Re: [PATCH v7 1/4] perf: Support PERF_SAMPLE_READ with inherit

[Peter Zijlstra]

On Fri, Jun 07, 2024 at 10:16:15AM +0000, Ben Gainey wrote:

> > > @@ -3532,11 +3544,18 @@ perf_event_context_sched_out(struct
> > > task_struct *task, struct task_struct *next)
> > >   perf_ctx_disable(ctx, false);
> > >  
> > >   /* PMIs are disabled; ctx->nr_pending is stable. */
> > > - if (local_read(&ctx->nr_pending) ||
> > > + if (ctx->nr_inherit_read ||
> > > +     next_ctx->nr_inherit_read ||
> > > +     local_read(&ctx->nr_pending) ||
> > >       local_read(&next_ctx->nr_pending)) {
> > 
> > This seems unfortunate, nr_pending and nr_inherit_read are both used
> > exclusively to inhibit this context switch optimization. Surely they
> > can
> > share the exact same counter.
> > 
> > That is, rename nr_pending and use it for both?
> 
> Sure, how about "nr_no_switch_fast" ?

Yeah, I suppose.


> Sure, presumably you are happy with just calling
> "perf_event_count(event, false)" everywhere it is currently used,
> rather than renaming it to something shorter and keeping the two
> functions?

Yeah, there aren't *that* many instances. Your current patch already
touches them all anyway.

> > That is, I would really rather you had:
> > 
> > static inline u64 perf_event_count(struct perf_event *event, bool
> > self)
> > {
> >  if (self)
> >  return local64_read(&event->count);
> > 
> >  return local64_read(&event->count) + local64_read(&event-
> > >child_count);
> > }
> > 
> > And then actually use that argument as intended.
> 
> 
> Fair point.
> 
> I was trying to avoid the 3 subsequent uses all having to repeat
> "from_sample && has_inherit_and_sample_read(&event->attr)", which feels
> a bit of a pit-trappy. 
> 
> I suppose I could pull that into a "use_self_value(from_sample,event)"?

IIRC they all originate in a single location around perf_output_read(),
that already has the event and could easily 'correct' the semantic
meaning by doing the above once or so.

> > 
> > > @@ -7205,13 +7232,14 @@ void perf_event__output_id_sample(struct
> > > perf_event *event,
> > >  
> > >  static void perf_output_read_one(struct perf_output_handle
> > > *handle,
> > >   struct perf_event *event,
> > > - u64 enabled, u64 running)
> > > + u64 enabled, u64 running,
> > > + bool from_sample)
> > >  {
> > >   u64 read_format = event->attr.read_format;
> > >   u64 values[5];
> > >   int n = 0;
> > >  
> > > - values[n++] = perf_event_count(event);
> > > + values[n++] = perf_event_count(event, from_sample);
> > 
> > ...observe the fail... from_sample != self-value-only
> 
> By fail you are referring to the difference in names?

The difference in meaning, one is from-sample, the other is self-value.
Per the extra condition squirrelled away they are not equivalent.