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Date: Tue, 19 Oct 2010 15:26:12 -0700 From: "H. Peter Anvin" <hpa@...or.com> To: Steven Rostedt <rostedt@...dmis.org> CC: Thomas Gleixner <tglx@...utronix.de>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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 10/19/2010 03:23 PM, Steven Rostedt wrote: > On Tue, 2010-10-19 at 14:48 -0700, H. Peter Anvin wrote: >> On 10/19/2010 02:23 PM, Steven Rostedt wrote: >>> >>> But it seemed that gcc for you inlined the code in the wrong spot. >>> Perhaps it's not a good idea to have the something like h - softirq_vec >>> in the parameter of the tracepoint. Not saying that your change is not >>> worth it. It is, because h - softirq_vec is used by others now too. >>> >> >> OK, first of all, there are some serious WTFs here: >> >> # define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t" >> >> A jump instruction is one of the worst possible NOPs. Why are we doing >> this? > > Good question. Safety? Jason? > > This is the initial jumps and are converted on boot up to a better nop. > But it makes absolutely no sense to insert an instruction that suboptimal and then convert it. Start out with a reasonable, universally acceptable, instruction, e.g. LEA on 32 bits and NOPL on 64 bits. >> >> The second thing that I found when implementing static_cpu_has() was >> that it is actually better to encapsulate the asm goto in a small inline >> which returns bool (true/false) -- gcc will happily optimize out the >> variable and only see it as a flow of control thing. I would be very >> curious if that wouldn't make gcc generate better code in cases like that. >> >> gcc 4.5.0 has a bug in that there must be a flowthrough case in the asm >> goto (you can't have it unconditionally branch one way or the other), so >> that should be the likely case and accordingly it should be annotated >> likely() so that gcc doesn't reorder. I suspect in the end one ends up >> with code like this: >> >> static __always_inline __pure bool __switch_point(...) >> { >> asm goto("1: " JUMP_LABEL_INITIAL_NOP >> /* ... patching stuff */ >> : : : : t_jump); >> return false; >> t_jump: >> return true; >> } >> >> #define SWITCH_POINT(x) unlikely(__switch_point(x)) >> >> I *suspect* this will resolve the need for hot/cold labels just fine. > > Interesting, we could try this. > It of course also have the nice property that it syntactically looks exactly like any other C conditional. -hpa -- 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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