linux-kernel - Re: [PATCH] KVM: x86/vPMU: refine kvm_pmu err msg when event creation failed

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Date:   Thu, 18 Jul 2019 13:45:18 +0800
From:   Like Xu <like.xu@...ux.intel.com>
To:     Joe Perches <joe@...ches.com>, Paolo Bonzini <pbonzini@...hat.com>,
        Avi Kivity <avi@...lladb.com>
Cc:     kvm@...r.kernel.org, Gleb Natapov <gleb@...hat.com>,
        like.xu@...ux.inetl.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/vPMU: refine kvm_pmu err msg when event creation
 failed

Hi Joe,

On 2019/7/18 13:18, Joe Perches wrote:
> On Thu, 2019-07-18 at 12:49 +0800, Like Xu wrote:
>> If a perf_event creation fails due to any reason of the host perf
>> subsystem, it has no chance to log the corresponding event for guest
>> which may cause abnormal sampling data in guest result. In debug mode,
>> this message helps to understand the state of vPMC and we should not
>> limit the number of occurrences.
> []
>> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> []
>> @@ -131,8 +131,8 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   						 intr ? kvm_perf_overflow_intr :
>>   						 kvm_perf_overflow, pmc);
>>   	if (IS_ERR(event)) {
>> -		printk_once("kvm_pmu: event creation failed %ld\n",
>> -			    PTR_ERR(event));
>> +		pr_debug("kvm_pmu: event creation failed %ld\n for pmc->idx = %d",
>> +			    PTR_ERR(event), pmc->idx);
> 
> Perhaps this was written as printk_once to avoid
> spamming the log with repeated messages.

The spamming case in practice from this messages is very rare but it's 
logically possible.

> 
> Maybe this should use pr_debug_ratelimited.
> (and it should also have a \n termination like:)

Thanks and it's my mistake, agree on '\n' usage.

> 
> 		pr_debug_ratelimited("kvm_pmu: event creation failed %ld for pmc->idx = %d\n",
> 				     PTR_ERR(event), pmc->idx);
> 
> Perhaps Avi Kivity remembers why he wrote it this way.
> https://lore.kernel.org/kvm/1305129333-7456-6-git-send-email-avi@redhat.com >>
>