Re: [PATCH v2] perf ftrace: Detect whether ftrace is enabled on system

[duchangbin <changbin.du@huawei.com>]

On Wed, Sep 11, 2024 at 09:31:02AM -0300, Arnaldo Carvalho de Melo wrote:
> On Wed, Sep 11, 2024 at 11:15:19AM +0100, James Clark wrote:
> > On 11/09/2024 11:01, Changbin Du wrote:
> > > To make error messages more accurate, this change detects whether ftrace is
> > > enabled on system by checking trace file "set_ftrace_pid".
> 
> <SNIP>
> 
> > > @@ -1583,6 +1601,11 @@ int cmd_ftrace(int argc, const char **argv)
> > >   	if (!check_ftrace_capable())
> > >   		return -1;
> > > +	if (!is_ftrace_supported()) {
> > > +		pr_err("ftrace is not supported on this system\n");
> > > +		return -ENOTSUP;
> > > +	}
> > > +
> > >   	ret = perf_config(perf_ftrace_config, &ftrace);
> > >   	if (ret < 0)
> > >   		return -1;
> > 
> > Reviewed-by: James Clark <james.clark@linaro.org>
> 
> Applied and added these comments:
> 
> Committer testing:
> 
> Doing it in an unprivileged toolbox container on Fedora 40:
> 
> Before:
> 
>   acme@number:~/git/perf-tools-next$ toolbox enter perf
>   ⬢[acme@toolbox perf-tools-next]$ sudo su -
>   ⬢[root@toolbox ~]# ~acme/bin/perf ftrace
>   failed to reset ftrace
>   ⬢[root@toolbox ~]#
> 
> After this patch:
> 
>   ⬢[root@toolbox ~]# ~acme/bin/perf ftrace
>   ftrace is not supported on this system
>   ⬢[root@toolbox ~]#
> 
> Maybe we could check if we are in such as situation, inside an
> unprivileged container, and provide a HINT line?
> 
I think we could check the mount status of tracefs first, and then check whether
the ftrace node exists. If there's a permission issue in container, maybe we
should also check the errno of access().

> Reviewed-by: James Clark <james.clark@linaro.org>
> Signed-off-by: Changbin Du <changbin.du@huawei.com>
> Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>

-- 
Cheers,
Changbin Du