Re: [PATCH 09/10] MCE: run through processors with more severe problemsfirst

[Hidetoshi Seto]

(2011/06/14 7:03), Tony Luck wrote:
>>> Or how about checking rip in each mces_seen?
>>
>> This is equivalent to what I did - but I think the code
>> will be cleaner. I'll give it a try.
> 
> Here's a patch on top of my previous series that just looks at
> mces_seen to choose the order. Obviously I'd fold this into the
> other patch for a final version - but this one lets you see what
> the "mce_nextcpu()" function would look like (and how removing
> the bitmaps cleans up the other parts of the code). It does look
> better to me.
> 
> Seto-san: Does this fit with what you were thinking?
> 
> Compile tested only.
> 
> -Tony
> 
> ---
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index a7a8c53..6b4176b 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -791,31 +791,47 @@ static void mce_reign(void)
>  
>  static atomic_t global_nwo;
>  
> -/*
> - * Keep separate bitmaps for cpus that have the option return from
> - * machine check handler (MCG_STATUS.RIPV == 1) and those for that
> - * cannot.
> - */
> -static cpumask_t	can_return;
> -static cpumask_t	cant_return;
> -
>  static int	monarch;
>  
>  /*
> - * next cpu choosing first from cant_return, and then from can_return
> + * Find next cpu that will run through the core of do_machine_check()
> + * checking all the banks of machine check registers. We first take
> + * cpus with serious problems (as indicated by MCG_STATUS_RIPV being
> + * clear in the mcgstatus register). A second pass through mces_seen
> + * is made to process the remaining cpus.
> + * We do this because some machine check banks are shared between cpus,
> + * and it is better to find the error on the cpu that has the problem
> + * and clear the bank so that the innocent bystanders do not have to
> + * worry about errors that do not affect them.

BTW in case of "no_way_out" events, we don't clear banks because they
could be carried over to the next boot (expecting logged as bootlog).
So we may need to have some trick for some known cases; e.g. ignore
observed AR by bystanders, anyway.

>   */
> -int mce_nextcpu(int this)
> +int mce_nextcpu(int cur)
>  {
> -	int next;
> +	struct mce	*m;
> +	int		cpu = cur;
> +	u64	mask = MCG_STATUS_MCIP;

Why do you check the MCG_STATUS_MCIP too here?
What happens if there is a problematic cpu that could not read
MCG register properly so indicates "PANIC with !MCIP"?

>  
> -	if (this == -1 || cpumask_test_cpu(this, &cant_return)) {
> -		next = cpumask_next(this, &cant_return);
> -		if (next >= nr_cpu_ids)
> -			next = cpumask_next(-1, &can_return);
> -		return next;
> +	if (cpu != -1) {
> +		m = &per_cpu(mces_seen, cpu);
> +		if (m->mcgstatus & MCG_STATUS_RIPV)
> +			mask |= MCG_STATUS_RIPV;
>  	}
>  
> -	return cpumask_next(this, &can_return);
> +	while (1) {
> +		cpu = cpumask_next(cpu, cpu_possible_mask);

possible? online?

Ah, I guess you assumed that all cpus checked in should have
mces_seen with MCIP while offline cpus have cleaned mces_seen.  

Though we know there might be races with cpu hotplug, now we
already use num_online_cpus() in this rendezvous mechanism,
it is OK to use cpu_online_mask here at the moment, I think.

Or we should invent new, better rendezvous mechanism...

> +		if (cpu >= nr_cpu_ids) {
> +			if (mask & MCG_STATUS_RIPV)
> +				return cpu;
> +			mask |= MCG_STATUS_RIPV;
> +			cpu = -1;
> +			continue;
> +		}
> +
> +		m = &per_cpu(mces_seen, cpu);
> +		if ((m->mcgstatus & (MCG_STATUS_MCIP|MCG_STATUS_RIPV)) == mask)
> +			break;
> +	}
> +
> +	return cpu;
>  }
>  
>  /*
> @@ -825,7 +841,7 @@ int mce_nextcpu(int this)
>   * one at a time.
>   * TBD double check parallel CPU hotunplug
>   */
> -static int mce_start(int *no_way_out, int noreturn)
> +static int mce_start(int *no_way_out)
>  {
>  	int order;
>  	int cpus = num_online_cpus();
> @@ -841,11 +857,6 @@ static int mce_start(int *no_way_out, int noreturn)
>  	smp_wmb();
>  	order = atomic_inc_return(&mce_callin);
>  
> -	if (noreturn)
> -		cpumask_set_cpu(smp_processor_id(), &cant_return);
> -	else
> -		cpumask_set_cpu(smp_processor_id(), &can_return);
> -
>  	/*
>  	 * Wait for everyone.
>  	 */
> @@ -951,8 +962,6 @@ static int mce_end(int order)
>  reset:
>  	atomic_set(&global_nwo, 0);
>  	atomic_set(&mce_callin, 0);
> -	cpumask_clear(&can_return);
> -	cpumask_clear(&cant_return);
>  	barrier();
>  
>  	/*
> @@ -1134,7 +1143,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>  	 * This way we don't report duplicated events on shared banks
>  	 * because the first one to see it will clear it.
>  	 */
> -	order = mce_start(&no_way_out, kill_it);
> +	order = mce_start(&no_way_out);
>  	for (i = 0; i < banks; i++) {
>  		__clear_bit(i, toclear);
>  		if (!mce_banks[i].ctl)
> 
> 

The rest looks good.


Thanks,
H.Seto

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