diff mbox series

mbuf: optimize memory loads during mbuf freeing

Message ID 1584383500-27482-1-git-send-email-akozyrev@mellanox.com (mailing list archive)
State Superseded, archived
Delegated to: Thomas Monjalon
Headers
Series mbuf: optimize memory loads during mbuf freeing |

Checks

Context Check Description
ci/checkpatch success coding style OK
ci/iol-mellanox-Performance success Performance Testing PASS
ci/iol-testing success Testing PASS
ci/travis-robot success Travis build: passed
ci/Intel-compilation success Compilation OK

Commit Message

Alexander Kozyrev March 16, 2020, 6:31 p.m. UTC 
  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(-)

Comments

Olivier Matz March 19, 2020, 9:30 a.m. UTC | #1 
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
Alexander Kozyrev March 20, 2020, 3:35 p.m. UTC | #2 
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
diff mbox series

Patch

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;
 		}
 
@@ -1352,10 +1351,9 @@  static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
 	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
 
 		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;
 		}