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

[Ben Gainey <Ben.Gainey@arm.com>]

On Fri, 2024-06-07 at 11:32 +0200, Peter Zijlstra wrote:
> On Thu, Jun 06, 2024 at 03:40:56PM +0100, Ben Gainey wrote:
> > This change allows events to use PERF_SAMPLE READ with inherit
> > so long as PERF_SAMPLE_TID is also set.
> > 
> > In this configuration, an event will be inherited into any
> > child processes / threads, allowing convenient profiling of a
> > multiprocess or multithreaded application, whilst allowing
> > profiling tools to collect per-thread samples, in particular
> > of groups of counters.
> 
> Perhaps a few words on *WHY* this is important.
> 
> > The read_format field of both PERF_RECORD_READ and
> > PERF_RECORD_SAMPLE
> > are changed by this new configuration, but calls to `read()` on the
> > same
> > event file descriptor are unaffected and continue to return the
> > cumulative total.
> 
> This is unfortunate. Up to this point they were the same. Also, I see
> no
> change to the uapi file. So were you trying to say that only
> read_format::value is changed to be the thread local value as opposed
> to
> the hierarchy total?
> 
> Please fix the wording to be unambiguous as to what is actually
> meant.
> Also try and justify why it is okay to break this symmetry.


Yes, the meaning of the PERF_RECORD_SAMPLE's read_format value changes
in this specific scenario to be a thread-local value rather than the
total.

I'll update and add some justification.

> 
> > @@ -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" ?


> 
> >   /*
> >   * Must not swap out ctx when there's pending
> >   * events that rely on the ctx->task relation.
> > + *
> > + * Likewise, when a context contains inherit +
> > + * SAMPLE_READ events they should be switched
> > + * out using the slow path so that they are
> > + * treated as if they were distinct contexts.
> >   */
> >   raw_spin_unlock(&next_ctx->lock);
> >   rcu_read_unlock();
> > @@ -4552,11 +4571,19 @@ static void __perf_event_read(void *info)
> >   raw_spin_unlock(&ctx->lock);
> >  }
> >  
> > -static inline u64 perf_event_count(struct perf_event *event)
> > +static inline u64 perf_event_count_cumulative(struct perf_event
> > *event)
> 
> I don't think you need this -- overly long and hard to type function
> name...

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?

> 
> >  {
> >   return local64_read(&event->count) + atomic64_read(&event-
> > >child_count);
> >  }
> >  
> > +static inline u64 perf_event_count(struct perf_event *event, bool
> > self_value_only)
> > +{
> > + if (self_value_only && has_inherit_and_sample_read(&event->attr))
> > + return local64_read(&event->count);
> 
> ... if this @self_value_only argument was actually used as such -- it
> isn't, see how you use 'from_sample' which is something else
> entirely.
> Which then also caused to you fix it up and make a mess with that &&
> has_inherit_and_sample_read() nonsense. (also, shorter function
> names,
> more good)
> 
> > +
> > + return perf_event_count_cumulative(event);
> > +}
> 
> 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)"?


> 
> > @@ -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?

> 
> >   if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED) {
> >   values[n++] = enabled +
> >   atomic64_read(&event->child_total_time_enabled);
> 


Thanks
Ben