Re: [RFC PATCH] x86-64: software IRQ masking and handling

[Tejun Heo]

Hello,

On 07/12/2010 03:18 AM, Linus Torvalds wrote:
> On Sun, Jul 11, 2010 at 3:03 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> 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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