Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone allocation

[Olivier Matz <olivier.matz@6wind.com>]

On Fri, 9 Jun 2017 10:02:55 +0100, Bruce Richardson <bruce.richardson@intel.com> wrote:
> On Thu, Jun 08, 2017 at 05:42:00PM +0100, Ananyev, Konstantin wrote:
> > 
> >   
> > > -----Original Message-----
> > > From: Richardson, Bruce
> > > Sent: Thursday, June 8, 2017 5:21 PM
> > > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > > Cc: Olivier Matz <olivier.matz@6wind.com>; Verkamp, Daniel <daniel.verkamp@intel.com>; dev@dpdk.org
> > > Subject: RE: [dpdk-dev] [PATCH v2] ring: use aligned memzone allocation
> > > 
> > > 
> > >   
> > > > -----Original Message-----
> > > > From: Ananyev, Konstantin
> > > > Sent: Thursday, June 8, 2017 5:13 PM
> > > > To: Richardson, Bruce <bruce.richardson@intel.com>
> > > > Cc: Olivier Matz <olivier.matz@6wind.com>; Verkamp, Daniel
> > > > <daniel.verkamp@intel.com>; dev@dpdk.org
> > > > Subject: RE: [dpdk-dev] [PATCH v2] ring: use aligned memzone allocation
> > > >
> > > >
> > > >  
> > > > > -----Original Message-----
> > > > > From: Richardson, Bruce
> > > > > Sent: Thursday, June 8, 2017 5:04 PM
> > > > > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > > > > Cc: Olivier Matz <olivier.matz@6wind.com>; Verkamp, Daniel
> > > > > <daniel.verkamp@intel.com>; dev@dpdk.org
> > > > > Subject: Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone
> > > > > allocation
> > > > >
> > > > > On Thu, Jun 08, 2017 at 04:35:20PM +0100, Ananyev, Konstantin wrote:  
> > > > > >
> > > > > >  
> > > > > > > -----Original Message-----
> > > > > > > From: Richardson, Bruce
> > > > > > > Sent: Thursday, June 8, 2017 4:25 PM
> > > > > > > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > > > > > > Cc: Olivier Matz <olivier.matz@6wind.com>; Verkamp, Daniel
> > > > > > > <daniel.verkamp@intel.com>; dev@dpdk.org
> > > > > > > Subject: Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone
> > > > > > > allocation
> > > > > > >
> > > > > > > On Thu, Jun 08, 2017 at 03:50:34PM +0100, Ananyev, Konstantin wrote:  
> > > > > > > >
> > > > > > > >  
> > > > > > > > > -----Original Message-----
> > > > > > > > > From: Richardson, Bruce
> > > > > > > > > Sent: Thursday, June 8, 2017 3:12 PM
> > > > > > > > > To: Olivier Matz <olivier.matz@6wind.com>
> > > > > > > > > Cc: Ananyev, Konstantin <konstantin.ananyev@intel.com>;
> > > > > > > > > Verkamp, Daniel <daniel.verkamp@intel.com>; dev@dpdk.org
> > > > > > > > > Subject: Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone
> > > > > > > > > allocation
> > > > > > > > >
> > > > > > > > > On Thu, Jun 08, 2017 at 04:05:26PM +0200, Olivier Matz wrote:  
> > > > > > > > > > On Thu, 8 Jun 2017 14:20:52 +0100, Bruce Richardson  
> > > > <bruce.richardson@intel.com> wrote:  
> > > > > > > > > > > On Thu, Jun 08, 2017 at 02:45:40PM +0200, Olivier Matz  
> > > > wrote:  
> > > > > > > > > > > > On Tue, 6 Jun 2017 15:56:28 +0100, Bruce Richardson  
> > > > <bruce.richardson@intel.com> wrote:  
> > > > > > > > > > > > > On Tue, Jun 06, 2017 at 02:19:21PM +0100, Ananyev,  
> > > > Konstantin wrote:  
> > > > > > > > > > > > > >
> > > > > > > > > > > > > >  
> > > > > > > > > > > > > > > -----Original Message-----
> > > > > > > > > > > > > > > From: Richardson, Bruce
> > > > > > > > > > > > > > > Sent: Tuesday, June 6, 2017 1:42 PM
> > > > > > > > > > > > > > > To: Ananyev, Konstantin
> > > > > > > > > > > > > > > <konstantin.ananyev@intel.com>
> > > > > > > > > > > > > > > Cc: Verkamp, Daniel <daniel.verkamp@intel.com>;
> > > > > > > > > > > > > > > dev@dpdk.org
> > > > > > > > > > > > > > > Subject: Re: [dpdk-dev] [PATCH v2] ring: use
> > > > > > > > > > > > > > > aligned memzone allocation
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > On Tue, Jun 06, 2017 at 10:59:59AM +0100, Ananyev,  
> > > > Konstantin wrote:  
> > > > > > > > > > > > > > > >  
> > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > >  
> > > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > > > The PROD/CONS_ALIGN values on x86-64 are
> > > > > > > > > > > > > > > > > > > set to 2 cache lines, so members  
> > > > > > > > > > > > > > > > > > of struct rte_ring are 128 byte aligned,  
> > > > > > > > > > > > > > > > > > >and therefore the whole struct needs
> > > > > > > > > > > > > > > > > > >128-byte alignment according to the ABI  
> > > > > > > > > > > > > > > > > > so that the 128-byte alignment of the fields  
> > > > can be guaranteed.  
> > > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > > Ah ok, missed the fact that rte_ring is 128B  
> > > > aligned these days.  
> > > > > > > > > > > > > > > > > > BTW, I probably missed the initial discussion,  
> > > > but what was the reason for that?  
> > > > > > > > > > > > > > > > > > Konstantin  
> > > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > > I don't know why PROD_ALIGN/CONS_ALIGN use 128
> > > > > > > > > > > > > > > > > byte alignment; it seems unnecessary if the
> > > > > > > > > > > > > > > > > cache line is only 64  
> > > > > > > bytes.  
> > > > > > > > > An  
> > > > > > > > > > > > > > > alternate  
> > > > > > > > > > > > > > > > > fix would be to just use cache line alignment  
> > > > for these fields (since memzones are already cache line aligned).  
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > Yes, had the same thought.
> > > > > > > > > > > > > > > >  
> > > > > > > > > > > > > > > > > Maybe there is some deeper  reason for the >=  
> > > > 128-byte alignment logic in rte_ring.h?  
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > Might be, would be good to hear opinion the author  
> > > > of that change.  
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > It gives improved performance for core-2-core  
> > > > transfer.  
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > You mean empty cache-line(s) after prod/cons, correct?
> > > > > > > > > > > > > > That's ok but why we can't keep them and whole  
> > > > rte_ring aligned on cache-line boundaries?  
> > > > > > > > > > > > > > Something like that:
> > > > > > > > > > > > > > struct rte_ring {
> > > > > > > > > > > > > >    ...
> > > > > > > > > > > > > >    struct rte_ring_headtail prod __rte_cache_aligned;
> > > > > > > > > > > > > >    EMPTY_CACHE_LINE   __rte_cache_aligned;
> > > > > > > > > > > > > >    struct rte_ring_headtail cons __rte_cache_aligned;
> > > > > > > > > > > > > >    EMPTY_CACHE_LINE   __rte_cache_aligned;
> > > > > > > > > > > > > > };
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Konstantin  
> > > > > > > > > > > > >
> > > > > > > > > > > > > Sure. That should probably work too.
> > > > > > > > > > > > >
> > > > > > > > > > > > > /Bruce  
> > > > > > > > > > > >
> > > > > > > > > > > > I also agree with Konstantin's proposal. One question
> > > > > > > > > > > > though: since it changes the alignment constraint of the
> > > > > > > > > > > > rte_ring structure, I think it is an ABI breakage: a
> > > > > > > > > > > > structure including the rte_ring structure inherits from  
> > > > this constraint.  
> > > > > > > > > > > >
> > > > > > > > > > > > How could we handle that, knowing this is probably a rare  
> > > > case?  
> > > > > > > > > > > >
> > > > > > > > > > > >  
> > > > > > > > > > > Is it an ABI break so long as we keep the resulting size
> > > > > > > > > > > and field placement of the structures the same? The
> > > > > > > > > > > alignment being reduced should not be a problem, as
> > > > > > > > > > > 128byte alignment is also valid as 64byte alignment, after  
> > > > all.  
> > > > > > > > > >
> > > > > > > > > > I'd say yes. Consider the following example:
> > > > > > > > > >
> > > > > > > > > > ---8<---
> > > > > > > > > > #include <stdio.h>
> > > > > > > > > > #include <stdlib.h>
> > > > > > > > > >
> > > > > > > > > > #define ALIGN 64
> > > > > > > > > > /* #define ALIGN 128 */
> > > > > > > > > >
> > > > > > > > > > /* dummy rte_ring struct */
> > > > > > > > > > struct rte_ring {
> > > > > > > > > > 	char x[128];
> > > > > > > > > > } __attribute__((aligned(ALIGN)));
> > > > > > > > > >
> > > > > > > > > > struct foo {
> > > > > > > > > > 	struct rte_ring r;
> > > > > > > > > > 	unsigned bar;
> > > > > > > > > > };
> > > > > > > > > >
> > > > > > > > > > int main(void)
> > > > > > > > > > {
> > > > > > > > > > 	struct foo array[2];
> > > > > > > > > >
> > > > > > > > > > 	printf("sizeof(ring)=%zu diff=%u\n",
> > > > > > > > > > 		sizeof(struct rte_ring),
> > > > > > > > > > 		(unsigned int)((char *)&array[1].r - (char  
> > > > *)array));  
> > > > > > > > > >
> > > > > > > > > > 	return 0;
> > > > > > > > > > }
> > > > > > > > > > ---8<---
> > > > > > > > > >
> > > > > > > > > > The size of rte_ring is always 128.
> > > > > > > > > > diff is 192 or 256, depending on the value of ALIGN.
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Olivier  
> > > > > > > >
> > > > > > > > About would it be an ABI breakage to 17.05 - I think would...
> > > > > > > > Though for me the actual breakage happens in 17.05 when rte_ring
> > > > > > > > alignment was increased from 64B 128B.
> > > > > > > > Now we just restoring it.
> > > > > > > >  
> > > > > > > Yes, ABI change was announced in advance and explicitly broken in  
> > > > 17.05.  
> > > > > > > There was no announcement of ABI break in 17.08 for rte_ring.
> > > > > > >  
> > > > > > > > >
> > > > > > > > > Yes, the diff will change, but that is after a recompile. If
> > > > > > > > > we have rte_ring_create function always return a 128-byte
> > > > > > > > > aligned structure, will any already-compiled apps fail to work
> > > > > > > > > if we also change the alignment of the rte_ring struct in the  
> > > > header?  
> > > > > > > >
> > > > > > > > Why 128B?
> > > > > > > > I thought we are discussing making rte_ring 64B aligned again?
> > > > > > > >
> > > > > > > > Konstantin  
> > > > > > >
> > > > > > > To avoid possibly breaking apps compiled against 17.05 when run
> > > > > > > against shared libs for 17.08. Having the extra alignment won't
> > > > > > > affect 17.08 apps, since they only require 64-byte alignment, but
> > > > > > > returning only 64-byte aligned memory for apps which expect
> > > > > > > 128byte aligned memory may cause issues.
> > > > > > >
> > > > > > > Therefore, we should reduce the required alignment to 64B, which
> > > > > > > should only affect any apps that do a recompile, and have memory
> > > > > > > allocation for rings return 128B aligned addresses to work with
> > > > > > > both 64B aligned and 128B aligned ring structures.  
> > > > > >
> > > > > > Ah, I see - you are talking just about rte_ring_create().
> > > > > > BTW, are you sure that right now it allocates rings 128B aligned?
> > > > > > As I can see it does just:
> > > > > > mz = rte_memzone_reserve(mz_name, ring_size, socket_id, mz_flags);
> > > > > > which means cache line alignment.
> > > > > >  
> > > > > It doesn't currently allocate with that alignment, which is something
> > > > > we need to fix - and what this patch was originally submitted to do.
> > > > > So I think this patch should be applied, along with a further patch to
> > > > > reduce the alignment going forward to avoid any other problems.  
> > > >
> > > > But if we going to reduce alignment anyway (patch #2) why do we need patch
> > > > #1 at all?  
> > > 
> > > Because any app compiled against 17.05 will use the old alignment value. Therefore patch 1 should be applied to 17.08 for backward
> > > compatibility, and backported to 17.05.1.  
> > 
> > Why then just no backport patch #2 to 17.05.1?
> >   
> Maybe so. I'm just a little wary about backporting changes like that to
> an older release, even though I'm not aware of any specific issues it
> might cause.


If we want to fully respect the API/ABI deprecation process, we should
have patch #1 in 17.05 and 17.08, a deprecation notice in 17.08, and patch
#2 starting from 17.11.

More pragmatically, it's quite difficult to foresee really big problems
due to the changes in patch #2. One I can see is:

- rte_ring.so: the dpdk ring library
- another_ring.so: a library based on dpdk ring. The struct another_ring
  is like the struct foo in my previous example.
- application: uses another_ring structure

After we apply patch #2 on dpdk, and recompile the another_ring library,
its ABI will change.


So I suggest to follow the deprecation process for that issue.

Olivier