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SubjectRe: [PATCH] specific do_timer_cpu value for nohz off mode
On Thu, 1 Dec 2011 10:37:40 -0600
Dimitri Sivanich <sivanich@sgi.com> wrote:

> +static ssize_t sysfs_store_do_timer_cpu(struct sys_device *dev,
> + struct sysdev_attribute *attr,
> + const char *buf, size_t size)
> +{
> + struct sysdev_ext_attribute *ea = SYSDEV_TO_EXT_ATTR(attr);
> + unsigned int new;
> + int rv;
> +
> +#ifdef CONFIG_NO_HZ
> + /* nohz mode not supported */
> + if (tick_nohz_enabled)
> + return -EINVAL;
> +#endif
> +
> + rv = kstrtouint(buf, 0, &new);
> + if (rv)
> + return rv;
> +
> + /* Protect against cpu-hotplug */
> + get_online_cpus();
> +
> + if (new >= nr_cpu_ids || !cpu_online(new)) {
> + put_online_cpus();
> + return -ERANGE;
> + }
> +
> + *(unsigned int *)(ea->var) = new;
> +
> + put_online_cpus();
> +
> + return size;
> +}

OK, I think this fixes one race. We modify tick_do_timer_cpu inside
get_online_cpus(). If that cpu goes offline then
tick_handover_do_timer() will correctly hand the timer functions over
to a new CPU, and tick_handover_do_timer() runs in the CPU hotplug
handler which I assume is locked by get_online_cpus(). Please check
all this.

Now, the above code can alter tick_do_timer_cpu while a timer interrupt
is actually executing on another CPU. Will this disrupt aything? I
think it might cause problems. If we take an interrupt on CPU 5 and
that CPU enters tick_periodic() and another CPU alters
tick_do_timer_cpu from 5 to 4 at exactly the correct time, tick_periodic()
might fail to run do_timer(). Or it might run do_timer() on both CPUs 4 and
5 concurrently?




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