Re: [PATCH 2/2 v4] mm/compaction: Disable compact_unevictable_allowed on RT

[Vlastimil Babka <vbabka@suse.cz>]

On 3/19/20 5:39 PM, Sebastian Andrzej Siewior 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.
> 
> Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Nit below:

> @@ -2572,6 +2577,28 @@ 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_minmax_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_minmax(table, write, buffer, lenp, ppos);
> +
> +	old = *(int *)table->data;
> +	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +	if (ret)
> +		return ret;
> +	if (old != *(int *)table->data)
> +		pr_warn_once("sysctl attribute %s changed changed by %s[%d]\n",

						  ^ "changed" twice

> +			     table->procname, current->comm,
> +			     task_pid_nr(current));
> +	return ret;
> +}
> +#endif
> +
>  /**
>   * proc_douintvec - read a vector of unsigned integers
>   * @table: the sysctl table