RE: [PATCH V1 01/13] selftests/resctrl: Fix feature detection

[Prakhya, Sai Praneeth]

Hi Reinette,

> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Sent: Monday, March 9, 2020 2:45 PM
> To: Prakhya, Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx>;
> shuah@xxxxxxxxxx; linux-kselftest@xxxxxxxxxxxxxxx
> Cc: tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; bp@xxxxxxxxx; Luck, Tony
> <tony.luck@xxxxxxxxx>; babu.moger@xxxxxxx; james.morse@xxxxxxx;
> Shankar, Ravi V <ravi.v.shankar@xxxxxxxxx>; Yu, Fenghua
> <fenghua.yu@xxxxxxxxx>; x86@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH V1 01/13] selftests/resctrl: Fix feature detection
> 
> Hi Sai,
> 
> On 3/6/2020 7:40 PM, Sai Praneeth Prakhya wrote:
> > From: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> >
> > The intention of the resctrl selftests is to only run the tests
> > associated with the feature(s) supported by the platform. Through
> > parsing of the feature flags found in /proc/cpuinfo it is possible to
> > learn which features are supported by the plaform.
> >
> > There are currently two issues with the platform feature detection
> > that together result in tests always being run, whether the platform
> > supports a feature or not. First, the parsing of the the feature flags
> > loads the line containing the flags in a buffer that is too small (256
> > bytes) to always contain all flags. The consequence is that the flags
> > of the features being tested for may not be present in the buffer.
> > Second, the actual test for presence of a feature has an error in the
> > logic, negating the test for a particular feature flag instead of
> > testing for the presence of a particular feature flag.
> >
> > These two issues combined results in all tests being run on all
> > platforms, whether the feature is supported or not.
> >
> > Fix these issue by (1) increasing the buffer size being used to parse
> > the feature flags, and (2) change the logic to test for presence of
> > the feature being tested for.
> >
> > Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> > Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
> > ---
> >  tools/testing/selftests/resctrl/resctrlfs.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/testing/selftests/resctrl/resctrlfs.c
> > b/tools/testing/selftests/resctrl/resctrlfs.c
> > index 19c0ec4045a4..226dd7fdcfb1 100644
> > --- a/tools/testing/selftests/resctrl/resctrlfs.c
> > +++ b/tools/testing/selftests/resctrl/resctrlfs.c
> > @@ -596,11 +596,11 @@ bool check_resctrlfs_support(void)
> >
> >  char *fgrep(FILE *inf, const char *str)  {
> > -	char line[256];
> >  	int slen = strlen(str);
> > +	char line[2048];
> >
> >  	while (!feof(inf)) {
> > -		if (!fgets(line, 256, inf))
> > +		if (!fgets(line, 2048, inf))
> >  			break;
> >  		if (strncmp(line, str, slen))
> >  			continue;
> > @@ -631,7 +631,7 @@ bool validate_resctrl_feature_request(char
> *resctrl_val)
> >  	if (res) {
> >  		char *s = strchr(res, ':');
> >
> > -		found = s && !strstr(s, resctrl_val);
> > +		found = s && strstr(s, resctrl_val);
> >  		free(res);
> >  	}
> >  	fclose(inf);
> >
> 
> Please note that this is only a partial fix. The current feature detection relies on
> the feature flags found in /proc/cpuinfo. Quirks and kernel boot parameters are
> not taken into account. This fix only addresses the parsing of feature flags. If a
> feature has been disabled via kernel boot parameter or quirk then the resctrl
> tests would still attempt to run the test for it.

That's a good point and makes sense to me. I think we could fix it in two ways
1. grep for strings in dmesg but that will still leave ambiguity in deciding b/w mbm and cqm because kernel prints "resctrl: L3 monitoring detected" for both the features
2. Check in "info" directory
	a. For cat_l3, we could search for info/L3
	b. For mba, we could search for info/MB
	c. For cqm and mbm, we could search for specified string in info/L3_MON/mon_features

I think option 2 might be better because it can handle all cases, please let me know what you think.

Regards,
Sai