From 6923aa0d8c629a7853822626877dcb11f4f1d354 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Wed, 1 Apr 2020 21:10:42 -0700 Subject: [PATCH] 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@linutronix.de: v5] Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/ Link: http://lkml.kernel.org/r/20200319165536.ovi75tsr2seared4@linutronix.de Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Andrew Morton Reviewed-by: Andrew Morton Acked-by: Mel Gorman Acked-by: Vlastimil Babka Cc: Thomas Gleixner Cc: Luis Chamberlain Cc: Kees Cook Cc: Iurii Zaikin Cc: Vlastimil Babka Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/ Link: http://lkml.kernel.org/r/20200303202225.nhqc3v5gwlb7x6et@linutronix.de Signed-off-by: Linus Torvalds --- Documentation/admin-guide/sysctl/vm.rst | 3 +++ kernel/sysctl.c | 29 ++++++++++++++++++++++++- mm/compaction.c | 4 ++++ 3 files changed, 35 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index 64aeee1009ca..0329a4d3fa9e 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -128,6 +128,9 @@ allowed to examine the unevictable lru (mlocked pages) for pages to compact. 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 diff --git a/kernel/sysctl.c b/kernel/sysctl.c index cb650bb9da68..8a176d8727a3 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -212,6 +212,11 @@ static int proc_do_cad_pid(struct ctl_table *table, int write, 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 *table, int write, 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 diff --git a/mm/compaction.c b/mm/compaction.c index 07947387244a..c589ead54fb3 100644 --- a/mm/compaction.c +++ b/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)