Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu_capacity

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

+ Phil Auld <pauld@redhat.com>

On 28/08/2020 19:26, Qais Yousef wrote:
> On 08/28/20 19:10, Dietmar Eggemann wrote:
>> On 28/08/2020 12:27, Qais Yousef wrote:
>>> On 08/28/20 10:00, vincent.donnefort@arm.com wrote:
>>>> From: Vincent Donnefort <vincent.donnefort@arm.com>

[...]

>> Can you remind me why we have all these helper functions like
>> sched_trace_rq_cpu_capacity?
> 
> struct rq is defined in kernel/sched/sched.h. It's not exported. Exporting
> these helper functions was the agreement to help modules trace internal info.
> By passing generic info you decouple the tracepoint from giving specific info
> and allow the modules to extract all the info they need from the same
> tracepoint. IE: if you need more than just cpu_capacity from this tracepoint,
> you can get that without having to continuously add extra arguments everytime
> you need an extra piece of info. Unless this info is not in the rq of course.

I think this decoupling is not necessary. The natural place for those
scheduler trace_event based on trace_points extension files is
kernel/sched/ and here the internal sched.h can just be included.

If someone really wants to build this as an out-of-tree module there is
an easy way to make kernel/sched/sched.h visible.

CFLAGS_sched_tp.o := -I$KERNEL_SRC/kernel/sched

all:
    make -C $KERNEL_SRC M=$(PWD) modules

This allowed me to build our trace_event extension module (sched_tp.c,
sched_events.h) out-of-tree and I was able to get rid of all the
sched_trace_foo() functions (in fair.c, include/linux/sched.h) and code
there content directly in foo.c

There are two things we would need exported from the kernel:

(1) cfs_rq_tg_path() to print the path of a taskgroup cfs_rq or se.

(2) sched_uclamp_used so uclamp_rq_util_with() can be used in
    sched_events.h.

I put Phil Auld on cc because of his trace_point
sched_update_nr_running_tp. I think Phil was using sched_tp as a base so
I can't see an issue why we can't also remove sched_trace_rq_nr_running().

>> In case we would let the extra code (which transforms trace points into
>> trace events) know the internals of struct rq we could handle those
>> things in the TRACE_EVENT and/or the register_trace_##name(void
>> (*probe)(data_proto), void *data) thing.
>> We always said when the internal things will change this extra code will
>> break. So that's not an issue.
> 
> The problem is that you need to export struct rq in a public header. Which we
> don't want to do. I have been trying to find out how to use BTF so we can
> remove these functions. Haven't gotten far away yet - but it should be doable
> and it's a question of me finding enough time to understand what was currently
> done and if I can re-use something or need to come up with extra infrastructure
> first.

Let's keep the footprint of these trace points as small as possible in
the scheduler code.

I'm putting the changes I described above in our monthly EAS integration
right now and when this worked out nicely I will share the patches on lkml.