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

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Message-ID: <4CBE2C1B.6070500@zytor.com>
Date:	Tue, 19 Oct 2010 16:39:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	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 03:27 PM, Peter Zijlstra wrote:
> 
> Due to not actually having a sane key type the above is not easy to
> implement, but I tried:
> 
> #define _SWITCH_POINT(x)\
> ({                                                              \
>         __label__ jl_enabled;                                   \
>         bool ret = true;                                        \
>         JUMP_LABEL(x, jl_enabled);                              \
>         ret = false;                                            \
> jl_enabled:                                                     \
>         ret;            })
> 
> #define SWITCH_POINT(x) unlikely(_SWITCH_POINT(x))
> 
> #define COND_STMT(key, stmt)                                    \
> do {                                                            \
>         if (SWITCH_POINT(key)) {                                \
>                 stmt;                                           \
>         }                                                       \
> } while (0)
> 
> 
> and that's still generating these double jumps.
> 

I just experimented with it, and the ({...}) construct doesn't work,
because it looks like a merged flow of control to gcc.

Replacing the ({ ... }) with an inline does indeed remove the double
jumps.

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index b67cb18..2ff829d 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -61,12 +61,22 @@ static inline int jump_label_text_reserved(void
*start, void *end)

 #endif

+static __always_inline __pure bool _SWITCH_POINT(void *x)
+{
+       asm goto("# SWITCH_POINT %0\n\t"
+                ".byte 0x66,0x66,0x66,0x66,0x90\n"
+                "1:"
+                : : "i" (x) : : jl_enabled);
+       return false;
+jl_enabled:
+       return true;
+}
+
+#define SWITCH_POINT(x)        unlikely(_SWITCH_POINT(x))
+
 #define COND_STMT(key, stmt)                                   \
 do {                                                           \
-       __label__ jl_enabled;                                   \
-       JUMP_LABEL(key, jl_enabled);                            \
-       if (0) {                                                \
-jl_enabled:                                                    \
+       if (SWITCH_POINT(key)) {                                \
                stmt;                                           \
        }                                                       \
 } while (0)


The key here seems to be to not use the JUMP_LABEL macro as implemented;
I have utterly failed to make JUMP_LABEL() do the right thing.


	-hpa
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