From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Subject: mm/compaction: Disable compact_unevictable_allowed on RT
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 issue a warning on RT if
it is changed.
[bigeasy@xxxxxxxxxxxxx: v5]
Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@xxxxxxxxxxxxx/
Link: http://lkml.kernel.org/r/20200319165536.ovi75tsr2seared4@xxxxxxxxxxxxx
Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@xxxxxxxxxxxxx/
Link: http://lkml.kernel.org/r/20200303202225.nhqc3v5gwlb7x6et@xxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Iurii Zaikin <yzaikin@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/sysctl/vm.rst | 3 ++
kernel/sysctl.c | 29 +++++++++++++++++++++-
mm/compaction.c | 4 +++
3 files changed, 35 insertions(+), 1 deletion(-)
--- a/Documentation/admin-guide/sysctl/vm.rst~mm-compaction-disable-compact_unevictable_allowed-on-rt
+++ a/Documentation/admin-guide/sysctl/vm.rst
@@ -128,6 +128,9 @@ allowed to examine the unevictable lru (
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 in order to avoid a page fault, due
+to compaction, which would block the task from becomming active until the fault
+is resolved.
dirty_background_bytes
--- a/kernel/sysctl.c~mm-compaction-disable-compact_unevictable_allowed-on-rt
+++ a/kernel/sysctl.c
@@ -212,6 +212,11 @@ static int proc_do_cad_pid(struct ctl_ta
void __user *buffer, size_t *lenp, loff_t *ppos);
static int proc_taint(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos);
+#ifdef CONFIG_COMPACTION
+static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,
+ int write, void __user *buffer,
+ size_t *lenp, loff_t *ppos);
+#endif
#endif
#ifdef CONFIG_PRINTK
@@ -1467,7 +1472,7 @@ static struct ctl_table vm_table[] = {
.data = &sysctl_compact_unevictable_allowed,
.maxlen = sizeof(int),
.mode = 0644,
- .proc_handler = proc_dointvec_minmax,
+ .proc_handler = proc_dointvec_minmax_warn_RT_change,
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_ONE,
},
@@ -2555,6 +2560,28 @@ int proc_dointvec(struct ctl_table *tabl
return do_proc_dointvec(table, write, buffer, lenp, ppos, NULL, NULL);
}
+#ifdef CONFIG_COMPACTION
+static int proc_dointvec_minmax_warn_RT_change(struct ctl_table *table,
+ int write, void __user *buffer,
+ size_t *lenp, loff_t *ppos)
+{
+ int ret, old;
+
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT) || !write)
+ return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+
+ old = *(int *)table->data;
+ ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+ if (ret)
+ return ret;
+ if (old != *(int *)table->data)
+ pr_warn_once("sysctl attribute %s changed by %s[%d]\n",
+ table->procname, current->comm,
+ task_pid_nr(current));
+ return ret;
+}
+#endif
+
/**
* proc_douintvec - read a vector of unsigned integers
* @table: the sysctl table
--- a/mm/compaction.c~mm-compaction-disable-compact_unevictable_allowed-on-rt
+++ a/mm/compaction.c
@@ -1594,7 +1594,11 @@ typedef enum {
* Allow userspace to control policy on scanning the unevictable LRU for
* compactable pages.
*/
+#ifdef CONFIG_PREEMPT_RT
+int sysctl_compact_unevictable_allowed __read_mostly = 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)
_
