Messages in this thread | | | Date | Mon, 12 Jul 2010 09:41:41 +0200 | From | Tejun Heo <> | Subject | Re: [RFC PATCH] x86-64: software IRQ masking and handling |
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Hello,
On 07/12/2010 03:18 AM, Linus Torvalds wrote: > 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.
I think I can pack everything into the space irq_count occupies now. 16 bit for pending, and a byte for enable and count each.
> Also, the preempt code is pretty optimized and uses "add". Tejun uses > "btrl" at least in some places, which is generally not a fast > instruction. So there's a few caveats there too. Which is why I'd > want numbers.
That can be replaced with bt + mov. I wasn't sure which would be cheaper tho.
Thanks.
-- tejun
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