Re: [PATCH v3 3/6] ARCv2: perf: Support sampling events using overflow interrupts

Peter Zijlstra
2015-08-26T14:25:53Z

On Wed, Aug 26, 2015 at 03:12:25PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 24, 2015 at 05:20:20PM +0300, Alexey Brodkin wrote:
> > @@ -295,6 +317,16 @@ static int arc_pmu_add(struct perf_event *event, int flags)
> > }
> >
> > write_aux_reg(ARC_REG_PCT_INDEX, idx);
> > +
> > + arc_pmu->act_counter[idx] = event;
> > +
> > + if (is_sampling_event(event)) {
> > + /* Mimic full counter overflow as other arches do */
> > + write_aux_reg(ARC_REG_PCT_INT_CNTL, (u32)arc_pmu->max_period);
> > + write_aux_reg(ARC_REG_PCT_INT_CNTH,
> > + (arc_pmu->max_period >> 32));
> > + }
> > +
>
> pmu::add should call pmu::start when PERF_EF_START, without that it
> should not start the counter, only schedule it.
>
> (although currently all pmu::add() calls will have EF_START set)
>
> > write_aux_reg(ARC_REG_PCT_CONFIG, 0);
> > write_aux_reg(ARC_REG_PCT_COUNTL, 0);
> > write_aux_reg(ARC_REG_PCT_COUNTH, 0);

Does the below clarify things a bit? If there's still some uncertainty
please say what/where and I'll try and expand.

---
include/linux/perf_event.h | 100 +++++++++++++++++++++++++++++++++++++++------
1 file changed, 87 insertions(+), 13 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2027809433b3..8f78a0b7bfe5 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -140,27 +140,60 @@ struct hw_perf_event {
};
#endif
};
+ /*
+ * If the event is a per task event, this will point to the task in
+ * question. See the comment in perf_event_alloc().
+ */
struct task_struct *target;
+
+/*
+ * hw_perf_event::state flags; used to track the PERF_EF_* state.
+ */
+#define PERF_HES_STOPPED 0x01 /* the counter is stopped */
+#define PERF_HES_UPTODATE 0x02 /* event->count up-to-date */
+#define PERF_HES_ARCH 0x04
+
int state;
+
+ /*
+ * The last observed hardware counter value, updated with a
+ * local64_cmpxchg() such that pmu::read() can be called nested.
+ */
local64_t prev_count;
+
+ /*
+ * The period to start the next sample with.
+ */
u64 sample_period;
+
+ /*
+ * The period we started this sample with.
+ */
u64 last_period;
+
+ /*
+ * However much is left of the current period; note that this is
+ * a full 64bit value and allows for generation of periods longer
+ * than hardware might allow.
+ */
local64_t period_left;
+
+ /*
+ * State for throttling the event, see __perf_event_overflow() and
+ * perf_adjust_freq_unthr_context().
+ */
u64 interrupts_seq;
u64 interrupts;

+ /*
+ * State for freq target events, see __perf_event_overflow() and
+ * perf_adjust_freq_unthr_context().
+ */
u64 freq_time_stamp;
u64 freq_count_stamp;
#endif
};

-/*
- * hw_perf_event::state flags
- */
-#define PERF_HES_STOPPED 0x01 /* the counter is stopped */
-#define PERF_HES_UPTODATE 0x02 /* event->count up-to-date */
-#define PERF_HES_ARCH 0x04
-
struct perf_event;

/*
@@ -210,7 +243,19 @@ struct pmu {

/*
* Try and initialize the event for this PMU.
- * Should return -ENOENT when the @event doesn't match this PMU.
+ *
+ * Returns:
+ * -ENOENT -- @event is not for this PMU
+ *
+ * -ENODEV -- @event is for this PMU but PMU not present
+ * -EBUSY -- @event is for this PMU but PMU temporarily unavailable
+ * -EINVAL -- @event is for this PMU but @event is not valid
+ * -EOPNOTSUPP -- @event is for this PMU, @event is valid, but not supported
+ * -EACCESS -- @event is for this PMU, @event is valid, but no privilidges
+ *
+ * 0 -- @event is for this PMU and valid
+ *
+ * Other error return values are allowed.
*/
int (*event_init) (struct perf_event *event);

@@ -221,27 +266,56 @@ struct pmu {
void (*event_mapped) (struct perf_event *event); /*optional*/
void (*event_unmapped) (struct perf_event *event); /*optional*/

+ /*
+ * Flags for ->add()/->del()/ ->start()/->stop(). There are
+ * matching hw_perf_event::state flags.
+ */
#define PERF_EF_START 0x01 /* start the counter when adding */
#define PERF_EF_RELOAD 0x02 /* reload the counter when starting */
#define PERF_EF_UPDATE 0x04 /* update the counter when stopping */

/*
- * Adds/Removes a counter to/from the PMU, can be done inside
- * a transaction, see the ->*_txn() methods.
+ * Adds/Removes a counter to/from the PMU, can be done inside a
+ * transaction, see the ->*_txn() methods.
+ *
+ * The add/del callbacks will reserve all hardware resources required
+ * to service the event, this includes any counter constraint
+ * scheduling etc.
+ *
+ * Called with IRQs disabled and the PMU disabled.
+ *
+ * ->add() called without PERF_EF_START should result in the same state
+ * as ->add() followed by ->stop().
+ *
+ * ->del() must always PERF_EF_UPDATE stop an event. If it calls
+ * ->stop() that must deal with already being stopped without
+ * PERF_EF_UPDATE.
*/
int (*add) (struct perf_event *event, int flags);
void (*del) (struct perf_event *event, int flags);

/*
- * Starts/Stops a counter present on the PMU. The PMI handler
- * should stop the counter when perf_event_overflow() returns
- * !0. ->start() will be used to continue.
+ * Starts/Stops a counter present on the PMU.
+ *
+ * The PMI handler should stop the counter when perf_event_overflow()
+ * returns !0. ->start() will be used to continue.
+ *
+ * Also used to change the sample period.
+ *
+ * ->stop() with PERF_EF_UPDATE will read the counter and update
+ * period/count values like ->read() would.
+ *
+ * ->start() with PERF_EF_RELOAD will reprogram the the counter
+ * value, must be preceded by a ->stop() with PERF_EF_UPDATE.
*/
void (*start) (struct perf_event *event, int flags);
void (*stop) (struct perf_event *event, int flags);

/*
* Updates the counter value of the event.
+ *
+ * For sampling capable PMUs this will also update the software period
+ * hw_perf_event::period_left field.
*/
void (*read) (struct perf_event *event);