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

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

> -----Original Message-----
> From: Xu, Ting <ting.xu@intel.com>
> Sent: Tuesday, July 21, 2020 6:16 AM
> To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>; dev@dpdk.org
> Cc: stable@dpdk.org
> Subject: RE: [PATCH v3] lib/table: fix cache alignment issue
> 
> 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.
> >
> 

My proposed solution, which IMO provides the cleanest and most readable way to fix / maintain this code:

#ifdef RTE_ARCH_64

struct rte_bucket_4_16 {
	//current definition of this struct
};

#else

struct rte_bucket_4_16 {
	//definition with padding fields for the 32-bit pointers to keep this struct to a multiple of 64 bytes
};

#endif

We need to apply the same for 8-byte key and 32-byte key hash functions from the same folder.

What do you think, Ting?

> 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