Re: [PATCH] xfstests: add a CGROUP configuration option

[Josef Bacik <josef@toxicpanda.com>]

On 2/17/20 11:38 AM, Brian Foster wrote:
> On Fri, Feb 14, 2020 at 03:34:31PM -0500, Josef Bacik wrote:
>> I want to add some extended statistic gathering for xfstests, but it's
>> tricky to isolate xfstests from the rest of the host applications.  The
>> most straightforward way to do this is to run every test inside of it's
>> own cgroup.  From there we can monitor the activity of tasks in the
>> specific cgroup using BPF.
>>
> 
> I'm curious what kind of info you're looking for from tests..
> 

Latencies.  We have all of these tests doing all sorts of interesting things, I 
want to track operation latencies with code we're actually testing so I can see 
if I've introduced a performance regression somewhere.  Since Facebook's whole 
fleet is on btrfs I want to make sure I'm only getting information from things 
being run by xfstests so I can easily go back and hunt down regressions that get 
introduced.  With BPF I can filter on cgroup membership, so I know I'm only 
recording stats I care about.

>> The support for this is pretty simple, allow users to specify
>> CGROUP=/path/to/cgroup.  We will create the path if it doesn't already
>> exist, and validate we can add things to cgroup.procs.  If we cannot
>> it'll be disabled, otherwise we will use this when we do _run_seq by
>> echo'ing the bash pid into cgroup.procs, which will cause any children
>> to run under that cgroup.
>>
> 
> Seems reasonable, but is there any opportunity to combine this with what
> we have in common/cgroup2? It's not clear to me if this cares about
> cgroup v1 or v2, but perhaps the cgroup2 checks could be built on top of
> a generic CGROUP var? I'm also wondering if we'd want to change runtime
> behavior purely based on the existence of the path as opposed to some
> kind of separate knob (in the event some future test requires the path
> for some reason without wanting to enable this mechanism).
> 

Oh I probably should have looked around, yeah we can definitely use this.  My 
initial thought is to just make CGROUP2_PATH exported always, we create 
/path/to/cgroup2/xfstests and point CGROUP2_PATH at that, and then any tests 
that use the cgroup2 path for their test will automatically be populated under 
that global xfstests directory, so I can still capture them with my scripts. 
Does that sound reasonable?  Thanks,

Josef