linux-kernel - Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

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Date:	Mon, 25 Oct 2010 15:12:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:	Jason Baron <jbaron@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Koki Sanagi <sanagi.koki@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	nhorman@...driver.com, scott.a.mcmillan@...el.com,
	laijs@...fujitsu.com, LKML <linux-kernel@...r.kernel.org>,
	eric.dumazet@...il.com, kaneshige.kenji@...fujitsu.com,
	David Miller <davem@...emloft.net>, izumi.taku@...fujitsu.com,
	kosaki.motohiro@...fujitsu.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"Luck, Tony" <tony.luck@...el.com>
Subject: Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

On 10/25/2010 03:01 PM, Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.com) wrote:
>> On 10/20/2010 08:27 AM, Jason Baron wrote:
>>>
>>> sure. The idea of the 'jmp 0' was simply to be an lcd for x86, if
>>> there's a better lcd for x86, I'll update it. But note, that since the
>>> 'jmp 0' is patched to a better nop at boot, we wouldn't see much gain.
>>> And in the boot path we are using 'text_poke_early()', so avoiding that
>>> isn't going to improve things much.
>>>
>>
>> It's still a completely unnecessary waste of startup time some
>> potentially significant fraction of the time.  Startup time matters,
>> especially as the number of tracepoints grow.
> 
> We're still waiting for input for the best single-5-byte-instruction nop that
> will work on all x86 variants. Please note that the GENERIC_NOP5 is actually two
> instructions one next to each other, which is not appropriate here.
> 

On 64 bits, use P6_NOP5; it seems to not suck on any platform.

On 32 bits, 3E 8D 74 26 00 (i.e. DS: + GENERIC_NOP4) seems to at least
do okay.

I can't say these are the *best* (in fact, they are guaranteed not the
best on some significant number of chips), but they haven't sucked on
any chips I have been able to measure -- and are way faster than JMP.

	-hpa
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