Messages in this thread | | | Subject | Re: [RFC PATCH] x86-64: software IRQ masking and handling | From | Eric Dumazet <> | Date | Mon, 12 Jul 2010 07:11:33 +0200 |
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Le dimanche 11 juillet 2010 à 18:18 -0700, Linus Torvalds a écrit : > On Sun, Jul 11, 2010 at 3:03 PM, Steven Rostedt <rostedt@goodmis.org> wrote: > > > > I have seen some hits with cli-sti. I was considering swapping all > > preempt_disable() with local_irq_save() in ftrace, but hackbench showed > > a 30% performance degradation when I did that. > > Yeah, but in that case you almost certainly keep the per-cpu cacheline > hot in the D$ L1 cache, and the stack tracer is presumably also not > taking any extra I$ L1 misses. So you're not seeing any of the > downsides. The upside of plain cli/sti is that they're small, and have > no D$ footprint. > > And it's possible that the interrupt flag - at least if/when > positioned right - wouldn't have any additional D$ footprint under > normal load either. IOW, if there is an existing per-cpu cacheline > that is effectively always already dirty and in the cache, > But that's something that really needs macro-benchmarks - exactly > because microbenchmarks don't show those effects since they are always > basically hot-cache. >
Some kernel dev incorrectly assume they own cpu caches...
This discussion reminds me I noticed a performance problem with placement of cpu_online_bits and cpu_online_mask on separate sections (and thus separate cache lines) and a network load.
static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly; const struct cpumask *const cpu_online_mask = to_cpumask(cpu_online_bits);
Two changes are possible :
1) Get rid of the cpu_online_mask (its a const pointer to a known target). I cant see a reason for its need it actually...
2) Dont use a the last const qualifier but __read_mostly to move cpu_online_mask on same section.
Rusty, could you comment on one or other way before I submit a patch ?
(Of course, possible/present/active have same problem)
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