Re: [PATCH v4 3/4] perf/core: Remove pmu linear searching code

[Ravi Bangoria <ravi.bangoria@amd.com>]

Hi Nathan,

On 25-May-23 3:11 AM, Nathan Chancellor wrote:
> Hi Ravi,
> 
> + arm64 KVM folks
> 
> On Thu, May 04, 2023 at 04:30:02PM +0530, Ravi Bangoria wrote:
>> Searching for the right pmu by iterating over all pmus is no longer
>> required since all pmus now *must* be present in the 'pmu_idr' list.
>> So, remove linear searching code.
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
>> ---
>>  kernel/events/core.c | 37 +++++++++++++------------------------
>>  1 file changed, 13 insertions(+), 24 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 0695bb9fbbb6..eba2b8595115 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -11630,38 +11630,27 @@ static struct pmu *perf_init_event(struct perf_event *event)
>>  	}
>>  
>>  again:
>> +	ret = -ENOENT;
>>  	rcu_read_lock();
>>  	pmu = idr_find(&pmu_idr, type);
>>  	rcu_read_unlock();
>> -	if (pmu) {
>> -		if (event->attr.type != type && type != PERF_TYPE_RAW &&
>> -		    !(pmu->capabilities & PERF_PMU_CAP_EXTENDED_HW_TYPE))
>> -			goto fail;
>> -
>> -		ret = perf_try_init_event(pmu, event);
>> -		if (ret == -ENOENT && event->attr.type != type && !extended_type) {
>> -			type = event->attr.type;
>> -			goto again;
>> -		}
>> +	if (!pmu)
>> +		goto fail;
>>  
>> -		if (ret)
>> -			pmu = ERR_PTR(ret);
>> +	if (event->attr.type != type && type != PERF_TYPE_RAW &&
>> +	    !(pmu->capabilities & PERF_PMU_CAP_EXTENDED_HW_TYPE))
>> +		goto fail;
>>  
>> -		goto unlock;
>> +	ret = perf_try_init_event(pmu, event);
>> +	if (ret == -ENOENT && event->attr.type != type && !extended_type) {
>> +		type = event->attr.type;
>> +		goto again;
>>  	}
>>  
>> -	list_for_each_entry_rcu(pmu, &pmus, entry, lockdep_is_held(&pmus_srcu)) {
>> -		ret = perf_try_init_event(pmu, event);
>> -		if (!ret)
>> -			goto unlock;
>> -
>> -		if (ret != -ENOENT) {
>> -			pmu = ERR_PTR(ret);
>> -			goto unlock;
>> -		}
>> -	}
>>  fail:
>> -	pmu = ERR_PTR(-ENOENT);
>> +	if (ret)
>> +		pmu = ERR_PTR(ret);
>> +
>>  unlock:
>>  	srcu_read_unlock(&pmus_srcu, idx);
>>  
>> -- 
>> 2.40.0
>>
> 
> My apologies if this has already been reported or fixed already, I did a
> search of lore.kernel.org and did not find anything. This patch as
> commit 9551fbb64d09 ("perf/core: Remove pmu linear searching code") in
> -next breaks starting QEMU with KVM enabled on two of my arm64 machines:
> 
> $ qemu-system-aarch64 \
>     -display none \
>     -nodefaults \
>     -machine virt,gic-version=max \
>     -append 'console=ttyAMA0 earlycon' \
>     -kernel arch/arm64/boot/Image.gz \
>     -initrd rootfs.cpio \
>     -cpu host \
>     -enable-kvm \
>     -m 512m \
>     -smp 8 \
>     -serial mon:stdio
> qemu-system-aarch64: PMU: KVM_SET_DEVICE_ATTR: No such device
> qemu-system-aarch64: failed to set irq for PMU
> 
> In the kernel log, I see
> 
> [   42.944952] kvm: pmu event creation failed -2
> 
> I am not sure if this issue is unexpected as a result of this change or
> if there is something that needs to change on the arm64 KVM side (it
> appears the kernel message comes from arch/arm64/kvm/pmu-emul.c).

Thanks for reporting it.

Based on these detail, I feel the pmu registration failed in the host,
most probably because pmu driver did not pass pmu name while calling
perf_pmu_register(). Consequently kvm also failed while trying to use
it for guest. Can you please check host kernel logs.

I'm sorry but I neither have Arm board to try myself not I'm familiar
with the Arm architecture. So I'll need your help to diagnose it.

Ravi