On 3/3/20 9:22 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.
>
> Make compact_unevictable_allowed = 0 default and issue a warning on RT
> if it is changed.
>
> Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@xxxxxxxxxxxxx/
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> v2…v3: - Allow to modify the value but issue a warning if it is changed.
>
> v1…v2: - Make the proc file RO instead removing it.
> - Mention this change in Documentation/…/vm.rst.
>
> Documentation/admin-guide/sysctl/vm.rst | 3 +++
> kernel/sysctl.c | 27 ++++++++++++++++++++++++-
> mm/compaction.c | 4 ++++
> 3 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 64aeee1009cab..0329a4d3fa9ec 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 982203101f961..3ace90b6ac57f 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_warn_RT_change(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp,
> + loff_t *ppos);
> +#endif
> #endif
>
> #ifdef CONFIG_PRINTK
> @@ -1484,7 +1489,7 @@ 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_warn_RT_change,
> .extra1 = SYSCTL_ZERO,
> .extra2 = SYSCTL_ONE,
> },
> @@ -2572,6 +2577,26 @@ 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_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(table, write, buffer, lenp, ppos);
Shouldn't you use her proc_dointvec_minmax() per Patch 1/2 ?
> +
> + old = *(int *)table->data;
> + ret = proc_dointvec(table, write, buffer, lenp, ppos);
And here.
> + if (ret)
> + return ret;
> + WARN_ONCE(old != *(int *)table->data, "sysctl attribute %s changed.",
> + table->procname);
> + 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 672d3c78c6abf..ba77809a1666e 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
> +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)
>
