Re: [PATCH 24/24] selftests/resctrl: Ignore failures from L2 CAT test with <= 2 bits

[Ilpo Järvinen]

On Thu, 2 Nov 2023, Reinette Chatre wrote:
> On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:
> > L2 CAT test with low number of bits tends to occasionally fail because
> > of what seems random variation. The margin is quite small to begin with
> > for <= 2 bits in CBM. At times, the result can even become negative.
> > While it would be possible to allow negative values for those cases, it
> > would be more confusing to user.
> > 
> > Ignore failures from the tests where <= 2 were used to avoid false
> > negative results.
> > 
> 
> I think the core message is that 2 or fewer bits should not be used. Instead
> of running the test and ignoring the results the test should perhaps just not
> be run.

I considered that but it often does work so it felt shame to now present
them when they're successful. Then I just had to decide how to deal with
the cases where they failed.

Also, if I make it to not run down to 1 bit, those numbers will never ever 
be seen by anyone. It doesn't say 2 and 1 bit results don't contain any 
information to a human reader who is able to do more informed decisions 
whether something is truly working or not. We could, hypothetically, have 
a HW issue one day which makes 1-bit L2 mask to misbehave and if the 
number is never seen by anyone, it's extremely unlikely to be caught 
easily.

They are just reliable enough for simple automated threshold currently. 
Maybe something else than average value would be, it would need to be 
explored but I suspect also the memory address of the buffer might affect 
the value, with L3 it definitely should because of how the things work but 
I don't know if that holds for L2 too. I have earlier tried playing with 
the buffer addresses with L3 but as I didn't immediately yield positive 
outcome to guard against outliers, I postponed that investigation (e.g., 
my alloc pattern might have been too straightforward and didn't provide 
enough entropy into the buffer start address because I just alloc'ed n x 
buf_size buffers back-to-back).

But I don't have very strong opinion on this so if you prefer I just stop 
at 3 bits, I can change it?

-- 
 i.