Re: [PATCH v2 2/7] perf: Use perf_pmu__open_file() and perf_pmu__scan_file()

[James Clark]

On 23/12/2022 03:45, Leo Yan wrote:
> On Thu, Dec 22, 2022 at 04:03:22PM +0000, James Clark wrote:
>> Remove some code that duplicates existing methods. This requires that
>> some consts are removed because one of the existing helper methods takes
>> a struct perf_pmu instead of a name which has a non const name field.
>> But except for the tests, the strings were already non const.
>>
>> No functional changes.
>>
>> Signed-off-by: James Clark <james.clark@xxxxxxx>
>> ---
>>  tools/perf/tests/evsel-roundtrip-name.c |  3 +-
>>  tools/perf/tests/parse-events.c         |  3 +-
>>  tools/perf/util/cputopo.c               |  9 +-----
>>  tools/perf/util/pmu-hybrid.c            | 27 +++-------------
>>  tools/perf/util/pmu-hybrid.h            |  2 +-
>>  tools/perf/util/pmu.c                   | 42 +++++++------------------
>>  tools/perf/util/pmu.h                   |  3 +-
>>  7 files changed, 24 insertions(+), 65 deletions(-)
>>
>> diff --git a/tools/perf/tests/evsel-roundtrip-name.c b/tools/perf/tests/evsel-roundtrip-name.c
>> index e94fed901992..9bccf177f481 100644
>> --- a/tools/perf/tests/evsel-roundtrip-name.c
>> +++ b/tools/perf/tests/evsel-roundtrip-name.c
>> @@ -103,8 +103,9 @@ static int test__perf_evsel__roundtrip_name_test(struct test_suite *test __maybe
>>  						 int subtest __maybe_unused)
>>  {
>>  	int err = 0, ret = 0;
>> +	char cpu_atom[] = "cpu_atom";
>>  
>> -	if (perf_pmu__has_hybrid() && perf_pmu__hybrid_mounted("cpu_atom"))
>> +	if (perf_pmu__has_hybrid() && perf_pmu__hybrid_mounted(cpu_atom))
> 
> After change the parameter 'name' to non const type in function
> perf_pmu__hybrid_mounted(), at here we still can pass string "cpu_atom"
> without warning, right?  If so, we don't need to define a local
> variable 'cpu_atom'.
> 
> [...]
> 
>> -bool perf_pmu__hybrid_mounted(const char *name)
>> +bool perf_pmu__hybrid_mounted(char *name)
>>  {
>> -	char path[PATH_MAX];
>> -	const char *sysfs;
>> -	FILE *file;
>> -	int n, cpu;
>> +	int cpu;
>> +	struct perf_pmu pmu = {.name = name};
>>  
>>  	if (strncmp(name, "cpu_", 4))
>>  		return false;
>>  
>> -	sysfs = sysfs__mountpoint();
>> -	if (!sysfs)
>> -		return false;
>> -
>> -	snprintf(path, PATH_MAX, CPUS_TEMPLATE_CPU, sysfs, name);
>> -	if (!file_available(path))
>> -		return false;
>> -
>> -	file = fopen(path, "r");
>> -	if (!file)
>> -		return false;
>> -
>> -	n = fscanf(file, "%u", &cpu);
>> -	fclose(file);
>> -	if (n <= 0)
>> -		return false;
>> -
>> -	return true;
>> +	return perf_pmu__scan_file(&pmu, "cpus", "%u", &cpu) > 0;
> 
> It's not necessarily to change the parameter 'name' to non const type,
> we can handle this case in two different ways.
> 
> The first option is to refine the code with using the function
> perf_pmu__pathname_scnprintf() which is introduced in patch 01, thus
> we can keep using fopen() and fscanf() to read out value from 'cpus'
> entry.
> 
> Another option is to define a local array 'pmu_name[PATH_MAX]' and
> then copy the input string to this array, something like:
> 
> bool perf_pmu__hybrid_mounted(const char *name)
> {
>   char pmu_name[PATH_MAX];
>   struct perf_pmu pmu;
>   int cpu;
> 
>   strncpy(pmu_name, name, sizeof(pmu_name));
>   pmu.name = pmu_name;
> 
>   return perf_pmu__scan_file(&pmu, "cpus", "%u", &cpu) > 0;
> }
> 
> We even can consider to dynamically allocate buffer and copy string from
> 'name' to the allocated buffer.
> 

I went with the string copy way and put the consts back in V3. One minor
difference is that I had to use strlcpy() instead of strncpy() to avoid
a compiler warning about the destination buffer being the same size.

James

[...]

>> -static struct perf_cpu_map *pmu_cpumask(const char *name)
>> +static struct perf_cpu_map *pmu_cpumask(char *name)
>>  {
>> -	char path[PATH_MAX];
>>  	struct perf_cpu_map *cpus;
>> -	const char *sysfs = sysfs__mountpoint();
>>  	const char *templates[] = {
>> -		CPUS_TEMPLATE_UNCORE,
>> -		CPUS_TEMPLATE_CPU,
>> +		"cpumask",
>> +		"cpus",
>>  		NULL
>>  	};
>>  	const char **template;
>> -
>> -	if (!sysfs)
>> -		return NULL;
>> +	struct perf_pmu pmu = {.name = name};
> 
> Here we also can define a local array and copy string from 'name' to
> the local array.  This can allow us to provide a friendly function and
> caller doesn't need to explictly pass non const string.
> 

Done