On Mon, Apr 20, 2009 at 01:42:49PM -0700, Stephen Hemminger wrote: > On Mon, 20 Apr 2009 20:25:14 +0200 > Eric Dumazet <dada1@xxxxxxxxxxxxx> wrote: > > > Stephen Hemminger a écrit : > > > This version of x_tables (ip/ip6/arp) locking uses a per-cpu > > > recursive lock that can be nested. It is sort of like existing kernel_lock, > > > rwlock_t and even old 2.4 brlock. > > > > > > "Reader" is ip/arp/ip6 tables rule processing which runs per-cpu. > > > It needs to ensure that the rules are not being changed while packet > > > is being processed. > > > > > > "Writer" is used in two cases: first is replacing rules in which case > > > all packets in flight have to be processed before rules are swapped, > > > then counters are read from the old (stale) info. Second case is where > > > counters need to be read on the fly, in this case all CPU's are blocked > > > from further rule processing until values are aggregated. > > > > > > The idea for this came from an earlier version done by Eric Dumazet. > > > Locking is done per-cpu, the fast path locks on the current cpu > > > and updates counters. This reduces the contention of a > > > single reader lock (in 2.6.29) without the delay of synchronize_net() > > > (in 2.6.30-rc2). > > > > > > The mutex that was added for 2.6.30 in xt_table is unnecessary since > > > there already is a mutex for xt[af].mutex that is held. > > > > > > Signed-off-by: Stephen Hemminger <shemminger@xxxxxxxxxx > > > > > > --- > > > Changes from earlier patches. > > > - function name changes > > > - disable bottom half in info_rdlock > > > > OK, but we still have a problem on machines with >= 250 cpus, > > because calling 250 times spin_lock() is going to overflow preempt_count, > > as each spin_lock() increases preempt_count by one. > > > > PREEMPT_MASK: 0x000000ff > > > > add_preempt_count() should warn us about this overflow if CONFIG_DEBUG_PREEMPT is set > > Wouldn't 256 or higher CPU system be faster without preempt? If there > are that many CPU's, it is faster to do the work on other cpu and avoid > the overhead of a hotly updated preempt count. The preempt count is maintained per-CPU, so has low overhead. The problem is that for CONFIG_PREEMPT builds, the preempt disabing is built into spin_lock(). Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
