linux-kernel - Re: [PATCH v2 2/4] perf ftrace: Add ftrace.tracer config option

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Date:   Tue, 31 Jan 2017 10:15:09 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Taeung Song <treeze.taeung@...il.com>
Cc:     linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wang Nan <wangnan0@...wei.com>
Subject: Re: [PATCH v2 2/4] perf ftrace: Add ftrace.tracer config option

Em Tue, Jan 31, 2017 at 09:46:28AM -0300, Arnaldo Carvalho de Melo escreveu:
> Also you are silently ignoring any unknown variable in this section, so
> if someone has this:
> 
> cat ~/.perfconfig
> 
> [ftrace]
> 
> 	trace = function
> 
> 
> I.e. forgets the 'r', it will keep using the current default,
> "function_graph" till the user checks the config and adds that missing
> 'r'...
> 
> I haven't changed this because I think this is so common that we should
> instead have a different error return for those config callbacks that
> will tell perf_config() to say something like:
> 
>   "unknown variable %s.%s\n", section_name, variable_name

So I read a bit more the config code and ended up with this:

static int perf_ftrace_config(const char *var, const char *value, void *cb)
{
        struct perf_ftrace *ftrace = cb;

        if (prefixcmp(var, "ftrace."))
                return 0;

        if (strcmp(var, "ftrace.tracer"))
                return -1;

        if (!strcmp(value, "function_graph") ||
            !strcmp(value, "function")) {
                ftrace->tracer = value;
                return 0;
        }

	pr_err("Please select \"function_graph\" (default) or \"function\"\n");
        return -1;
}

I.e. it will ignore variables for other sections and will stop at an unknown
variable or value for ftrace.tracer:

[root@...et ~]# cat ~/.perfconfig 
[ftrace]
	trace = function
[root@...et ~]# perf ftrace usleep 1
Error: wrong config key-value pair ftrace.trace=function
[root@...et ~]# cat ~/.perfconfig 
[ftrace]
	tracer = functin
[root@...et ~]# perf ftrace usleep 1
Please select "function_graph" (default) or "function"
Error: wrong config key-value pair ftrace.tracer=functin
[root@...et ~]# cat ~/.perfconfig 
[ftrace]
	tracer = function
[root@...et ~]# perf ftrace usleep 1 | head -5
          <idle>-0     [000] d...  3855.820847: switch_mm_irqs_off <-__schedule
           <...>-18550 [000] d...  3855.820849: finish_task_switch <-__schedule
           <...>-18550 [000] d...  3855.820851: smp_irq_work_interrupt <-irq_work_interrupt
           <...>-18550 [000] d...  3855.820851: irq_enter <-smp_irq_work_interrupt
           <...>-18550 [000] d...  3855.820851: rcu_irq_enter <-irq_enter
[root@...et ~]#

- Arnaldo