Re: [dpdk-dev] [dpdk-stable] [PATCH v4] lib/table: fix cache alignment issue

["Dumitrescu, Cristian" <cristian.dumitrescu@intel.com>]

> -----Original Message-----
> From: David Marchand <david.marchand@redhat.com>
> Sent: Wednesday, July 29, 2020 2:28 PM
> To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> Cc: Xu, Ting <ting.xu@intel.com>; dev <dev@dpdk.org>; dpdk stable
> <stable@dpdk.org>; Kevin Traynor <ktraynor@redhat.com>; Luca Boccassi
> <bluca@debian.org>
> Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH v4] lib/table: fix cache
> alignment issue
> 
> On Wed, Jul 29, 2020 at 3:14 PM Dumitrescu, Cristian
> <cristian.dumitrescu@intel.com> wrote:
> > > Please correct me if I am wrong, but it simply means this part of the
> > > table library never worked for 32-bit.
> > > It seems more adding 32-bit support rather than a fix and then I
> > > wonder if it has its place in rc3.
> > >
> >
> > Functionally. the code works, but performance is affected.
> >
> > The only thing that prevents the code from working is the check in the
> table create function that checks the size of the above structure is 64 bytes,
> which caught this issue.
> 
> Yes, and that's my point.
> It was not working.
> It was not tested.
> 
> 

Not sure when this code was last tested on 32-bit systems, I'll let the validation folks comment on this, but I cannot rule out a change in compiler behavior either.

This is a low complexity and low impact change, hence low risk IMO.

> This patch asks for backport in stable branches, I will let Kevin and
> Luca comment.
> 
> 
> >
> > >
> > >
> > > Now, looking at the details:
> > >
> > > For 64-bit on my x86, we have:
> > >
> > > struct rte_bucket_4_8 {
> > >     uint64_t                   signature;            /*     0     8 */
> > >     uint64_t                   lru_list;             /*     8     8 */
> > >     struct rte_bucket_4_8 *    next;                 /*    16     8 */
> > >     uint64_t                   next_valid;           /*    24     8 */
> > >     uint64_t                   key[4];               /*    32    32 */
> > >     /* --- cacheline 1 boundary (64 bytes) --- */
> > >     uint8_t                    data[];               /*    64     0 */
> > >
> > >     /* size: 64, cachelines: 1, members: 6 */
> > > };
> > >
> > >
> > > For 32-bit, we have:
> > >
> > > struct rte_bucket_4_8 {
> > >     uint64_t                   signature;            /*     0     8 */
> > >     uint64_t                   lru_list;             /*     8     8 */
> > >     struct rte_bucket_4_8 *    next;                 /*    16     4 */
> > >     uint64_t                   next_valid;           /*    20     8 */
> > >     uint64_t                   key[4];               /*    28    32 */
> > >     uint8_t                    data[];               /*    60     0 */
> > >
> > >     /* size: 60, cachelines: 1, members: 6 */
> > >     /* last cacheline: 60 bytes */
> > > } __attribute__((__packed__));
> > >
> > > ^^ it is interesting that a packed attribute ends up here.
> > > I saw no such attribute in the library code.
> > > Compiler black magic at work I guess...
> > >
> >
> > Where do you see the packet attribute? I don't see it in the code.
> 
> That's pahole reporting this.
> Maybe the tool extrapolates this attribute based on the next_valid
> field placement... I don't know.
> 
> > A packet attribute would explain this issue, i.e. why did the compiler decide
> not to insert an expected padfing of 4 bytes right after the "next" field, that
> would allow the field "next_valid" to be aligned to its natural boundary of 8
> bytes.
> 
> Or a 64-bit field on 32-bit has a special alignment that I am not aware of.
> 
> 
> >
> > >
> > > >
> > > > Fixes: 8aa327214c ("table: hash")
> > > > Cc: stable@dpdk.org
> > > >
> > > > Signed-off-by: Ting Xu <ting.xu@intel.com>
> > > >
> > > > ---
> > > > v3->v4: Change design based on comment
> > > > v2->v3: Rebase
> > > > v1->v2: Correct patch time
> > > > ---
> > > >  lib/librte_table/rte_table_hash_key16.c | 17 +++++++++++++++++
> > > >  lib/librte_table/rte_table_hash_key32.c | 17 +++++++++++++++++
> > > >  lib/librte_table/rte_table_hash_key8.c  | 16 ++++++++++++++++
> > > >  3 files changed, 50 insertions(+)
> > > >
> > > > diff --git a/lib/librte_table/rte_table_hash_key16.c
> > > b/lib/librte_table/rte_table_hash_key16.c
> > > > index 2cca1c924..c4384b114 100644
> > > > --- a/lib/librte_table/rte_table_hash_key16.c
> > > > +++ b/lib/librte_table/rte_table_hash_key16.c
> > > > @@ -33,6 +33,7 @@
> > > >
> > > >  #endif
> > > >
> > > > +#ifdef RTE_ARCH_64
> > > >  struct rte_bucket_4_16 {
> > > >         /* Cache line 0 */
> > > >         uint64_t signature[4 + 1];
> > > > @@ -46,6 +47,22 @@ struct rte_bucket_4_16 {
> > > >         /* Cache line 2 */
> > > >         uint8_t data[0];
> > > >  };
> > > > +#else
> > > > +struct rte_bucket_4_16 {
> > > > +       /* Cache line 0 */
> > > > +       uint64_t signature[4 + 1];
> > > > +       uint64_t lru_list;
> > > > +       struct rte_bucket_4_16 *next;
> > > > +       uint32_t pad;
> > > > +       uint64_t next_valid;
> > > > +
> > > > +       /* Cache line 1 */
> > > > +       uint64_t key[4][2];
> > > > +
> > > > +       /* Cache line 2 */
> > > > +       uint8_t data[0];
> > > > +};
> > > > +#endif
> > >
> > > The change could simply be:
> > >
> > > @@ -38,6 +38,9 @@ struct rte_bucket_4_16 {
> > >         uint64_t signature[4 + 1];
> > >         uint64_t lru_list;
> > >         struct rte_bucket_4_16 *next;
> > > +#ifndef RTE_ARCH_64
> > > +       uint32_t pad;
> > > +#endif
> > >         uint64_t next_valid;
> > >
> > >         /* Cache line 1 */
> > >
> > > It avoids duplicating the whole structure definition (we could miss
> > > updating one side of the #ifdef later).
> > > Idem for the other "8" and "32" structures.
> 
> 
> What about this comment?
> 
> 
> --
> David Marchand