On Thu, May 19, 2016 at 7:45 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > On Thu, 2016-05-19 at 18:50 -0700, Cong Wang wrote: >> On Thu, May 19, 2016 at 5:35 AM, Eric Dumazet <eric.duma...@gmail.com> wrote: >> > >> > These stats are using u64 or u32 fields, so reading integral values >> > should not prevent writers from doing concurrent updates if the kernel >> > arch is a 64bit one. >> > >> > Being able to atomically fetch all counters like packets and bytes sent >> > at the expense of interfering in fast path (queue and dequeue packets) >> > is simply not worth the pain, as the values are generally stale after 1 >> > usec. >> >> I think one purpose of this lock is to make sure we have an atomic >> snapshot of these counters as a whole. IOW, we may need another >> lock rather than the qdisc root lock to guarantee this. > > Right, this was stated in the changelog. > > I played a bit at changing qdisc->__state to a seqcount. > > But this would add 2 additional smp_wmb() barriers. > > Not sure if any application would actually care if it reads > > Sent 22954160104777 bytes 1808585171 pkt (dropped 0, overlimits 0 > requeues 26021) > > Instead of > > Sent 22954160104777 bytes 1808585172 pkt (dropped 0, overlimits 0 > requeues 26021) > > ?
Sometimes I use bytes/pkts to valuate the average size of the packets. ;) It doesn't have to be that accurate but we don't know how inaccurate it is without lock?
