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SubjectRe: [PATCH] specific do_timer_cpu value for nohz off mode
On Tue, 8 Nov 2011 13:11:49 -0600
Dimitri Sivanich <sivanich@sgi.com> wrote:

> Resending this.

Not enough times apparently :(

>
> Allow manual override of the tick_do_timer_cpu.
>
> While not necessarily harmful, doing jiffies updates on an application cpu
> does cause some extra overhead that HPC benchmarking people notice. They
> prefer to have OS activity isolated to certain cpus. They like reproducibility
> of results, and having jiffies updates bouncing around introduces variability.

This doesn't really explain what the patch does. "override" with what?
What effects can the user expect to see from doing
the-action-which-you-didn't-describe?

> ===================================================================
> --- linux.orig/kernel/time/tick-sched.c
> +++ linux/kernel/time/tick-sched.c
> @@ -834,6 +834,104 @@ void tick_cancel_sched_timer(int cpu)
> }
> #endif
>
> +
> +#ifdef CONFIG_SYSFS
> +/**
> + * sysfs_show_do_timer_cpu - sysfs interface for tick_do_timer_cpu
> + * @dev: unused
> + * @buf: char buffer where value of tick_do_timer_cpu is copied
> + *
> + * Provides sysfs interface for showing the current tick_do_timer_cpu.
> + */
> +static ssize_t
> +sysfs_show_do_timer_cpu(struct sys_device *dev,
> + struct sysdev_attribute *attr, char *buf)
> +{
> + ssize_t count = 0;
> +
> + count = snprintf(buf, PAGE_SIZE, "%d\n", tick_do_timer_cpu);
> +
> + return count;

Save some trees:

return snprintf(buf, PAGE_SIZE, "%d\n", tick_do_timer_cpu);

> +}
> +
> +/**
> + * sysfs_override_do_timer_cpu - manually override tick_do_timer_cpu
> + * @dev: unused
> + * @buf: cpu number of desired tick_do_timer_cpu
> + * @count: length of buffer
> + *
> + * Takes input from sysfs interface for manually overriding the selected
> + * tick_do_timer_cpu. Only applicable when not running in nohz mode.
> + */
> +static ssize_t
> +sysfs_override_do_timer_cpu(struct sys_device *dev,
> + struct sysdev_attribute *attr,
> + const char *buf, size_t count)
> +{
> + char b[16];
> + size_t ret = count;
> + int c;
> +
> +#ifdef CONFIG_NO_HZ
> + /* nohz mode not supported */
> + if (tick_nohz_enabled)
> + return -EINVAL;
> +#endif
> + /* strings from sysfs write are not 0 terminated! */

hm. Aren't they?

> + if (count >= sizeof(b))
> + return -EINVAL;
> +
> + /* strip off \n: */
> + if (buf[count-1] == '\n')
> + count--;
> + if (count < 1)
> + return -EINVAL;
> +
> + memcpy(b, buf, count);
> + b[count] = 0;
> +
> + if (sscanf(b, "%d", &c) != 1)
> + return -EINVAL;

You should be able to use strim() in here, and eliminate b[]. Not that
the \n needed to be removed anyway! I think it's OK to modify the
incoming memory for sysfs write handlers.

Also, kstrto*() should be used to detect and reject input of the form
"42foo".

But surely we have a helper function somewhere to read an integer out
of a sysfs-written buffer. If not, we should!

> + if (!cpu_online(c))
> + return -EINVAL;
> +
> + tick_do_timer_cpu = c;
> +
> + return ret;
> +}
> +
> +/*
> + * Sysfs setup bits:
> + */
> +static SYSDEV_ATTR(jiffies_cpu, 0644, sysfs_show_do_timer_cpu,
> + sysfs_override_do_timer_cpu);
> +
> +static struct sysdev_class timekeeping_sysclass = {
> + .name = "timekeeping",
> +};

The patch shouod add some user-facing documentation for the user-facing
feature.

> +static struct sys_device device_timekeeping = {
> + .id = 0,
> + .cls = &timekeeping_sysclass,
> +};
> +
> +static int __init init_timekeeping_sysfs(void)
> +{
> + int error = sysdev_class_register(&timekeeping_sysclass);
> +
> + if (!error)
> + error = sysdev_register(&device_timekeeping);
> + if (!error)
> + error = sysdev_create_file(
> + &device_timekeeping,
> + &attr_jiffies_cpu);
> + return error;
> +}
> +
> +device_initcall(init_timekeeping_sysfs);
> +#endif /* SYSFS */



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