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

[Vlastimil Babka <vbabka@suse.cz>]

On 1/15/2020 5:10 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.

Fine, this makes sense on RT I guess. There might be some trade-off for
high-order allocation latencies though. We could perhaps migrate such mlocked
pages to pages allocated without __GFP_MOVABLE during the mlock() to at least
somewhat prevent them being scattered all over the zones. For MCL_FUTURE,
allocate them as unmovable from the beginning. But that can wait until issues
are reported.
I assume you have similar solution for NUMA balancing and whatever else can
cause minor faults?

> Make compact_unevictable_allowed = 0 default and remove it from /proc on
> RT.

Removing it is maybe going too far in terms of RT kernel differences confusing
users? Change the default sure, perhaps making it read-only, but removing?

> Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/

In any case the sysctl Documentation/ should be updated? And perhaps also the
mlock manpage as you noted in the older thread above?

Thanks,
Vlastimil

> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  kernel/sysctl.c | 3 ++-
>  mm/compaction.c | 4 ++++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 70665934d53e2..d08bd51a0fbc3 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1488,6 +1488,7 @@ static struct ctl_table vm_table[] = {
>  		.extra1		= &min_extfrag_threshold,
>  		.extra2		= &max_extfrag_threshold,
>  	},
> +#ifndef CONFIG_PREEMPT_RT
>  	{
>  		.procname	= "compact_unevictable_allowed",
>  		.data		= &sysctl_compact_unevictable_allowed,
> @@ -1497,7 +1498,7 @@ static struct ctl_table vm_table[] = {
>  		.extra1		= SYSCTL_ZERO,
>  		.extra2		= SYSCTL_ONE,
>  	},
> -
> +#endif
>  #endif /* CONFIG_COMPACTION */
>  	{
>  		.procname	= "min_free_kbytes",
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 672d3c78c6abf..b2c804c35ae56 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1590,7 +1590,11 @@ typedef enum {
>   * Allow userspace to control policy on scanning the unevictable LRU for
>   * compactable pages.
>   */
> +#ifdef CONFIG_PREEMPT_RT
> +#define sysctl_compact_unevictable_allowed 0
> +#else
>  int sysctl_compact_unevictable_allowed __read_mostly = 1;
> +#endif
>  
>  static inline void
>  update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
>