linux-kernel - Re: [PATCH] perf/x86: descriptive failure messages for PMU init

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Date:   Mon, 15 Apr 2019 08:42:04 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH] perf/x86: descriptive failure messages for PMU init

Hi Peter,

Peter Zijlstra <peterz@...radead.org> writes:

> On Fri, Apr 12, 2019 at 03:09:17PM -0400, Bandan Das wrote:
>> 
>> There's a default warning message that gets printed, however,
>> there are various failure conditions:
>>  - a msr read can fail
>>  - a msr write can fail
>>  - a msr has an unexpected value
>>  - all msrs have unexpected values (disable PMU)
>> 
>> Also, commit commit 005bd0077a79 ("perf/x86: Modify error message in
>> virtualized environment") completely removed printing the msr in
>> question but these messages could be helpful for debugging vPMUs as
>> well. Add them back and change them to pr_debugs, this keeps the
>> behavior the same for baremetal.
>> 
>> Lastly, use %llx to silence checkpatch
>
> Yuck... if you're debugging a hypervisor, you can bloody well run your
> own kernel with additional print slattered around.
>
> The whole make an exception for virt bullshit was already pushing it,
> this is just insane.
>

The only virt specific parts are the pr_debugs which I can remove and
replace with unconditional pr_err()s as suggested by Jiri. Is that ok ?

Bandan

>> @@ -266,12 +282,30 @@ static bool check_hw_exists(void)
>>  	return true;
>>  
>>  msr_fail:
>> -	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
>> +	if (virt)
>>  		pr_cont("PMU not available due to virtualization, using software events only.\n");
>> -	} else {
>> -		pr_cont("Broken PMU hardware detected, using software events only.\n");
>> -		pr_err("Failed to access perfctr msr (MSR %x is %Lx)\n",
>> -		       reg, val_new);
>> +	switch (status) {
>> +	case READ_FAIL:
>> +		if (virt)
>> +			pr_debug("Failed to read perfctr msr (MSR %x)\n", reg);
>> +		else
>> +			pr_err("Failed to read perfctr msr (MSR %x)\n", reg);
>> +		break;
>> +	case WRITE_FAIL:
>> +		if (virt)
>> +			pr_debug("Failed to write perfctr msr (MSR %x, wrote: %llx, read: %llx)\n",
>> +				 reg, val, val_new);
>> +		else
>> +			pr_err("Failed to write perfctr msr (MSR %x, wrote: %llx, read: %llx)\n",
>> +				 reg, val, val_new);
>> +		break;
>> +	case PMU_FAIL:
>> +		/* fall through for default message */
>> +	default:
>> +		if (virt)
>> +			pr_debug("Broken PMU hardware detected, using software events only.\n");
>> +		else
>> +			pr_cont("Broken PMU hardware detected, using software events only.\n");
>>  	}