[LTP] [PATCH v4 1/4] lib: add new cgroup test API

[Richard Palethorpe]

Hello Li,

Li Wang <liwang@redhat.com> writes:

> On Wed, Sep 23, 2020 at 7:03 PM Richard Palethorpe <rpalethorpe@suse.de>
> wrote:
>
>> Hello,
>>
>> Li Wang <liwang@redhat.com> writes:
>>
>> > Many of our LTP tests need Control Group in the configuration,
>> > this patch makes cgroup unified mounting at setup phase to be
>> > possible. The method is extracted from mem.h with the purpose
>> > of extendible for further developing, and trying to compatible
>> > the current two versions of cgroup,
>> >
>> > It's hard to make all APIs be strictly consistent because there
>> > are many differences between v1 and v2. But it capsulate the detail
>> > of cgroup mounting in high-level functions, which will be easier
>> > to use cgroup without considering too much technical thing.
>> >
>> > Btw, test get passed on RHEL7(x86_64), RHEL8(ppc64le),
>> > Fedora32(x86_64).
>>
>> This appears to be broken on SUSE.
>>
>> > +enum tst_cgroup_ver tst_cgroup_version(void)
>> > +{
>> > +     if (tst_cgroup_check("cgroup2")) {
>> > +             if (!tst_is_mounted("cgroup2") && tst_is_mounted("cgroup"))
>>
>> Unfortunately this is backwards: if a cgroup controller is mounted
>> (e.g. memory) in the v1 separated hierarchy then it is not available in
>> the cgroup v2 unified hierarchy even though the v2 unified hierarchy
>> exists.
>>
>
> That's on purpose because we don't want the user to mix v1 and v2 in using.
> In other words, if a version of cgroup has been chosen on a SUT then LTP
> follows in the same version.
>
> The cgroup lib choose cgroup version likes:
>       * system doesn't support any cgroup(v1, v2): TCONF
>       * system only support cgroup v1: choose v1
>       * system only support cgroup v2: choose v2
>       * system support v1 & v2 but mounting v1: chosse v1   <=== this is
> what you met

Perhaps you meant:

if (tst_is_mounted("cgroup"))
   cg_ver = TST_CGROUP_V1;
else
   cg_ver = TST_CGROUP_V2;

There is no point trying to use V2 if the controller is active in V1.

>       * system support v1 & v2 but mounting v2: choose v2
>       * system support v1 & v2 but mounting none: choose v2
>
>
>
>>
>> So ksm{03,04} fails with:
>>
>> safe_file_ops.c:299: BROK: Failed to close FILE
>> '/tmp/cgroup_mem/cgroup.subtree_control' at tst_cgroup.c:301: ENOENT (2)
>>
>> because we have the memory controller (in fact all the controllers)
>> mounted in the V1 way, but we also have the cgroup2 mounted. If I
>>
>
> Hmm, this is a situation that system mount v1&v2 at the same time.
>
> So the ksm03/4 choose v2 automatically but hit a problem in cgroup_v2 using.
> I pesonally think it mainly imputes to the SUT config in cgroup, because
> that
> makes cgroup_v2 has no actuall controllers in the cgroup.controllers. After
> umounting all V1 hierarchies, the controllers get back into V2.
>

yes

>
>
>> unmount the memory controller from V1 then the test passes.
>>
>> Another potential problem is that it may be possible to remove access to
>> controllers some other way in which case you have to check
>> cgroup.controllers to see what is available. OTOH maybe this is not
>> possible because you mount a new cgroup2 hierarchy?
>>
>
> I'm not sure about this.
>
> To be honest, I hope a Linux distribution provides a pure way in cgroup
> using, which means just mounting V1 or V2 but not to mix together. Or, do
> you think LTP should handle the situation for the v1&v2 mixing mount?

For now, I think we just need to use V1 if it is mounted. I don't like
the fact we have both mounted, but it seems most software can handle it,
so LTP should as well.

-- 
Thank you,
Richard.