Re: [PATCH v2] mm/compaction: Disable compact_unevictable_allowed on RT

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

On Mon, 2 Mar 2020 18:35:16 +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 RO on RT.

hm, that's a bit sad but I guess it's tolerable.

> ...
>
> index 64aeee1009cab..bbfa59d25eec3 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -128,6 +128,7 @@ allowed to examine the unevictable lru (mlocked pages) for pages to compact.
>  This should be used on systems where stalls for minor page faults are an
>  acceptable trade for large contiguous free memory.  Set to 0 to prevent
>  compaction from moving pages that are unevictable.  Default value is 1.
> +On CONFIG_PREEMPT_RT the default value is 0.

This doesn't mention that the file is unwritable on -rt, and it doesn't
explain *why* -rt has different behaviour.

> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1483,7 +1483,11 @@ static struct ctl_table vm_table[] = {
>  		.procname	= "compact_unevictable_allowed",
>  		.data		= &sysctl_compact_unevictable_allowed,
>  		.maxlen		= sizeof(int),
> +#ifdef CONFIG_PREEMPT_RT
> +		.mode		= 0444,
> +#else
>  		.mode		= 0644,
> +#endif

This is non-backward-compatible and introduces a possibility that
tested-on-non-rt userspace will fail on -rt kernels.  It might be
better to accept the writes, but to ignore them.  Probably with a
pr_warn_once() to let people know what we did.

But do we really need to take the option away from -rt users?  Perhaps
someone wants this feature and can accept the latency hit.  How about
switching the default and otherwise leaving the kernel behaviour as-is
and simply emitting a warning letting -rt users know that they might
not want to enable this?