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

Hidetoshi Seto
2009-06-08T01:18:11Z

> 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