On Mon, Jun 15, 2015 at 01:47:26PM -0700, Stephen Hemminger wrote: > On Mon, 15 Jun 2015 18:34:13 +0000 > "Ananyev, Konstantin" <konstantin.ananyev at intel.com> wrote: > > > > > > > > -----Original Message----- > > > From: Richardson, Bruce > > > Sent: Monday, June 15, 2015 5:23 PM > > > To: Ananyev, Konstantin > > > Cc: Olivier MATZ; dev at dpdk.org; Damjan Marion (damarion) > > > Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline > > > > > > On Mon, Jun 15, 2015 at 05:10:44PM +0100, Ananyev, Konstantin wrote: > > > > > > > > > > > > > -----Original Message----- > > > > > From: Richardson, Bruce > > > > > Sent: Monday, June 15, 2015 5:02 PM > > > > > To: Ananyev, Konstantin > > > > > Cc: Olivier MATZ; dev at dpdk.org; Damjan Marion (damarion) > > > > > Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline > > > > > > > > > > On Mon, Jun 15, 2015 at 04:59:55PM +0100, Ananyev, Konstantin wrote: > > > > > > > > > > > > > > > > > > > > > > > > As I can see, vector TX is the only one that calls > > > > > > __rte_pktmbuf_prefree_seg() directly. > > > > > > All others use rte_pktmbuf_free_seg(), that does ' m->next = NULL' > > > > > > anyway. > > > > > > For vector TX - yes, need to verify that it would not introduce a > > > > > > slowdown. > > > > > > Konstantin > > > > > > > > > > > > > > > > But if the function is only directly called from one place, and that > > > > > doesn't > > > > > have a problem, why would we bother making any change at all? > > > > > > > > > > > > For future usages? > > > > But sure, if you believe that we can safely remove 'm->next = NULL' at > > > > RX path, > > > > without any changes in the __rte_pktmbuf_prefree_seg() - > > > > that seems fine to me. > > > > Konstantin > > > > > > > > > > If we find it's not safe, we can add in the change to > > > __rte_pktmbuf_prefree_seg > > > as you suggest. > > > > > > One other question: based on this, do you think it's safe to also remove > > > the > > > assignment to NULL from the pktmbuf_alloc function? I suspect it should > > > be safe, and > > > that should help any traffic-generator type applications that use that > > > function > > > extensively. > > > > So it will be setup to NULL, either by: > > - mbuf constructor. > > - TX full-path free descriptors code. > > - upper layer code that uses 'next' pointer explicitly. > > ? > > > > I can't come-up with some breakage scenario off-hand. > > But that means that we have to should avoid resetting tx_offload in > > pktmbuf_alloc() too, right? > > Otherwise there probably wouldn't be any real gain. > > Konstantin > > > > > > > > /Bruce > > > > > > The issue is when mbuf is used once by something and it sets it to non-NULL > packet is sent then freed. > then the free packet is picked up by one of the drivers using "fast path" Rx > code. > > IMHO the "fast path" Rx code has to go away. All the special allocation code > should call rte_pktmuf_reset and then only set the fields that are special > for the received packet. The compiler should be able to remove the redundant > stores, and it would prevent bugs I am seeing where some fields are not set > in some drivers.
Is the better option here not to just fix the drivers rather than making the fast-path RX code go away? Can you give us a report out on what fields are not getting correctly set, and when using what drivers, and we can look and see where our gaps in coverage are? /Bruce
