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Date: Fri, 7 Jun 2024 13:02:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Ben Gainey <Ben.Gainey@arm.com>
Cc: "alexander.shishkin@linux.intel.com" <alexander.shishkin@linux.intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Mark Rutland <Mark.Rutland@arm.com>,
	"acme@kernel.org" <acme@kernel.org>,
	"mingo@redhat.com" <mingo@redhat.com>,
	James Clark <James.Clark@arm.com>,
	"adrian.hunter@intel.com" <adrian.hunter@intel.com>,
	"namhyung@kernel.org" <namhyung@kernel.org>,
	"irogers@google.com" <irogers@google.com>,
	"jolsa@kernel.org" <jolsa@kernel.org>,
	"linux-perf-users@vger.kernel.org" <linux-perf-users@vger.kernel.org>
Subject: Re: [PATCH v7 1/4] perf: Support PERF_SAMPLE_READ with inherit
Message-ID: <20240607110214.GQ8774@noisy.programming.kicks-ass.net>
References: <20240606144059.365633-1-ben.gainey@arm.com>
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 <20240607093254.GN8774@noisy.programming.kicks-ass.net>
 <451afb8eb03f1519c482a84a6c1cbd1e62222988.camel@arm.com>
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In-Reply-To: <451afb8eb03f1519c482a84a6c1cbd1e62222988.camel@arm.com>

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.