Re: [PATCH 2/2 v3] mm/compaction: Disable compact_unevictable_allowed on RT

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 3 Mar 2020 21:22:25 +0100 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> Since commit
>     5bbe3547aa3ba ("mm: allow compaction of unevictable pages")
> 
> it is allowed to examine mlocked pages and compact them by default.
> On -RT even minor pagefaults are problematic because it may take a few
> 100us to resolve them and until then the task is blocked.
> 
> Make compact_unevictable_allowed = 0 default and issue a warning on RT
> if it is changed.

Fair enough, I guess.

> @@ -2572,6 +2577,26 @@ int proc_dointvec(struct ctl_table *table, int write,
>  	return do_proc_dointvec(table, write, buffer, lenp, ppos, NULL, NULL);
>  }
>  
> +#ifdef CONFIG_COMPACTION
> +static int proc_dointvec_warn_RT_change(struct ctl_table *table, int write,
> +					void __user *buffer, size_t *lenp,
> +					loff_t *ppos)
> +{
> +	int ret, old;
> +
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || !write)
> +		return proc_dointvec(table, write, buffer, lenp, ppos);
> +
> +	old = *(int *)table->data;
> +	ret = proc_dointvec(table, write, buffer, lenp, ppos);
> +	if (ret)
> +		return ret;
> +	WARN_ONCE(old != *(int *)table->data, "sysctl attribute %s changed.",
> +		  table->procname);

The WARN will include a stack trace which just isn't interesting.  A
pr_warn() would be better?

> +	return ret;
> +}
> +#endif