linux-kernel - Re: [PATCH] perf, tools: Add PERF

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Date:	Mon, 15 Sep 2014 16:22:59 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	jolsa@...hat.com, linux-kernel@...r.kernel.org,
	namhyung@...nel.org, mingo@...nel.org, peterz@...radead.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf, tools: Add PERF_PID

Em Tue, Sep 09, 2014 at 05:03:53PM -0700, Andi Kleen escreveu:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> It's currently difficult to filter out perf itself using a filter.
> This can give cascading effects during IO tracing when the IO
> perf does itself causes more trace output.
> 
> The best way to filter is to use the pid. But it's difficult to get the pid
> of perf without using hacks.
> 
> Add a PERF_PID meta variable to the perf filter that contains the current pid.
> 
> With this patch the following works
> 
> % perf record -e syscalls:sys_enter_write -a --filter 'common_pid != PERF_PID' ...
> 
> This won't work for more complex perf pipe lines with multiple processes,
> but at least solves the problem nicely for a single perf.
 
> @@ -983,6 +984,13 @@ int parse_filter(const struct option *opt, const char *str,
>  		return -1;
>  	}
>  
> +	/* Assume a pid has not more than 8 characters */

> +	pid = strstr(last->filter, "PERF_PID");
> +	if (pid) {
> +		char buf[9];
> +		snprintf(buf, 9, "%8d", getpid());
> +		memcpy(pid, buf, 8);

Can we please remove this assumption?

If the get to be more than 8 chars we'll be just truncating stuff and
possibly filtering some other, unrelated process, no?

       pid_t getpid(void);

typedef __pid_t pid_t;
__STD_TYPE __PID_T_TYPE __pid_t;	/* Type of process identifications.  */
/usr/include/bits/typesizes.h:#define __PID_T_TYPE __S32_TYPE
/usr/include/bits/types.h:#define	__S32_TYPE		int
[acme@zoo linux]$ uname -a
Linux zoo.ghostprotocols.net 3.17.0-rc1+ #2 SMP Thu Aug 21 12:25:42 BRT 2014 x86_64 x86_64 x86_64 GNU/Linux

[acme@zoo ~]$ cat a.c
#include <limits.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
	printf("%8d", INT_MAX);
}
[acme@zoo ~]$ make a
cc     a.c   -o a
[acme@zoo ~]$ ./a | wc -c
10
[acme@zoo ~]$

The feature is needed, no doubt about it, thanks for working on it.

I guess we need the equivalent of (python regexp) s/PERF_PID/("%d" % getpid())/g
to properly implement this.

- Arnaldo
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