Re: [PATCH -v4] x86: MCE: Re-implement MCE log ring buffer as per-CPUring buffer

[Hidetoshi Seto]

Huang Ying wrote:
> On Fri, 2009-06-05 at 16:28 +0800, Hidetoshi Seto wrote:
>> Huang Ying wrote:
>>> On Fri, 2009-06-05 at 15:01 +0800, Hidetoshi Seto wrote:
>>>> Huang Ying wrote:
>>>>>  struct mce_log {
>>>>> -	char signature[12]; /* "MACHINECHECK" */
>>>>> +	char signature[12]; /* "MACHINECHEC2" */
>>>>>  	unsigned len;	    /* = MCE_LOG_LEN */
>>>>> -	unsigned next;
>>>>>  	unsigned flags;
>>>>>  	unsigned pad0;
>>>>> -	struct mce entry[MCE_LOG_LEN];
>>>>> +	struct mce_log_cpu *mcelog_cpus;
>>>>>  };
>>>> What is this *mcelog_cpus to be used for?
>>>> It seems it will point one of per-CPU buffers (maybe cpu#0's buffer)
>>>> if I have read the following mce_log_init() correctly.
>>> It is mainly used by something like kdump, which can search
>>> "MACHINECHEC2", and analyze mce_log. mcelog_cpus can help kdump find the
>>> real mcelog storage.
>> Hum, but it help tools only to find one of buffers, not to find all.
>>
>> I think it would be better help for tools if we have another signature
>> on struct mce_log_cpu, e.g.: 
>>
>>  +struct mce_log_cpu {
>> + 	char signature[**];	/* "MCE_LOG_CPU_VER_1" or so */
>> +	__u32 cpuid;		/* cpuid or extcpu, same as struct mce */
>>  +	int head;
>>  +	int tail;
>>  +	unsigned long flags;
>>  +	struct mce entry[MCE_LOG_LEN];
>>  +};
>>
>> How about this?
> 
> +	mcelog.mcelog_cpus = &per_cpu_var(mce_log_cpus);
> 
> So mcelog.mcelog_cpus are pointed to the buffers of all CPUs, not just
> that of one CPU. You can find them by analyzing PER cpu data structure.

I thought that the signature is used to find the structure without such
analyzing.  If a tool can analyze PER cpu data, then it likely know
where the mce_log_cpu is, so mcelog.mcelog_cpus will not be required.

#define per_cpu_var(var) per_cpu__##var

#define per_cpu(var, cpu) \
        (*SHIFT_PERCPU_PTR(&per_cpu_var(var), per_cpu_offset(cpu)))

OK, I have mistook per_cpu_var() for per_cpu(). 
Then mcelog.mcelog_cpus does not point any of the buffers, but tells where
the buffer is locating in each of PER cpu data.
Tools have to get per_cpu_offset(cpu) to know where the PER cpu data
allocated.  According to pcpu_alloc_bootmem(), it might consider NUMA.
Anyway It will take a bit more effort.

So still I think the mcelog.mcelog_cpus is mostly pointless.


Thanks,
H.Seto

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