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Date: Mon, 25 Oct 2010 20:39:48 -0400 From: Steven Rostedt <rostedt@...dmis.org> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: "H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>, 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 Mon, 2010-10-25 at 18:55 -0400, Mathieu Desnoyers wrote: > * H. Peter Anvin (hpa@...or.com) wrote: > > 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. > > Cool, thanks for the info! Steven and Jason should probably update their > respective infrastructure to use the 32-bit 5-byte nop you propose rather than > the 5-byte jump. Actually, I was thinking that we could take any 5 byte nop. The alternate code is executed _before_ SMP is enabled. Thus we should not have any cases where something could be executing in midstream. -- Steve -- 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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