On Tue, 3 Mar 2020 21:22:25 +0100 Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> 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
