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

[Huang Ying <ying.huang@intel.com>]

On Mon, 2009-06-08 at 09:17 +0800, Hidetoshi Seto wrote:
> 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.

Emm, it seem hard to analyze mce_log_cpu from per_cpu_var(mce_log_cpus),
how about following method:

struct mce_log {
    char signature[12];
    ...
    struct mce_log_cpu *mcelog_cpus[];
};

void mcelog_init(void)
{
    int cpu;
    mcelog.mcelog_cpus = kmalloc(sizeof(void *) * num_possible_cpus();
    for_each_possible_cpu(cpu)
	mcelog.mcelog_cpus[cpu] = &per_cpu(mce_log_cpus, cpu);
}

Best Regards,
Huang Ying