Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Don Zickus]

On Tue, Apr 21, 2015 at 10:07:00AM -0400, Ulrich Obergfell wrote:
> 
> Chris,
> 
> I think it would also be nice to check the plausibility of the user input.
> 
> +int proc_watchdog_cpumask(struct ctl_table *table, int write,
> +                          void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +        int err;
> +
> +        mutex_lock(&watchdog_proc_mutex);
> +        err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> +        if (!err && write) {
> +                /* Remove impossible cpus to keep sysctl output cleaner. */
> +                cpumask_and(watchdog_cpumask, watchdog_cpumask,
> +                            cpu_possible_mask);
> +
> +                if (watchdog_enabled && watchdog_thresh)
> +                        smpboot_update_cpumask_percpu_thread(&watchdog_threads,
> +                                                             watchdog_cpumask);
> +        }
> +        mutex_unlock(&watchdog_proc_mutex);
> +        return err;
> +}
> 
> I think the user should only be allowed to specify a mask that is a subset of
> tick_nohz_full_mask as only those CPUs don't have a watchdog thread by default.
> In other words, the user should not be able to interfere with housekeeping CPUs.

Hi Uli,

I am not sure that is necessary.  This was supposed to be a debugging
interface for nohz (and possibly other technologies).  I think restricting
it to just tick_nohz makes it difficult to try out new things or debug
certain problems.

Personally, I feel anyone who will use this sys interface will need to do so
at their own risk.


Cheers,
Don

> 
> For example, add a plausibility check like so:
> 
>   save watchdog_cpumask because proc_do_large_bitmap() is going to change it
> 
>   proc_do_large_bitmap()
> 
>   // return an error if the user-specified mask includes a housekeeping CPU
>   if (watchdog_cpumask and 'negated tick_nohz_full_mask') {
>       restore saved watchdog_cpumask
>       return -EINVAL
>   }
> 
> 
> Regards,
> 
> Uli
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