On Wed, Jun 06, 2012 at 05:19:04PM +0200, Eric Dumazet wrote:
> 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.
Yes, of course.
> 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.
>
>
I understand, and not arguing about that.
But why do you say at most 1 packet?
Consider get_stats doing:
u64_stats_update_begin(&stats->syncp);
stats->tx_bytes += skb->len;
on 64 bit at this point
tx_packets might get incremented any number of times, no?
stats->tx_packets++;
u64_stats_update_end(&stats->syncp);
now tx_bytes and tx_packets are out of sync by more than 1.
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