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Date: Mon, 12 Jul 2010 09:41:41 +0200 From: Tejun Heo <tj@...nel.org> To: Linus Torvalds <torvalds@...ux-foundation.org> CC: Steven Rostedt <rostedt@...dmis.org>, Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, the arch/x86 maintainers <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>, Christoph Lameter <cl@...ux-foundation.org>, Frederic Weisbecker <fweisbec@...il.com> Subject: Re: [RFC PATCH] x86-64: software IRQ masking and handling Hello, On 07/12/2010 03:18 AM, Linus Torvalds wrote: > On Sun, Jul 11, 2010 at 3:03 PM, Steven Rostedt <rostedt@...dmis.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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