Re: [dpdk-dev] [PATCH] mbuf: optimize memory loads during mbuf freeing

[Alexander Kozyrev <akozyrev@mellanox.com>]

You are right, Olivier, thanks for your suggestion - it looks even better.
I've tested this version and the performance is great - will send a v2 shortly.

Regards,
Alex

> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: Thursday, March 19, 2020 5:30
> To: Alexander Kozyrev <akozyrev@mellanox.com>
> Cc: dev@dpdk.org; Slava Ovsiienko <viacheslavo@mellanox.com>; Matan
> Azrad <matan@mellanox.com>; Thomas Monjalon
> <thomas@monjalon.net>; stable@dpdk.org
> Subject: Re: [PATCH] mbuf: optimize memory loads during mbuf freeing
> 
> Hi,
> 
> On Mon, Mar 16, 2020 at 06:31:40PM +0000, Alexander Kozyrev wrote:
> > Introduction of pinned external buffers doubled memory loads in the
> > rte_pktmbuf_prefree_seg() function. Analysis of the generated assembly
> > code shows unnecessary load of the pool field of the rte_mbuf structure.
> > Here is the snippet of the assembly for "if (!RTE_MBUF_DIRECT(m))":
> > Before the change the code was:
> > 	movq  0x18(%rbx), %rax // load the ol_flags field
> > 	test %r13, %rax	       // check if ol_flags equals to 0x60...0
> > 	jz 0x9a8718 <Block 2>  // jump out to "if (m->next != NULL)"
> > After the change the code becomed:
> > 	movq  0x18(%rbx), %rax // load ol_flags
> > 	test %r14, %rax	       // check if ol_flags equals to 0x60...0
> > 	jnz 0x9bea38 <Block 2> // jump in to "if
> (!RTE_MBUF_HAS_EXTBUF(m)"
> > 	movq  0x48(%rbx), %rax // load the pool field
> > 	jmp 0x9bea78 <Block 7> // jump out to "if (m->next != NULL)"
> > Look like this absolutely unneeded memory load of the pool field is an
> > optimization for the external buffer case in GCC (4.8.5), since Clang
> > generates the same assembly for both before and after the chenge
> versions.
> > Plus, GCC favors the extrnal buffer case over the simple case.
> > This assembly code layout causes the performance degradation because
> > the
> > rte_pktmbuf_prefree_seg() function is a part of a very hot path.
> > Workaround this compilation issue by moving the check for pinned
> > buffer apart from the check for external buffer and restore the
> > initial code flow that favors the direct mbuf case over the external one.
> >
> > Fixes: 6ef1107ad4c6 ("mbuf: detach mbuf with pinned external buffer")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Alexander Kozyrev <akozyrev@mellanox.com>
> > Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
> > ---
> >  lib/librte_mbuf/rte_mbuf.h | 14 ++++++--------
> >  1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > index 34679e0..ab9d3f5 100644
> > --- a/lib/librte_mbuf/rte_mbuf.h
> > +++ b/lib/librte_mbuf/rte_mbuf.h
> > @@ -1335,10 +1335,9 @@ static inline int
> __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
> >  	if (likely(rte_mbuf_refcnt_read(m) == 1)) {
> >
> >  		if (!RTE_MBUF_DIRECT(m)) {
> > -			if (!RTE_MBUF_HAS_EXTBUF(m) ||
> > -			    !RTE_MBUF_HAS_PINNED_EXTBUF(m))
> > -				rte_pktmbuf_detach(m);
> > -			else if (__rte_pktmbuf_pinned_extbuf_decref(m))
> > +			rte_pktmbuf_detach(m);
> > +			if (RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> > +			    __rte_pktmbuf_pinned_extbuf_decref(m))
> >  				return NULL;
> >  		}
> >
> [...]
> 
> Reading the previous code again, it was correct but not easy to understand,
> especially the:
> 
>   if (!RTE_MBUF_HAS_EXTBUF(m) || !RTE_MBUF_HAS_PINNED_EXTBUF(m))
> 
> Knowing we already checked it is not a direct mbuf, it is equivalent to:
> 
>   if (!RTE_MBUF_HAS_PINNED_EXTBUF(m))
> 
> I think the objective was to avoid an access to the pool flags if not necessary.
> 
> Completely removing the test as you did is also functionally OK, because
> rte_pktmbuf_detach() also does the check, and the code is even clearer.
> 
> I wonder however if doing this wouldn't avoid an access to the pool flags for
> mbufs which have the IND_ATTACHED flags:
> 
> 		if (!RTE_MBUF_DIRECT(m)) {
> 			rte_pktmbuf_detach(m);
> 			if (RTE_MBUF_HAS_EXTBUF(m) &&
> 			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> 			    __rte_pktmbuf_pinned_extbuf_decref(m))
> 				return NULL;
> 		}
> 
> What do you think?
> 
> Nit: if you wish to send a v2, there are few english fixes that could be done
> (becomed, chenge, extrnal)
> 
> Thanks