Date:   Thu, 29 Sep 2022 16:08:19 +0200
From:   Pierre Gondois <pierre.gondois@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v1 1/1] arm_pmu: acpi: Pre-allocate pmu structures

Hello Mark,

On 9/28/22 17:47, Mark Rutland wrote:
> Hi Pierre,
> 
> Thanks for this, and sorry for the delayed reply.
> 
> On Mon, Sep 12, 2022 at 05:51:04PM +0200, Pierre Gondois wrote:
>> On an Ampere Altra,
>> Running a preemp_rt kernel based on v5.19-rc3-rt4 on an Ampere Altra
>> triggers:
>> [   12.642389] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
>> [   12.642402] in_atomic(): 0, irqs_disabled(): 128, non_block: 0, pid: 24, name: cpuhp/0
>> [   12.642406] preempt_count: 0, expected: 0
>> [   12.642409] RCU nest depth: 0, expected: 0
>> [   12.642411] 3 locks held by cpuhp/0/24:
>> [   12.642414] #0: ffffd8a22c8870d0 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun (linux/kernel/cpu.c:754)
>> [   12.642429] #1: ffffd8a22c887120 (cpuhp_state-up){+.+.}-{0:0}, at: cpuhp_thread_fun (linux/kernel/cpu.c:754)
>> [   12.642436] #2: ffff083e7f0d97b8 ((&c->lock)){+.+.}-{3:3}, at: ___slab_alloc (linux/mm/slub.c:2954)
>> [   12.642458] irq event stamp: 42
>> [   12.642460] hardirqs last enabled at (41): finish_task_switch (linux/./arch/arm64/include/asm/irqflags.h:35)
>> [   12.642471] hardirqs last disabled at (42): cpuhp_thread_fun (linux/kernel/cpu.c:776 (discriminator 1))
>> [   12.642476] softirqs last enabled at (0): copy_process (linux/./include/linux/lockdep.h:191)
>> [   12.642484] softirqs last disabled at (0): 0x0
>> [   12.642495] CPU: 0 PID: 24 Comm: cpuhp/0 Tainted: G        W         5.19.0-rc3-rt4-custom-piegon01-rt_0 #142
>> [   12.642500] Hardware name: WIWYNN Mt.Jade Server System B81.03001.0005/Mt.Jade Motherboard, BIOS 1.08.20220218 (SCP: 1.08.20220218) 2022/02/18
>> [   12.642506] Call trace:
>> [   12.642508] dump_backtrace (linux/arch/arm64/kernel/stacktrace.c:200)
>> [   12.642514] show_stack (linux/arch/arm64/kernel/stacktrace.c:207)
>> [   12.642517] dump_stack_lvl (linux/lib/dump_stack.c:107)
>> [   12.642523] dump_stack (linux/lib/dump_stack.c:114)
>> [   12.642527] __might_resched (linux/kernel/sched/core.c:9929)
>> [   12.642531] rt_spin_lock (linux/kernel/locking/rtmutex.c:1732 (discriminator 4))
>> [   12.642536] ___slab_alloc (linux/mm/slub.c:2954)
>> [   12.642539] __slab_alloc.isra.0 (linux/mm/slub.c:3116)
>> [   12.642543] kmem_cache_alloc_trace (linux/mm/slub.c:3207)
>> [   12.642549] __armpmu_alloc (linux/./include/linux/slab.h:600)
>> [   12.642558] armpmu_alloc_atomic (linux/drivers/perf/arm_pmu.c:927)
>> [   12.642562] arm_pmu_acpi_cpu_starting (linux/drivers/perf/arm_pmu_acpi.c:204)
>> [   12.642568] cpuhp_invoke_callback (linux/kernel/cpu.c:192)
>> [   12.642571] cpuhp_thread_fun (linux/kernel/cpu.c:777 (discriminator 3))
>> [   12.642573] smpboot_thread_fn (linux/kernel/smpboot.c:164 (discriminator 3))
>> [   12.642580] kthread (linux/kernel/kthread.c:376)
>> [   12.642584] ret_from_fork (linux/arch/arm64/kernel/entry.S:868)
>>
>> arm_pmu_acpi_cpu_starting() is called in the STARTING hotplug section,
>> which runs with interrupts disabled. To avoid allocating memory and
>> sleeping in this function, the pmu structures must be pre-allocated.
>>
>> On ACPI systems, the count of PMUs is unknown until CPUs are
>> hotplugged, cf:
>> commit 0dc1a1851af1 ("arm_pmu: add armpmu_alloc_atomic()")
>>
>> At most #PMU_IRQs pmu structures will be used and thus need to be
>> pre-allocated.
>> In arm_pmu_acpi_cpu_starting() subcalls, after checking the cpuid,
>> decide to use or re-use a pre-allocated pmu structure. Thus the
>> pre-allocated pmu struct can be seen as a pool.
>> When probing, search and free unused pmu structures.
> 
> I think in retrospect I was trying to be too clever with
> arm_pmu_acpi_cpu_starting() handling boot-time CPUs and late hotplug, and we
> can make this simpler by handling the boot-time probing synchronously within
> arm_pmu_acpi_probe(), removing a bunch of state.
> 
> I had a go at that, and in testing (in a QEMU TCG VM) atop arm64/for-next/core,
> that seems to work (even with a faked-up heterogenous config). I've pushed that
> to my `arm_pmu/acpi/rework` branch at:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm_pmu/acpi/rework
>    git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm_pmu/acpi/rework
> 
> ... does that work for you?

Thanks for the branch and for looking at this. I think there is an issue for late hotplug
CPUs. Indeed the pmu structure allocation is done for the online CPUs at the
time of probing. This let rooms for the case where none of the CPUs of a PMU is booted
at startup.
I tried the patch on a Juno-r2 with the 'maxcpus=1 apci=force' parameters. When late
hotplugging CPU1 (which has a different pmu than CPU0), no pmu structure is found and
the cpuhp state machine fails (since arm_pmu_acpi_cpu_starting() failed).

This current patch is blindly allocating numerous pmu structures based on the number
of pmu irq that can be seen in the MADT table. I currently don't see another solution.

Regards,
Pierre

> 
> Thanks,
> Mark.
> 
>>
>> Platforms used to test this patch:
>> - Juno-r2 (2 clusters: 2 big and 4 little CPUs) with 2 PMUs and
>>    one interrupt for each CPU
>> - Ampere Altra with 1 PMU and one interrupt for the 160 CPUs
>>
>> Link: https://lore.kernel.org/all/20210810134127.1394269-2-valentin.schneider@arm.com
>> Reported-by: Valentin Schneider <vschneid@...hat.com>
>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
>> ---
>>   drivers/perf/arm_pmu.c       |  17 +-----
>>   drivers/perf/arm_pmu_acpi.c  | 114 ++++++++++++++++++++++++++++++-----
>>   include/linux/perf/arm_pmu.h |   1 -
>>   3 files changed, 103 insertions(+), 29 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
>> index 59d3980b8ca2..731e793dfef5 100644
>> --- a/drivers/perf/arm_pmu.c
>> +++ b/drivers/perf/arm_pmu.c
>> @@ -861,16 +861,16 @@ static void cpu_pmu_destroy(struct arm_pmu *cpu_pmu)
>>   					    &cpu_pmu->node);
>>   }
>>   
>> -static struct arm_pmu *__armpmu_alloc(gfp_t flags)
>> +struct arm_pmu *armpmu_alloc(void)
>>   {
>>   	struct arm_pmu *pmu;
>>   	int cpu;
>>   
>> -	pmu = kzalloc(sizeof(*pmu), flags);
>> +	pmu = kzalloc(sizeof(*pmu), GFP_KERNEL);
>>   	if (!pmu)
>>   		goto out;
>>   
>> -	pmu->hw_events = alloc_percpu_gfp(struct pmu_hw_events, flags);
>> +	pmu->hw_events = alloc_percpu_gfp(struct pmu_hw_events, GFP_KERNEL);
>>   	if (!pmu->hw_events) {
>>   		pr_info("failed to allocate per-cpu PMU data.\n");
>>   		goto out_free_pmu;
>> @@ -916,17 +916,6 @@ static struct arm_pmu *__armpmu_alloc(gfp_t flags)
>>   	return NULL;
>>   }
>>   
>> -struct arm_pmu *armpmu_alloc(void)
>> -{
>> -	return __armpmu_alloc(GFP_KERNEL);
>> -}
>> -
>> -struct arm_pmu *armpmu_alloc_atomic(void)
>> -{
>> -	return __armpmu_alloc(GFP_ATOMIC);
>> -}
>> -
>> -
>>   void armpmu_free(struct arm_pmu *pmu)
>>   {
>>   	free_percpu(pmu->hw_events);
>> diff --git a/drivers/perf/arm_pmu_acpi.c b/drivers/perf/arm_pmu_acpi.c
>> index 96ffadd654ff..599d8be78950 100644
>> --- a/drivers/perf/arm_pmu_acpi.c
>> +++ b/drivers/perf/arm_pmu_acpi.c
>> @@ -17,6 +17,8 @@
>>   
>>   static DEFINE_PER_CPU(struct arm_pmu *, probed_pmus);
>>   static DEFINE_PER_CPU(int, pmu_irqs);
>> +static unsigned int preallocated_pmus_count;
>> +static struct arm_pmu **preallocated_pmus;
>>   
>>   static int arm_pmu_acpi_register_irq(int cpu)
>>   {
>> @@ -187,30 +189,108 @@ static int arm_pmu_acpi_parse_irqs(void)
>>   	return err;
>>   }
>>   
>> +/* Count the number of different PMU IRQs for the input PMU. */
>> +static int count_pmu_irqs(struct arm_pmu *pmu)
>> +{
>> +	struct pmu_hw_events __percpu *hw_events = pmu->hw_events;
>> +	unsigned int num_irqs = 0;
>> +	int cpu, probed_cpu, irq;
>> +
>> +	for_each_cpu(cpu, &pmu->supported_cpus) {
>> +		irq = per_cpu(hw_events->irq, cpu);
>> +		for_each_cpu(probed_cpu, &pmu->supported_cpus) {
>> +			if (irq == per_cpu(hw_events->irq, probed_cpu)) {
>> +				if (probed_cpu == cpu)
>> +					num_irqs++;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +
>> +	return num_irqs;
>> +}
>> +
>> +/*
>> + * Count the number of different PMU IRQs across all the PMUs of the system
>> + * to get an upper bound of the number of struct arm_pmu to pre-allocate.
>> + */
>> +static int count_all_pmu_irqs(void)
>> +{
>> +	unsigned int num_irqs = 0;
>> +	int cpu, probed_cpu, irq;
>> +
>> +	for_each_possible_cpu(cpu) {
>> +		irq = per_cpu(pmu_irqs, cpu);
>> +		for_each_possible_cpu(probed_cpu) {
>> +			if (irq == per_cpu(pmu_irqs, probed_cpu)) {
>> +				if (probed_cpu == cpu)
>> +					num_irqs++;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +
>> +	return num_irqs;
>> +}
>> +
>> +static unsigned int pmu_preallocate(void)
>> +{
>> +	unsigned int i, num_irqs = count_all_pmu_irqs();
>> +
>> +	preallocated_pmus = kcalloc(num_irqs, sizeof(*preallocated_pmus),
>> +					GFP_KERNEL);
>> +	if (!preallocated_pmus) {
>> +		pr_err("Failed to pre-allocate %d pmu struct\n", num_irqs);
>> +		return -ENOMEM;
>> +	}
>> +
>> +	for (i = 0; i < num_irqs; i++) {
>> +		preallocated_pmus[i] = armpmu_alloc();
>> +		if (!preallocated_pmus[i])
>> +			return -ENOMEM;
>> +	}
>> +
>> +	preallocated_pmus_count = num_irqs;
>> +	return 0;
>> +}
>> +
>>   static struct arm_pmu *arm_pmu_acpi_find_alloc_pmu(void)
>>   {
>>   	unsigned long cpuid = read_cpuid_id();
>>   	struct arm_pmu *pmu;
>> -	int cpu;
>> +	unsigned int i;
>>   
>> -	for_each_possible_cpu(cpu) {
>> -		pmu = per_cpu(probed_pmus, cpu);
>> -		if (!pmu || pmu->acpi_cpuid != cpuid)
>> -			continue;
>> +	for (i = 0; i < preallocated_pmus_count; i++) {
>> +		pmu = preallocated_pmus[i];
>>   
>> -		return pmu;
>> +		if (!pmu->acpi_cpuid) {
>> +			pmu->acpi_cpuid = cpuid;
>> +			return pmu;
>> +		} else if (pmu->acpi_cpuid == cpuid)
>> +			return pmu;
>>   	}
>>   
>> -	pmu = armpmu_alloc_atomic();
>> -	if (!pmu) {
>> -		pr_warn("Unable to allocate PMU for CPU%d\n",
>> -			smp_processor_id());
>> -		return NULL;
>> -	}
>> +	pr_err("Unable to find pre-allocated PMU for CPU%d\n",
>> +		smp_processor_id());
>> +	return NULL;
>> +}
>>   
>> -	pmu->acpi_cpuid = cpuid;
>> +static void pmu_free_unused_preallocated(struct arm_pmu *pmu)
>> +{
>> +	int i, unused_num_irqs = count_pmu_irqs(pmu);
>>   
>> -	return pmu;
>> +	if (unused_num_irqs <= 1)
>> +		return;
>> +	else if (unused_num_irqs >= preallocated_pmus_count) {
>> +		pr_err("Trying to free %d pmu struct when %d are allocated\n",
>> +				unused_num_irqs, preallocated_pmus_count);
>> +		return;
>> +	}
>> +
>> +	unused_num_irqs--;
>> +	for (i = 0; i < unused_num_irqs; i++)
>> +		armpmu_free(preallocated_pmus[preallocated_pmus_count - i - 1]);
>> +	preallocated_pmus_count -= unused_num_irqs;
>>   }
>>   
>>   /*
>> @@ -311,6 +391,8 @@ int arm_pmu_acpi_probe(armpmu_init_fn init_fn)
>>   		if (!pmu || pmu->name)
>>   			continue;
>>   
>> +		pmu_free_unused_preallocated(pmu);
>> +
>>   		ret = init_fn(pmu);
>>   		if (ret == -ENODEV) {
>>   			/* PMU not handled by this driver, or not present */
>> @@ -351,6 +433,10 @@ static int arm_pmu_acpi_init(void)
>>   	if (ret)
>>   		return ret;
>>   
>> +	ret = pmu_preallocate();
>> +	if (ret)
>> +		return ret;
>> +
>>   	ret = cpuhp_setup_state(CPUHP_AP_PERF_ARM_ACPI_STARTING,
>>   				"perf/arm/pmu_acpi:starting",
>>   				arm_pmu_acpi_cpu_starting, NULL);
>> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
>> index 0407a38b470a..049908af3595 100644
>> --- a/include/linux/perf/arm_pmu.h
>> +++ b/include/linux/perf/arm_pmu.h
>> @@ -173,7 +173,6 @@ void kvm_host_pmu_init(struct arm_pmu *pmu);
>>   
>>   /* Internal functions only for core arm_pmu code */
>>   struct arm_pmu *armpmu_alloc(void);
>> -struct arm_pmu *armpmu_alloc_atomic(void);
>>   void armpmu_free(struct arm_pmu *pmu);
>>   int armpmu_register(struct arm_pmu *pmu);
>>   int armpmu_request_irq(int irq, int cpu);
>> -- 
>> 2.25.1
>>

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