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Date: Mon, 24 Feb 2014 12:28:39 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Vince Weaver <vincent.weaver@...ne.edu>
Cc: Dave Jones <davej@...hat.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>,
Paul Mackerras <paulus@...ba.org>,
Steven Rostedt <rostedt@...dmis.org>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: x86_pmu_start WARN_ON.
On Fri, Feb 21, 2014 at 03:18:38PM -0500, Vince Weaver wrote:
> I've applied the patch and have been unable to trigger the warning with
> either my testcase or a few hours of fuzzing.
Yay.
> My only comment on the patch is it could always use some comments.
>
> The perf_event code is really hard to follow as is, without adding
> more uncommented special cases.
Does the below help a bit? Or is there anywhere in particular you want
more comments?
---
Subject: perf, x86: Add a few more comments
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Feb 24 12:26:21 CET 2014
Add a few comments on the ->add(), ->del() and ->*_txn()
implementation.
Requested-by: Vince Weaver <vincent.weaver@...ne.edu>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
arch/x86/kernel/cpu/perf_event.c | 49 +++++++++++++++++++++++++++------------
arch/x86/kernel/cpu/perf_event.h | 8 +++---
2 files changed, 39 insertions(+), 18 deletions(-)
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -892,7 +892,6 @@ static void x86_pmu_enable(struct pmu *p
* hw_perf_group_sched_in() or x86_pmu_enable()
*
* step1: save events moving to new counters
- * step2: reprogram moved events into new counters
*/
for (i = 0; i < n_running; i++) {
event = cpuc->event_list[i];
@@ -918,6 +917,9 @@ static void x86_pmu_enable(struct pmu *p
x86_pmu_stop(event, PERF_EF_UPDATE);
}
+ /*
+ * step2: reprogram moved events into new counters
+ */
for (i = 0; i < cpuc->n_events; i++) {
event = cpuc->event_list[i];
hwc = &event->hw;
@@ -1043,7 +1045,7 @@ static int x86_pmu_add(struct perf_event
/*
* If group events scheduling transaction was started,
* skip the schedulability test here, it will be performed
- * at commit time (->commit_txn) as a whole
+ * at commit time (->commit_txn) as a whole.
*/
if (cpuc->group_flag & PERF_EVENT_TXN)
goto done_collect;
@@ -1058,6 +1060,10 @@ static int x86_pmu_add(struct perf_event
memcpy(cpuc->assign, assign, n*sizeof(int));
done_collect:
+ /*
+ * Commit the collect_events() state. See x86_pmu_del() and
+ * x86_pmu_*_txn().
+ */
cpuc->n_events = n;
cpuc->n_added += n - n0;
cpuc->n_txn += n - n0;
@@ -1183,28 +1189,38 @@ static void x86_pmu_del(struct perf_even
* If we're called during a txn, we don't need to do anything.
* The events never got scheduled and ->cancel_txn will truncate
* the event_list.
+ *
+ * XXX assumes any ->del() called during a TXN will only be on
+ * an event added during that same TXN.
*/
if (cpuc->group_flag & PERF_EVENT_TXN)
return;
+ /*
+ * Not a TXN, therefore cleanup properly.
+ */
x86_pmu_stop(event, PERF_EF_UPDATE);
for (i = 0; i < cpuc->n_events; i++) {
- if (event == cpuc->event_list[i]) {
-
- if (i >= cpuc->n_events - cpuc->n_added)
- --cpuc->n_added;
+ if (event == cpuc->event_list[i])
+ break;
+ }
- if (x86_pmu.put_event_constraints)
- x86_pmu.put_event_constraints(cpuc, event);
+ if (WARN_ON_ONCE(i == cpuc->n_events)) /* called ->del() without ->add() ? */
+ return;
- while (++i < cpuc->n_events)
- cpuc->event_list[i-1] = cpuc->event_list[i];
+ /* If we have a newly added event; make sure to decrease n_added. */
+ if (i >= cpuc->n_events - cpuc->n_added)
+ --cpuc->n_added;
+
+ if (x86_pmu.put_event_constraints)
+ x86_pmu.put_event_constraints(cpuc, event);
+
+ /* Delete the array entry. */
+ while (++i < cpuc->n_events)
+ cpuc->event_list[i-1] = cpuc->event_list[i];
+ --cpuc->n_events;
- --cpuc->n_events;
- break;
- }
- }
perf_event_update_userpage(event);
}
@@ -1598,7 +1614,8 @@ static void x86_pmu_cancel_txn(struct pm
{
__this_cpu_and(cpu_hw_events.group_flag, ~PERF_EVENT_TXN);
/*
- * Truncate the collected events.
+ * Truncate collected array by the number of events added in this
+ * transaction. See x86_pmu_add() and x86_pmu_*_txn().
*/
__this_cpu_sub(cpu_hw_events.n_added, __this_cpu_read(cpu_hw_events.n_txn));
__this_cpu_sub(cpu_hw_events.n_events, __this_cpu_read(cpu_hw_events.n_txn));
@@ -1609,6 +1626,8 @@ static void x86_pmu_cancel_txn(struct pm
* Commit group events scheduling transaction
* Perform the group schedulability test as a whole
* Return 0 if success
+ *
+ * Does not cancel the transaction on failure; expects the caller to do this.
*/
static int x86_pmu_commit_txn(struct pmu *pmu)
{
--- a/arch/x86/kernel/cpu/perf_event.h
+++ b/arch/x86/kernel/cpu/perf_event.h
@@ -130,9 +130,11 @@ struct cpu_hw_events {
unsigned long running[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
int enabled;
- int n_events;
- int n_added;
- int n_txn;
+ int n_events; /* the # of events in the below arrays */
+ int n_added; /* the # last events in the below arrays;
+ they've never been enabled yet */
+ int n_txn; /* the # last events in the below arrays;
+ added in the current transaction */
int assign[X86_PMC_IDX_MAX]; /* event to counter assignment */
u64 tags[X86_PMC_IDX_MAX];
struct perf_event *event_list[X86_PMC_IDX_MAX]; /* in enabled order */
--
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