Re: [dpdk-stable] [PATCH v3] lib/table: fix cache alignment issue

["Xu, Ting" <ting.xu@intel.com>]

Hi, Cristian

> -----Original Message-----
> From: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> Sent: Monday, July 20, 2020 10:38 PM
> To: Xu, Ting <ting.xu@intel.com>; dev@dpdk.org
> Cc: stable@dpdk.org
> Subject: RE: [PATCH v3] lib/table: fix cache alignment issue
> 
> 
> 
> > -----Original Message-----
> > From: Xu, Ting <ting.xu@intel.com>
> > Sent: Thursday, July 9, 2020 2:48 AM
> > To: dev@dpdk.org
> > Cc: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>; Xu, Ting
> > <ting.xu@intel.com>; stable@dpdk.org
> > Subject: [PATCH v3] lib/table: fix cache alignment issue
> >
> > When create softnic hash table with 16 keys, it failed on 32bit
> > environment because of the structure rte_bucket_4_16 alignment issue.
> > Add __rte_cache_aligned to ensure correct cache align.
> >
> > Fixes: 8aa327214c ("table: hash")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Ting Xu <ting.xu@intel.com>
> >
> > ---
> > v2->v3: Rebase
> > v1->v2: Correct patch time
> > ---
> >  lib/librte_table/rte_table_hash_key16.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/lib/librte_table/rte_table_hash_key16.c
> > b/lib/librte_table/rte_table_hash_key16.c
> > index 2cca1c924..5e1665c15 100644
> > --- a/lib/librte_table/rte_table_hash_key16.c
> > +++ b/lib/librte_table/rte_table_hash_key16.c
> > @@ -44,7 +44,7 @@ struct rte_bucket_4_16 {
> >  	uint64_t key[4][2];
> >
> >  	/* Cache line 2 */
> > -	uint8_t data[0];
> > +	uint8_t data[0] __rte_cache_aligned;
> >  };
> >
> >  struct rte_table_hash {
> > --
> > 2.17.1
> 
> Hi Ting,
> 
> This fix is breaking the execution for systems with cache line of 128 bytes, as
> typically (on 64-bit systems) this structure would be 64-byte in size and
> adding the __rte_cache_aligned would force doubling the size of this
> structure through padding enforced by the compiler.
> 
> Can you please confirm this is caused by check below failing in the table
> create function:
> 	sizeof(struct rte_bucket_4_16) % 64) != 0
> 

The result of sizeof(struct rte_bucket_4_16) is 124 byte in this case, and this is the direct reason causing this failure.

> Since all the other fields in this data structure are explicitly declared as 64-bit
> fields, due to the alignment rules I was expecting the compiler to add a 32-bit
> padding field after the "next" field, which is a pointer that would only take 32
> bits on 32-bit systems. I am not sure why this did not take place in your case,
> any thoughts?
> 

It shows that the size of the field struct rte_bucket_4_16 *next in struct rte_bucket_4_16 is only 32 bits. And there is no padding added by the compiler in my and the reporter's case.
I tried to add a 32 bits pad field after the field next manually, and the result is correct then.
Is it because in 32-bit system, the compiler will not extend the 32 bits pointer to 64 bits, since the 32 bits size has already matched the cache line?

> Not sure why we would run Soft NIC on 32-bit systems, might be better to
> disable Soft NIC for 32-bit systems.
> 

To be honest, I do not know why we should run softnic on 32-bit system, I was just assigned this specific bug. It seems there is a complete test case for validation team to test softnic in 32-bit system.
I am not sure is it OK to tell the validation team that we should disable softnic in 32-bit system now. Or we should fix this issue this time and discuss about the problem later?

Thanks!

> Thanks,
> Cristian