On Mon, 2 Mar 2020 18:35:16 +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 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?
