbuf_from_indirect(buf)->pool;
> > > > -	return buf->pool;
> > > > -}
> > > >
> > > >  /**
> > > > - * Get memory region (MR) <-> memory pool (MP) association from
> > > >txq- mp2mr[].
> > > > - * Add MP to txq->mp2mr[] if it's not registered yet. If mp2mr[]
> > > >is  full,
> > > > - * remove an entry first.
> > > > + * Add memory region (MR) <-> memory pool (MP) association to
> > > > + txq-
> > > >mp2mr[].
> > > > + * If mp2mr[] is full, remove an entry first.
> > > >   *
> > > >   * @param txq
> > > >   *   Pointer to Tx queue structure.
> > > >   * @param[in] mp
> > > > - *   Memory pool for which a memory region lkey must be returned.
> > > > + *   Memory pool for which a memory region lkey must be added
> > > > + * @param[in] i
> > > > + *   Index in memory pool (MP) where to add memory region (MR)
> > > >   *
> > > >   * @return
> > > > - *   mr->lkey on success, (uint32_t)-1 on failure.
> > > > + *   Added mr->lkey on success, (uint32_t)-1 on failure.
> > > >   */
> > > > -uint32_t
> > > > -mlx4_txq_mp2mr(struct txq *txq, struct rte_mempool *mp)
> > > > +uint32_t mlx4_txq_add_mr(struct txq *txq, struct rte_mempool *mp,
> > > > +uint32_t i)
> > > >  {
> > > > -	unsigned int i;
> > > >  	struct ibv_mr *mr;
> > > >
> > > > -	for (i = 0; (i != RTE_DIM(txq->mp2mr)); ++i) {
> > > > -		if (unlikely(txq->mp2mr[i].mp == NULL)) {
> > > > -			/* Unknown MP, add a new MR for it. */
> > > > -			break;
> > > > -		}
> > > > -		if (txq->mp2mr[i].mp == mp) {
> > > > -			assert(txq->mp2mr[i].lkey != (uint32_t)-1);
> > > > -			assert(txq->mp2mr[i].mr->lkey == txq-
> > > >mp2mr[i].lkey);
> > > > -			return txq->mp2mr[i].lkey;
> > > > -		}
> > > > -	}
> > > >  	/* Add a new entry, register MR first. */
> > > >  	DEBUG("%p: discovered new memory pool \"%s\" (%p)",
> > > >  	      (void *)txq, mp->name, (void *)mp); diff --git
> > > >a/drivers/net/mlx4/mlx4_rxtx.h b/drivers/net/mlx4/mlx4_rxtx.h index
> > > > e10bbca..719ef45 100644
> > > > --- a/drivers/net/mlx4/mlx4_rxtx.h
> > > > +++ b/drivers/net/mlx4/mlx4_rxtx.h
> > > > @@ -53,6 +53,7 @@
> > > >
> > > >  #include "mlx4.h"
> > > >  #include "mlx4_prm.h"
> > > > +#include "mlx4_utils.h"
> > >
> > > Why?
> > >
> > > >
> > > >  /** Rx queue counters. */
> > > >  struct mlx4_rxq_stats {
> > > > @@ -160,7 +161,6 @@ void mlx4_rx_queue_release(void *dpdk_rxq);
> > > >
> > > >  /* mlx4_rxtx.c */
> > > >
> > > > -uint32_t mlx4_txq_mp2mr(struct txq *txq, struct rte_mempool *mp);
> > > > uint16_t mlx4_tx_burst(void *dpdk_txq, struct rte_mbuf **pkts,
> > > >  		       uint16_t pkts_n);
> > > >  uint16_t mlx4_rx_burst(void *dpdk_rxq, struct rte_mbuf **pkts, @@
> > > > -169,6 +169,8 @@ uint16_t mlx4_tx_burst_removed(void *dpdk_txq,
> > > struct rte_mbuf **pkts,
> > > >  			       uint16_t pkts_n);
> > > >  uint16_t mlx4_rx_burst_removed(void *dpdk_rxq, struct rte_mbuf
> **pkts,
> > > >  			       uint16_t pkts_n);
> > > > +uint32_t mlx4_txq_add_mr(struct txq *txq, struct rte_mempool *mp,
> > > > +				unsigned int i);
> > > >
> > > >  /* mlx4_txq.c */
> > > >
> > > > @@ -177,4 +179,52 @@ int mlx4_tx_queue_setup(struct rte_eth_dev
> > > *dev, uint16_t idx,
> > > >  			const struct rte_eth_txconf *conf);  void
> > > > mlx4_tx_queue_release(void *dpdk_txq);
> > > >
> > > > +/**
> > > > + * Get memory pool (MP) from mbuf. If mbuf is indirect, the pool
> > > > +from which
> > > > + * the cloned mbuf is allocated is returned instead.
> > > > + *
> > > > + * @param buf
> > > > + *   Pointer to mbuf.
> > > > + *
> > > > + * @return
> > > > + *   Memory pool where data is located for given mbuf.
> > > > + */
> > > > +static __rte_always_inline struct rte_mempool *
> > > > +mlx4_txq_mb2mp(struct rte_mbuf *buf) {
> > > > +	if (unlikely(RTE_MBUF_INDIRECT(buf)))
> > > > +		return rte_mbuf_from_indirect(buf)->pool;
> > > > +	return buf->pool;
> > > > +}
> > > > +
> > > > +/**
> > > > + * Get memory region (MR) <-> memory pool (MP) association from
> > > > +txq-
> > > >mp2mr[].
> > > > + * Call mlx4_txq_add_mr() if MP is not registered yet.
> > > > + *
> > > > + * @param txq
> > > > + *   Pointer to Tx queue structure.
> > > > + * @param[in] mp
> > > > + *   Memory pool for which a memory region lkey must be returned.
> > > > + *
> > > > + * @return
> > > > + *   mr->lkey on success, (uint32_t)-1 on failure.
> > > > + */
> > > > +static __rte_always_inline uint32_t
> > >
> > > Note __rte_always_inline is defined in rte_common.h and should be
> > > explicitly included (however don't do that, see below).
> > >
> > > > +mlx4_txq_mp2mr(struct txq *txq, struct rte_mempool *mp) {
> > > > +	unsigned int i;
> > > > +
> > > > +	for (i = 0; (i != RTE_DIM(txq->mp2mr)); ++i) {
> > > > +		if (unlikely(txq->mp2mr[i].mp == NULL)) {
> > > > +			/* Unknown MP, add a new MR for it. */
> > > > +			break;
> > > > +		}
> > > > +		if (txq->mp2mr[i].mp == mp) {
> > > > +			assert(txq->mp2mr[i].lkey != (uint32_t)-1);
> > > > +			assert(txq->mp2mr[i].mr->lkey == txq-
> > > >mp2mr[i].lkey);
> > >
> > > assert() requires assert.h (but don't include it, see subsequent
> suggestion).
> > >
> > > > +			return txq->mp2mr[i].lkey;
> > > > +		}
> > > > +	}
> > > > +	return mlx4_txq_add_mr(txq, mp, i); }
> > > >  #endif /* MLX4_RXTX_H_ */
> > >
> > > So as described above, these functions do not need the
> > > __rte_always_inline, please remove it. They also do not need to be
> > > located in a header file; the reason it's the case for their mlx5
> > > counterparts is that they have to be shared between
> > > vectorized/non-vectorized code. No such requirement here, you should
> move them back to their original spot.
> > >
> >
> > Static function mlx4_txq_mp2mr() must be in a header file because it is
> shared by 2 files: mlx4_txq.c and mlx4_rxtx.c.
> > It is not related to vectorized/non-vectorized code in mlx5.
> > Having said that -__rte_always_inline is required as well otherwise
> > compilation fails with
> > drivers/net/mlx4/mlx4_rxtx.h:200:1: error: 'mlx4_txq_mp2mr' defined
> > but not used [-Werror=unused-function] for files which include
> > mlx4_rxtx.h
> 
> All right, then what you were looking or was static inline, not *force* inline.
> The former is a hint, the latter doesn't leave much of a choice to the
> compiler, it means you're sure this way brings the most performance,
> however for this patch I really think inlining plays a really minor part (even
> changes anything at all) compared to dividing this function, which is the real
> performance improvement.

Without inline I get ~0.2Mpps degradation on my setup, therefore I suggest keeping inline.
The final call is yours. Please let me know if to leave inline or remove.

> 
> > > My suggestion for this performance improvement is to move
> > > mlx4_txq_add_mr() to a different file, mlx4_mr.c looks like a good
> > > candidate. This fact will ensure it's never inlined and far away
> > > from the data path.
> > >
> >
> > Function mlx4_txq_add_mr() is relatively small.
> > What do you say about preceding it with __attribute((noinline)) instead of
> creating a new file?
> 
> What I mean is you should declare mlx4_txq_add_mr() which does the heavy
> lifting inside mlx4_mr.c and provide its definition in mlx4.h instead of
> mlx4_rxtx.h.
> 
> Then, mlx4_txq_mp2mr() can remain defined in mlx4_rxtx.c in its original
> spot as a non-static function with its public declaration remaining in
> mlx4_rxtx.h for users outside of this file.
> 

That's done before. mlx4_txq_mp2mr() is defined in mlx4_rxtx.c as described.

> The fact mlx4_txq_mp2mr() remains defined in that file *before*
> mlx4_post_send()'s definition where it's needed allows the compiler to
> optimize it away as if it was static inline thanks to -O3, that is, unless it thinks
> doing so would hurt performance, but as a (now) small function this
> shouldn't be an issue.
> 
> Other reasons includes that doing so would make a smaller diff that focuses
> on the performance improvement itself. The extra performance brought by a
> statically inlined version of mlx4_txq_mp2mr() is not needed in mlx4_txq.c,
> whose only purpose is to set up queues.

I have moved mlx4_txq_add_mr() to mlx_mr.c. One of the disadvantages of moving it is that now 
it requires adding #include "mlx4_rxtx.h" in the C file in order to know the details of txq struct.
I liked mlx4_mr.c for being encapsulated from any Tx/Rx specific structures and just handling MR issues.

> 
> --
> Adrien Mazarguil
> 6WIND