linux-kernel - Re: [RFC/HACK] x86: Fast return to kernel

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Date:	Sun, 04 May 2014 12:59:46 -0700
From:	"H. Peter Anvin" <h.peter.anvin@...el.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC/HACK] x86: Fast return to kernel

On 05/04/2014 11:40 AM, Ingo Molnar wrote:
> 
> * Andy Lutomirski <luto@...capital.net> wrote:
> 
>>> That said, regular *device* interrupts do often return to kernel 
>>> mode (the idle loop in particular), so if you have any way to 
>>> measure that, that might be interesting, and might show some of 
>>> the same advantages.
>>
>> I can try something awful involving measuring latency of 
>> hardware-timed packets on a SolarFlare card, but I'll have 
>> calibration issues.  I suppose I could see if 'ping' gets faster.  
>> In general, this will speed up interrupts that wake userspace from 
>> idle by about 100ns on my box, since it's presumably the same size 
>> and the speedup per loop in my silly benchmark.
> 
> To simulate high rate device IRQ you can generate very high frequency 
> lapic IRQs by using hrtimers, that's generating a ton of per CPU lapic 
> IRQs.
> 

The bigger question is if that helps in measuring the actual latency.
It should get more data points, to be sure.

Maybe let userspace sit in a tight loop doing RDTSC, and look for data
points too far apart to have been uninterrupted?

	-hpa


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