Re: [LTP] [PATCH 0/3] cpuset_regression_test: convert and improve

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello Joerg,

Joerg Vehlow <lkml@jv-coder.de> writes:

> Hi Richard,
>
> On 6/23/2021 1:11 PM, Richard Palethorpe wrote:
>> Hello Joerg,
>>
>> Joerg Vehlow <lkml@jv-coder.de> writes:
>>
>>> Hi,
>>>
>>> this is more or less a v2 of a patch I send previously (patch 3).
>>> I know that richard is not entirely happy with this patch, I will
>>> give it another try anyway...
>>> It is either this patch or another patch, that has to look through
>>> the cgroup hierarchy, to check if there is any group,that explicitely
>>> uses cpu 0.
>> If it is already being used then can you set it?
> The test can use any cpu, that is not explicitly assigned to a group
> already.
> What I mean by "either this or another patch" (and forgot to type),
> the alternative
> patch has to check if anything is using cpu 0 explicitly and then fail
> with TCONF.
> Or it has to look for an used cpu core. That would be another possibility...
>
>>
>>> To me, it is a better solution to just change groups for a short time,
>>> and check if the bug exists. If ltp tests are running, the chance, that
>>> there is anything running, that really needs a correct cpuset is very low.
>>> I am comming from a system, where cgroups are setup by a container launcher,
>>> that just happens to assign cpus to the containers - not even exclusively.
>>> LTP tests are used as some part of the testsuite, to test as close to a
>>> production system as possible.
>> I was thinking that if you are already using CPU sets then you either
>> don't have the bug or you won't hit it on your setup(s)? If so then the
>> test is redundant.
> True about the "don't hit it part", at least with the setup, but I
> guess the reason for a regression test,
> is to prevent regressions. This was clearly a bug in the kernel and
> not only an inconvenience. And since
> there  is not "the one kernel source", I think it is important to run
> tests like this for as many different
> kernels as possible. Apart from the already setup cgroups, there may
> be other uses of cgroups as well,
> that try to set them up the other way around (first exclusive, then cpus).
>>
>>> The only way I could think of a process misbehaving by disabeling cpu pinning,
>>> would be a badly written multithread application, that cannot correctly run,
>>> if threads are really running in parallel, but this would also require a scheduling
>>> policy, that makes scheduling points predicatable. While I know that software like
>>> that exists (in fact I was working on something like that in the past), I think it
>>> is highly unlikely, that it is running parallel to ltp.
>>> And even then, this could be mitigated by not just setting cpu binding to undefined,
>>> but to one fixed core. But with the changes in patch 2, this is not
>>> possible.
>>>
>>> But anyhow ltp fiddles with lots of critical system parameters during it's runtime,
>>> there is no guarantee, that an application that requires some very specific kernel
>>> runtime settings survives this. That's why I would still vote for this patch.
>>>
>>> Jörg
>> I still think it has a small chance of causing problems for us. There
>> are some heterogeneous CPU systems where control software should run on
>> a given CPU. I don't know whether CGroups are used to control that or if
>> it would matter if the process is moved temporarily. It's just a small
>> risk I would avoid if the test is not really worth it.
> I get that, but these systems may have to opt-out of some tests anyway.
>>
>> OTOH the patch looks good otherwise, so it should be merged if no one
>> else agrees with me.
> Ok, lets see what the others have to say :)
>
> Jörg

So a few months later there are no comments. The patch-set as a whole
looks a like an improvement. So let's merge it.


-- 
Thank you,
Richard.

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