Re: [PATCH] sched_ext: Add trace point to track sched_ext core events

[Changwoo Min]

Hi Andrea,

Thank you for the review!

On 25. 2. 27. 16:38, Andrea Righi wrote:
> Hi Changwoo,
>
> On Wed, Feb 26, 2025 at 11:33:27PM +0900, Changwoo Min wrote:
> > Add tracing support, which may be useful for debugging sched_ext schedulers
> > that trigger a certain event.
> >
> > Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
> > ---
> >   include/trace/events/sched_ext.h | 21 +++++++++++++++++++++
> >   kernel/sched/ext.c               |  4 ++++
> >   2 files changed, 25 insertions(+)
> >
> > diff --git a/include/trace/events/sched_ext.h b/include/trace/events/sched_ext.h
> > index fe19da7315a9..88527b9316de 100644
> > --- a/include/trace/events/sched_ext.h
> > +++ b/include/trace/events/sched_ext.h
> > @@ -26,6 +26,27 @@ TRACE_EVENT(sched_ext_dump,
> >   	)
> >   );
> >   
> > +TRACE_EVENT(sched_ext_add_event,
> > +	    TP_PROTO(const char *name, int offset, __u64 added),
> > +	    TP_ARGS(name, offset, added),
> > +
> > +	TP_STRUCT__entry(
> > +		__string(name, name)
> > +		__field(	int,		offset		)
> > +		__field(	__u64,		added		)
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__assign_str(name);
> > +		__entry->offset		= offset;
> > +		__entry->added		= added;
> > +	),
> > +
> > +	TP_printk("name %s offset %d added %llu",
> > +		  __get_str(name), __entry->offset, __entry->added
> > +	)
> > +);
>
> Isn't the name enough to determine which event has been triggered? What are
> the benefits of reporting also the offset within struct scx_event_stats?

@name and @offset are duplicated information. However, I thought
having two is more convenient from the users' point of view
because they have different pros and cons.

@offset is quick to compare and can be used easily in the BPF
code, but the offset of an event can change across kernel
versions when new events are added. @offset would be good to
write a quick trace hook for debugging.

On the other hand, @name won't change across kernel versions,
which is good. However, it requires more code to acutally read
the string in the BPF code (__data_loc for string is a 32-bit
integer encoding string length and location).

Does it make sense to you?

Regards,
Changwoo Min