Re: [PATCH] mm: Disable NUMA_BALANCING_DEFAULT_ENABLED and TRANSPARENT_HUGEPAGE on PREEMPT_RT

[David Hildenbrand <david@redhat.com>]

On 28.10.21 16:33, Sebastian Andrzej Siewior wrote:
> TRANSPARENT_HUGEPAGE:
> There are potential non-deterministic delays to an RT thread if a critical
> memory region is not THP-aligned and a non-RT buffer is located in the same
> hugepage-aligned region. It's also possible for an unrelated thread to migrate
> pages belonging to an RT task incurring unexpected page faults due to memory
> defragmentation even if khugepaged is disabled.
> 
> Regular HUGEPAGEs are not affected by this can be used.
> 
> NUMA_BALANCING:
> There is a non-deterministic delay to mark PTEs PROT_NONE to gather NUMA fault
> samples, increased page faults of regions even if mlocked and non-deterministic
> delays when migrating pages.
> 
> [Mel Gorman worded 99% of the commit description].
> 
> Link: https://lore.kernel.org/all/20200304091159.GN3818@techsingularity.net/
> Link: https://lore.kernel.org/all/20211026165100.ahz5bkx44lrrw5pt@linutronix.de/
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  init/Kconfig | 2 +-
>  mm/Kconfig   | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index edc0a0228f143..f83e7a0478f7c 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -911,7 +911,7 @@ config NUMA_BALANCING
>  	bool "Memory placement aware NUMA scheduler"
>  	depends on ARCH_SUPPORTS_NUMA_BALANCING
>  	depends on !ARCH_WANT_NUMA_VARIABLE_LOCALITY
> -	depends on SMP && NUMA && MIGRATION
> +	depends on SMP && NUMA && MIGRATION && !PREEMPT_RT
>  	help
>  	  This option adds support for automatic NUMA aware memory/task placement.
>  	  The mechanism is quite primitive and is based on migrating memory when
> diff --git a/mm/Kconfig b/mm/Kconfig
> index c150a0c6fce2c..5c5508fafcec5 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -374,7 +374,7 @@ config NOMMU_INITIAL_TRIM_EXCESS
>  
>  config TRANSPARENT_HUGEPAGE
>  	bool "Transparent Hugepage Support"
> -	depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
> +	depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE && !PREEMPT_RT
>  	select COMPACTION
>  	select XARRAY_MULTI
>  	help
> 

Makes perfect sense to me. I was also wondering how "dangerous"
MIGRATION (and everything depending on it, like CMA, compaction,
MEMORY_HOTREMOVE, ...) is for RT recently.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb