[PATCH v2 0/2] Fix Arm system PMU hotplug issues

[PATCH v2 0/2] Fix Arm system PMU hotplug issues

[Robin Murphy]

Hi all,

After a converstaion with Thomas a while back, it felt like the best way
forward here is just to resolve the pressing preemption violation, and
not make things more complicated by trying to manage theoretical races
within perf core from a distance.

Thus v2 simply nukes the offending logic and brings those drivers in
line with the status quo. Due to the abrupt change in tactic I've not
kept the review tags given for v1.

Robin.


Robin Murphy (2):
  perf/arm-cci: Remove broken race mitigation
  perf/arm-ccn: Remove broken race mitigation

 drivers/perf/arm-cci.c | 16 +++++++---------
 drivers/perf/arm-ccn.c | 23 +++++++++++------------
 2 files changed, 18 insertions(+), 21 deletions(-)

-- 
2.21.0.dirty

[PATCH v2 2/2] perf/arm-ccn: Remove broken race mitigation

[Robin Murphy]

Like arm-cci, arm-ccn has the same issue of disabling preemption around
operations which can take mutexes. Again, remove the definite bug by
simply not trying to fight the theoretical races. And since we are
touching the hotplug handling code, take the opportunity to streamline
it, as there's really no need to store a full-sized cpumask to keep
track of a single CPU ID.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/perf/arm-ccn.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/drivers/perf/arm-ccn.c b/drivers/perf/arm-ccn.c
index 2ae76026e947..a0214308b0cd 100644
--- a/drivers/perf/arm-ccn.c
+++ b/drivers/perf/arm-ccn.c
@@ -167,7 +167,7 @@ struct arm_ccn_dt {
 
 	struct hrtimer hrtimer;
 
-	cpumask_t cpu;
+	unsigned int cpu;
 	struct hlist_node node;
 
 	struct pmu pmu;
@@ -559,7 +559,7 @@ static ssize_t arm_ccn_pmu_cpumask_show(struct device *dev,
 {
 	struct arm_ccn *ccn = pmu_to_arm_ccn(dev_get_drvdata(dev));
 
-	return cpumap_print_to_pagebuf(true, buf, &ccn->dt.cpu);
+	return cpumap_print_to_pagebuf(true, buf, cpumask_of(ccn->dt.cpu));
 }
 
 static struct device_attribute arm_ccn_pmu_cpumask_attr =
@@ -759,7 +759,7 @@ static int arm_ccn_pmu_event_init(struct perf_event *event)
 	 * mitigate this, we enforce CPU assignment to one, selected
 	 * processor (the one described in the "cpumask" attribute).
 	 */
-	event->cpu = cpumask_first(&ccn->dt.cpu);
+	event->cpu = ccn->dt.cpu;
 
 	node_xp = CCN_CONFIG_NODE(event->attr.config);
 	type = CCN_CONFIG_TYPE(event->attr.config);
@@ -1215,15 +1215,15 @@ static int arm_ccn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
 	struct arm_ccn *ccn = container_of(dt, struct arm_ccn, dt);
 	unsigned int target;
 
-	if (!cpumask_test_and_clear_cpu(cpu, &dt->cpu))
+	if (cpu != dt->cpu)
 		return 0;
 	target = cpumask_any_but(cpu_online_mask, cpu);
 	if (target >= nr_cpu_ids)
 		return 0;
 	perf_pmu_migrate_context(&dt->pmu, cpu, target);
-	cpumask_set_cpu(target, &dt->cpu);
+	dt->cpu = target;
 	if (ccn->irq)
-		WARN_ON(irq_set_affinity_hint(ccn->irq, &dt->cpu) != 0);
+		WARN_ON(irq_set_affinity_hint(ccn->irq, cpumask_of(dt->cpu)));
 	return 0;
 }
 
@@ -1299,29 +1299,28 @@ static int arm_ccn_pmu_init(struct arm_ccn *ccn)
 	}
 
 	/* Pick one CPU which we will use to collect data from CCN... */
-	cpumask_set_cpu(get_cpu(), &ccn->dt.cpu);
+	ccn->dt.cpu = raw_smp_processor_id();
 
 	/* Also make sure that the overflow interrupt is handled by this CPU */
 	if (ccn->irq) {
-		err = irq_set_affinity_hint(ccn->irq, &ccn->dt.cpu);
+		err = irq_set_affinity_hint(ccn->irq, cpumask_of(ccn->dt.cpu));
 		if (err) {
 			dev_err(ccn->dev, "Failed to set interrupt affinity!\n");
 			goto error_set_affinity;
 		}
 	}
 
+	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
+					 &ccn->dt.node);
+
 	err = perf_pmu_register(&ccn->dt.pmu, name, -1);
 	if (err)
 		goto error_pmu_register;
 
-	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
-					 &ccn->dt.node);
-	put_cpu();
 	return 0;
 
 error_pmu_register:
 error_set_affinity:
-	put_cpu();
 error_choose_name:
 	ida_simple_remove(&arm_ccn_pmu_ida, ccn->dt.id);
 	for (i = 0; i < ccn->num_xps; i++)
-- 
2.21.0.dirty

Re: [PATCH v2 2/2] perf/arm-ccn: Remove broken race mitigation

[Suzuki K Poulose]

Hi Robin,

On 03/04/2019 18:10, Robin Murphy wrote:
> Like arm-cci, arm-ccn has the same issue of disabling preemption around
> operations which can take mutexes. Again, remove the definite bug by
> simply not trying to fight the theoretical races. And since we are
> touching the hotplug handling code, take the opportunity to streamline
> it, as there's really no need to store a full-sized cpumask to keep
> track of a single CPU ID.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>   drivers/perf/arm-ccn.c | 23 +++++++++++------------
>   1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/perf/arm-ccn.c b/drivers/perf/arm-ccn.c
> index 2ae76026e947..a0214308b0cd 100644
> --- a/drivers/perf/arm-ccn.c
> +++ b/drivers/perf/arm-ccn.c

>   	/* Pick one CPU which we will use to collect data from CCN... */
> -	cpumask_set_cpu(get_cpu(), &ccn->dt.cpu);
> +	ccn->dt.cpu = raw_smp_processor_id();
>   
>   	/* Also make sure that the overflow interrupt is handled by this CPU */
>   	if (ccn->irq) {
> -		err = irq_set_affinity_hint(ccn->irq, &ccn->dt.cpu);
> +		err = irq_set_affinity_hint(ccn->irq, cpumask_of(ccn->dt.cpu));
>   		if (err) {
>   			dev_err(ccn->dev, "Failed to set interrupt affinity!\n");
>   			goto error_set_affinity;
>   		}
>   	}
>   
> +	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
> +					 &ccn->dt.node);
> +
>   	err = perf_pmu_register(&ccn->dt.pmu, name, -1);
>   	if (err)
>   		goto error_pmu_register;

Should we not remove the above instance, in case we fail to register
the PMU ? Similarly for the CCI driver, we may have to reset the g_cci_pmu
if we fail.

Cheers
Suzuki


>   
> -	cpuhp_state_add_instance_nocalls(CPUHP_AP_PERF_ARM_CCN_ONLINE,
> -					 &ccn->dt.node);
> -	put_cpu();
>   	return 0;
>   
>   error_pmu_register:
>   error_set_affinity:
> -	put_cpu();
>   error_choose_name:
>   	ida_simple_remove(&arm_ccn_pmu_ida, ccn->dt.id);
>   	for (i = 0; i < ccn->num_xps; i++)
> 

[PATCH v2 1/2] perf/arm-cci: Remove broken race mitigation

[Robin Murphy]

Uncore PMU drivers face an awkward cyclic dependency wherein:

 - They have to pick a valid online CPU to associate with before
   registering the PMU device, since it will get exposed to userspace
   immediately.
 - The PMU registration has to be be at least partly complete before
   hotplug events can be handled, since trying to migrate an
   uninitialised context would be bad.
 - The hotplug handler has to be ready as soon as a CPU is chosen, lest
   it go offline without the user-visible cpumask value getting updated.

The arm-cci driver has tried to solve this by using get_cpu() to pick
the current CPU and prevent it from disappearing while both
registrations are performed, but that results in taking mutexes with
preemption disabled, which makes certain configurations very unhappy:

[ 1.983337] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:2004
[ 1.983340] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0
[ 1.983342] Preemption disabled at:
[ 1.983353] [<ffffff80089801f4>] cci_pmu_probe+0x1dc/0x488
[ 1.983360] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.20-rt8-yocto-preempt-rt #1
[ 1.983362] Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
[ 1.983364] Call trace:
[ 1.983369] dump_backtrace+0x0/0x158
[ 1.983372] show_stack+0x24/0x30
[ 1.983378] dump_stack+0x80/0xa4
[ 1.983383] ___might_sleep+0x138/0x160
[ 1.983386] __might_sleep+0x58/0x90
[ 1.983391] __rt_mutex_lock_state+0x30/0xc0
[ 1.983395] _mutex_lock+0x24/0x30
[ 1.983400] perf_pmu_register+0x2c/0x388
[ 1.983404] cci_pmu_probe+0x2bc/0x488
[ 1.983409] platform_drv_probe+0x58/0xa8

It is not feasible to resolve all the possible races outside of the perf
core itself, so address the immediate bug by following the example of
nearly every other PMU driver and not even trying to do so. Registering
the hotplug notifier first should minimise the window in which things
can go wrong, so that's about as much as we can reasonably do here. This
also revealed an additional race in assigning the global pointer too
late relative to the hotplug notifier, which gets fixed in the process.

Reported-by: "Li, Meng" <Meng.Li@windriver.com>
Reported-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/perf/arm-cci.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/perf/arm-cci.c b/drivers/perf/arm-cci.c
index bfd03e023308..7a29ed88bd4f 100644
--- a/drivers/perf/arm-cci.c
+++ b/drivers/perf/arm-cci.c
@@ -1684,19 +1684,17 @@ static int cci_pmu_probe(struct platform_device *pdev)
 	raw_spin_lock_init(&cci_pmu->hw_events.pmu_lock);
 	mutex_init(&cci_pmu->reserve_mutex);
 	atomic_set(&cci_pmu->active_events, 0);
-	cci_pmu->cpu = get_cpu();
-
-	ret = cci_pmu_init(cci_pmu, pdev);
-	if (ret) {
-		put_cpu();
-		return ret;
-	}
 
+	cci_pmu->cpu = raw_smp_processor_id();
+	g_cci_pmu = cci_pmu;
 	cpuhp_setup_state_nocalls(CPUHP_AP_PERF_ARM_CCI_ONLINE,
 				  "perf/arm/cci:online", NULL,
 				  cci_pmu_offline_cpu);
-	put_cpu();
-	g_cci_pmu = cci_pmu;
+
+	ret = cci_pmu_init(cci_pmu, pdev);
+	if (ret)
+		return ret;
+
 	pr_info("ARM %s PMU driver probed", cci_pmu->model->name);
 	return 0;
 }
-- 
2.21.0.dirty

Re: [PATCH v2 1/2] perf/arm-cci: Remove broken race mitigation

[Suzuki K Poulose]

On 03/04/2019 18:10, Robin Murphy wrote:
> Uncore PMU drivers face an awkward cyclic dependency wherein:
> 
>   - They have to pick a valid online CPU to associate with before
>     registering the PMU device, since it will get exposed to userspace
>     immediately.
>   - The PMU registration has to be be at least partly complete before
>     hotplug events can be handled, since trying to migrate an
>     uninitialised context would be bad.
>   - The hotplug handler has to be ready as soon as a CPU is chosen, lest
>     it go offline without the user-visible cpumask value getting updated.
> 
> The arm-cci driver has tried to solve this by using get_cpu() to pick
> the current CPU and prevent it from disappearing while both
> registrations are performed, but that results in taking mutexes with
> preemption disabled, which makes certain configurations very unhappy:
> 
> [ 1.983337] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:2004
> [ 1.983340] in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: swapper/0
> [ 1.983342] Preemption disabled at:
> [ 1.983353] [<ffffff80089801f4>] cci_pmu_probe+0x1dc/0x488
> [ 1.983360] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.20-rt8-yocto-preempt-rt #1
> [ 1.983362] Hardware name: ZynqMP ZCU102 Rev1.0 (DT)
> [ 1.983364] Call trace:
> [ 1.983369] dump_backtrace+0x0/0x158
> [ 1.983372] show_stack+0x24/0x30
> [ 1.983378] dump_stack+0x80/0xa4
> [ 1.983383] ___might_sleep+0x138/0x160
> [ 1.983386] __might_sleep+0x58/0x90
> [ 1.983391] __rt_mutex_lock_state+0x30/0xc0
> [ 1.983395] _mutex_lock+0x24/0x30
> [ 1.983400] perf_pmu_register+0x2c/0x388
> [ 1.983404] cci_pmu_probe+0x2bc/0x488
> [ 1.983409] platform_drv_probe+0x58/0xa8
> 
> It is not feasible to resolve all the possible races outside of the perf
> core itself, so address the immediate bug by following the example of
> nearly every other PMU driver and not even trying to do so. Registering
> the hotplug notifier first should minimise the window in which things
> can go wrong, so that's about as much as we can reasonably do here. This
> also revealed an additional race in assigning the global pointer too
> late relative to the hotplug notifier, which gets fixed in the process.
> 
> Reported-by: "Li, Meng" <Meng.Li@windriver.com>
> Reported-by: Corentin Labbe <clabbe.montjoie@gmail.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Thanks for fixing this.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Re: [PATCH v2 0/2] Fix Arm system PMU hotplug issues

[Corentin Labbe]

On Wed, Apr 03, 2019 at 06:08:58PM +0100, Robin Murphy wrote:
> Hi all,
> 
> After a converstaion with Thomas a while back, it felt like the best way
> forward here is just to resolve the pressing preemption violation, and
> not make things more complicated by trying to manage theoretical races
> within perf core from a distance.
> 
> Thus v2 simply nukes the offending logic and brings those drivers in
> line with the status quo. Due to the abrupt change in tactic I've not
> kept the review tags given for v1.
> 
> Robin.
> 
> 
> Robin Murphy (2):
>   perf/arm-cci: Remove broken race mitigation
>   perf/arm-ccn: Remove broken race mitigation
> 
>  drivers/perf/arm-cci.c | 16 +++++++---------
>  drivers/perf/arm-ccn.c | 23 +++++++++++------------
>  2 files changed, 18 insertions(+), 21 deletions(-)
> 
> -- 
> 2.21.0.dirty
> 

Hello

Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Tested-on: sun8i-a83t-bananapi-m3

Thanks for having fixed this problem.
Regards