Re: [PATCH] mm/compaction: Disable compact_unevictable_allowed on RT
From: Vlastimil Babka <hidden>
Date: 2020-01-15 22:04:23
Also in:
linux-mm
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-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org/
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
quoted hunk
Signed-off-by: Sebastian Andrzej Siewior <redacted> --- 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)