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Subject[PATCH 3/3] perf: Fix mux_interval hrtimer wreckage
Thomas stumbled over the hrtimer_forward_now() in
perf_event_mux_interval_ms_store() and noticed its broken-ness.

You cannot just change the expiry time of an active timer, it will
destroy the red-black tree order and cause havoc.

Change it to (re)start the timer instead, (re)starting a timer will
dequeue and enqueue a timer and therefore preserve rb-tree order.

Since we cannot enqueue remotely, wrap the thing in
cpu_function_call(), this however mandates that we restrict ourselves
to online cpus. Also serialize the entire setting so we don't get
multiple concurrent threads trying to update to different values.

Also fix a problem in perf_mux_hrtimer_restart(), checking against
hrtimer_active() can actually loose us the timer when timer->state ==
HRTIMER_STATE_CALLBACK and the callback has already decided NORESTART.

Furthermore it doesn't make any sense to test
hrtimer_callback_running() when we already tested hrtimer_active(),
but with the above change, we explicitly must call it when
callback_running.

Lastly, rename a few functions:

s/perf_cpu_hrtimer_/perf_mux_hrtimer_/ -- because I could not find
the mux timer function

s/\<hr\>/timer/ -- because that's the normal way of calling things.

Fixes: 62b856397927 ("perf: Add sysfs entry to adjust multiplexing interval per PMU")
Cc: Stephane Eranian <eranian@google.com>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/events/core.c | 62 ++++++++++++++++++++++++++++-----------------------
1 file changed, 35 insertions(+), 27 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -51,9 +51,11 @@

static struct workqueue_struct *perf_wq;

+typedef int (*remote_function_f)(void *);
+
struct remote_function_call {
struct task_struct *p;
- int (*func)(void *info);
+ remote_function_f func;
void *info;
int ret;
};
@@ -86,7 +88,7 @@ static void remote_function(void *data)
* -EAGAIN - when the process moved away
*/
static int
-task_function_call(struct task_struct *p, int (*func) (void *info), void *info)
+task_function_call(struct task_struct *p, remote_function_f func, void *info)
{
struct remote_function_call data = {
.p = p,
@@ -110,7 +112,7 @@ task_function_call(struct task_struct *p
*
* returns: @func return value or -ENXIO when the cpu is offline
*/
-static int cpu_function_call(int cpu, int (*func) (void *info), void *info)
+static int cpu_function_call(int cpu, remote_function_f func, void *info)
{
struct remote_function_call data = {
.p = NULL,
@@ -747,7 +749,7 @@ perf_cgroup_mark_enabled(struct perf_eve
/*
* function must be called with interrupts disbled
*/
-static enum hrtimer_restart perf_cpu_hrtimer_handler(struct hrtimer *hr)
+static enum hrtimer_restart perf_mux_hrtimer_handler(struct hrtimer *hr)
{
struct perf_cpu_context *cpuctx;
enum hrtimer_restart ret = HRTIMER_NORESTART;
@@ -771,7 +773,7 @@ static enum hrtimer_restart perf_cpu_hrt
}

/* CPU is going down */
-void perf_cpu_hrtimer_cancel(int cpu)
+void perf_mux_hrtimer_cancel(int cpu)
{
struct perf_cpu_context *cpuctx;
struct pmu *pmu;
@@ -798,11 +800,11 @@ void perf_cpu_hrtimer_cancel(int cpu)
local_irq_restore(flags);
}

-static void __perf_cpu_hrtimer_init(struct perf_cpu_context *cpuctx, int cpu)
+static void __perf_mux_hrtimer_init(struct perf_cpu_context *cpuctx, int cpu)
{
- struct hrtimer *hr = &cpuctx->hrtimer;
+ struct hrtimer *timer = &cpuctx->hrtimer;
struct pmu *pmu = cpuctx->ctx.pmu;
- int timer;
+ u64 interval;

/* no multiplexing needed for SW PMU */
if (pmu->task_ctx_nr == perf_sw_context)
@@ -812,29 +814,29 @@ static void __perf_cpu_hrtimer_init(stru
* check default is sane, if not set then force to
* default interval (1/tick)
*/
- timer = pmu->hrtimer_interval_ms;
- if (timer < 1)
- timer = pmu->hrtimer_interval_ms = PERF_CPU_HRTIMER;
+ interval = pmu->hrtimer_interval_ms;
+ if (interval < 1)
+ interval = pmu->hrtimer_interval_ms = PERF_CPU_HRTIMER;

- cpuctx->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * timer);
+ cpuctx->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * interval);

- hrtimer_init(hr, CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED);
- hr->function = perf_cpu_hrtimer_handler;
+ hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED);
+ timer->function = perf_mux_hrtimer_handler;
}

-static void perf_cpu_hrtimer_restart(struct perf_cpu_context *cpuctx)
+static int perf_mux_hrtimer_restart(struct perf_cpu_context *cpuctx)
{
- struct hrtimer *hr = &cpuctx->hrtimer;
+ struct hrtimer *timer = &cpuctx->hrtimer;
struct pmu *pmu = cpuctx->ctx.pmu;

/* not for SW PMU */
if (pmu->task_ctx_nr == perf_sw_context)
- return;
+ return 0;

- if (hrtimer_active(hr))
- return;
+ if (hrtimer_is_queued(timer))
+ return 0;

- hrtimer_start(hr, cpuctx->hrtimer_interval, HRTIMER_MODE_REL_PINNED);
+ hrtimer_start(timer, cpuctx->hrtimer_interval, HRTIMER_MODE_REL_PINNED);
}

void perf_pmu_disable(struct pmu *pmu)
@@ -1913,7 +1915,7 @@ group_sched_in(struct perf_event *group_

if (event_sched_in(group_event, cpuctx, ctx)) {
pmu->cancel_txn(pmu);
- perf_cpu_hrtimer_restart(cpuctx);
+ perf_mux_hrtimer_restart(cpuctx);
return -EAGAIN;
}

@@ -1960,7 +1962,7 @@ group_sched_in(struct perf_event *group_

pmu->cancel_txn(pmu);

- perf_cpu_hrtimer_restart(cpuctx);
+ perf_mux_hrtimer_restart(cpuctx);

return -EAGAIN;
}
@@ -2233,7 +2235,7 @@ static int __perf_event_enable(void *inf
*/
if (leader != event) {
group_sched_out(leader, cpuctx, ctx);
- perf_cpu_hrtimer_restart(cpuctx);
+ perf_mux_hrtimer_restart(cpuctx);
}
if (leader->attr.pinned) {
update_group_times(leader);
@@ -7143,6 +7145,8 @@ perf_event_mux_interval_ms_show(struct d
return snprintf(page, PAGE_SIZE-1, "%d\n", pmu->hrtimer_interval_ms);
}

+static DEFINE_MUTEX(mux_interval_mutex);
+
static ssize_t
perf_event_mux_interval_ms_store(struct device *dev,
struct device_attribute *attr,
@@ -7162,17 +7166,21 @@ perf_event_mux_interval_ms_store(struct
if (timer == pmu->hrtimer_interval_ms)
return count;

+ mutex_lock(&mux_interval_mutex);
pmu->hrtimer_interval_ms = timer;

/* update all cpuctx for this PMU */
- for_each_possible_cpu(cpu) {
+ get_online_cpus();
+ for_each_online_cpu(cpu) {
struct perf_cpu_context *cpuctx;
cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
cpuctx->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * timer);

- if (hrtimer_active(&cpuctx->hrtimer))
- hrtimer_forward_now(&cpuctx->hrtimer, cpuctx->hrtimer_interval);
+ cpu_function_call(cpu,
+ (remote_function_f)perf_mux_hrtimer_restart, cpuctx);
}
+ put_online_cpus();
+ mutex_unlock(&mux_interval_mutex);

return count;
}
@@ -7277,7 +7285,7 @@ int perf_pmu_register(struct pmu *pmu, c
lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
cpuctx->ctx.pmu = pmu;

- __perf_cpu_hrtimer_init(cpuctx, cpu);
+ __perf_mux_hrtimer_init(cpuctx, cpu);

cpuctx->unique_pmu = pmu;
}



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