On Thu, Mar 16, 2017 at 3:50 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > On Thu, 2017-03-16 at 15:33 -0700, Alexander Duyck wrote: >> On Thu, Mar 16, 2017 at 3:05 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > >> > It is not clear why this patch is needed . >> > >> > What you describe here is the case we might receive packets for a socket >> > coming from different interfaces ? >> > >> > If skb->napi_id is a sender_cpu, why should we prevent overwriting the >> > sk_napi_id with it, knowing that busy polling will simply ignore the >> > invalid value ? >> > >> > Do not get me wrong : >> > >> > I simply try to understand why the test about napi_id validity is now >> > done twice : >> > >> > 1) At the time we are writing into sk->sk_napi_id >> >> I would argue that this is the one piece we were missing. >> >> > 2) At busy polling time when we read sk->sk_napi_id >> >> Unless there was something recently added I don't think this was ever >> checked. Instead we start digging into the hash looking for the ID >> that won't ever be there. Maybe we should add something to napi_by_id >> that just returns NULL in these cases. > > But this is exactly what should happen. > > For invalid ID, we return NULL from napi_by_id() > > No need to add code for that, since the function is meant to deal with > valid cases.
I don't know. My concern here is about the cost of going through all that code just for something that we know shouldn't be valid. If nothing else I might update sk_can_busy_loop so that it doesn't try busy looping on a sk_napi_id that is NR_CPU or less. >> On top of that I think there end up being several spots where once we >> lock in a non-NAPI ID it is stuck such as the sk_mark_napi_id_once >> call. I figure we are better off locking in an actual NAPI ID rather >> than getting a sender_cpu stuck in there. > > Are you referring to sk_mark_napi_id_once() ? > > Since this is only used by UDP, I would be OK to avoid the 'locking' for > 'sender_cpu' ids. What I probably can do is go through and replace all the spots where we where checking for sk_napi_id being 0, and instead replace it with a check against NR_CPUS.
