On Wed, 2012-06-06 at 17:49 +0300, Michael S. Tsirkin wrote: > On Wed, Jun 06, 2012 at 03:10:10PM +0200, Eric Dumazet wrote: > > On Wed, 2012-06-06 at 14:13 +0300, Michael S. Tsirkin wrote: > > > > > We currently do all stats either on napi callback or from > > > start_xmit callback. > > > This makes them safe, yes? > > > > Hmm, then _bh() variant is needed in virtnet_stats(), as explained in > > include/linux/u64_stats_sync.h section 6) > > > > * 6) If counter might be written by an interrupt, readers should block interrupts. > > * (On UP, there is no seqcount_t protection, a reader allowing interrupts could > > * read partial values) > > > > Yes, its tricky... > > Sounds good, but I have a question: this realies on counters > being atomic on 64 bit. > Would not it be better to always use a seqlock even on 64 bit? > This way counters would actually be correct and in sync. > As it is if we want e.g. average packet size, > we can not rely e.g. on it being bytes/packets. When this stuff was discussed, we chose to have a nop on 64bits. Your point has little to do with 64bit stats, it was already like that with 'long int' counters. Consider average driver doing : dev->stats.rx_bytes += skb->len; dev->stats.rx_packets++; A concurrent reader can read an updated rx_bytes and a 'previous' rx_packets one. 'fixing' this requires a lot of work and memory barriers (in all drivers), for a very litle gain (at most one packet error) u64_stats_sync was really meant to be 0-cost on 64bit arches. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
