Re: [PATCH 2/2 v3] mm/compaction: Disable compact_unevictable_allowed on RT

[Vlastimil Babka <vbabka@suse.cz>]

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@linutronix.de/
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> 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)
>