| Message ID | 20200115161035.893221-1-bigeasy@linutronix.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/compaction: Disable compact_unevictable_allowed on RT | expand |
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) >
On 2020-01-15 23:04:19 [+0100], Vlastimil Babka wrote: > 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? I've found this one while testing. Could you please point to the NUMA balancing that might be an issue? > > 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? Okay. I will make it RO then. > > 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? Sure. Let me add the sysctl documentation to this patch and then I will look into the manpage. > Thanks, > Vlastimil Sebastian
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)
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 remove it from /proc on RT. Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/ Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- kernel/sysctl.c | 3 ++- mm/compaction.c | 4 ++++ 2 files changed, 6 insertions(+), 1 deletion(-)