Re: [dpdk-dev] [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: Wednesday, July 22, 2020 5:17 AM
> 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: 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?
> 

Thanks for your advice. I think it makes sense.
I have updated a new patch version based on this method, could you please help review?

Thanks!

> > 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