Re: [Patch 2/7] Add sysctl for schedstats

[Nick Piggin <nickpiggin@yahoo.com.au>]

Shailabh Nagar wrote:
> schedstats-sysctl.patch
> 
> Add sysctl option for controlling schedstats collection
> dynamically. Delay accounting leverages schedstats for
> cpu delay statistics.
> 

I'd sort of rather not tie this in with schedstats if possible.
Schedstats adds a reasonable amount of cache footprint and
branches in hot paths. Most of schedstats stuff is something that
hardly anyone will use.

Sure you can share common code though...

>  
> Index: linux-2.6.16-rc4/kernel/sched.c
> ===================================================================
> --- linux-2.6.16-rc4.orig/kernel/sched.c	2006-02-27 01:20:04.000000000 -0500
> +++ linux-2.6.16-rc4/kernel/sched.c	2006-02-27 01:52:52.000000000 -0500
> @@ -382,11 +382,56 @@ static inline void task_rq_unlock(runque
>  }
>  
>  #ifdef CONFIG_SCHEDSTATS
> +
> +int schedstats_sysctl = 0;		/* schedstats turned off by default */

Should be read mostly.

> +static DEFINE_PER_CPU(int, schedstats) = 0;
> +

When the above is in the read mostly section, you won't need this at all.

You don't intend to switch the sysctl with great frequency, do you?

> +static void schedstats_set(int val)
> +{
> +	int i;
> +	static spinlock_t schedstats_lock = SPIN_LOCK_UNLOCKED;
> +
> +	spin_lock(&schedstats_lock);
> +	schedstats_sysctl = val;
> +	for (i = 0; i < NR_CPUS; i++)
> +		per_cpu(schedstats, i) = val;
> +	spin_unlock(&schedstats_lock);
> +}
> +
> +static int __init schedstats_setup_enable(char *str)
> +{
> +	schedstats_sysctl = 1;
> +	schedstats_set(schedstats_sysctl);
> +	return 1;
> +}
> +
> +__setup("schedstats", schedstats_setup_enable);
> +
> +int schedstats_sysctl_handler(ctl_table *table, int write, struct file *filp,
> +			void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int ret, prev = schedstats_sysctl;
> +	struct task_struct *g, *t;
> +
> +	ret = proc_dointvec(table, write, filp, buffer, lenp, ppos);
> +	if ((ret != 0) || (prev == schedstats_sysctl))
> +		return ret;
> +	if (schedstats_sysctl) {
> +		read_lock(&tasklist_lock);
> +		do_each_thread(g, t) {
> +			memset(&t->sched_info, 0, sizeof(t->sched_info));
> +		} while_each_thread(g, t);
> +		read_unlock(&tasklist_lock);
> +	}
> +	schedstats_set(schedstats_sysctl);

You don't clear the rq's schedstats stuff here.

And clearing this at all is not really needed for the schedstats interface.
You have a timestamp and a set of accumulated values, so it is easy to work
out deltas. So do you need this?

-- 
SUSE Labs, Novell Inc.
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