On Wed, Jul 29, 2020 at 3:14 PM Dumitrescu, Cristian
<[email protected]> 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.
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: [email protected]
> > >
> > > Signed-off-by: Ting Xu <[email protected]>
> > >
> > > ---
> > > 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
