答复: [PATCH] mm/compaction:let proactive compaction order configurable

["Chu,Kaiping" <chukaiping@baidu.com>]

Hi Oleksandr,
Please see my answer inline.

BR,
Chu Kaiping

-----邮件原件-----
发件人: Oleksandr Natalenko <oleksandr@natalenko.name> 
发送时间: 2021年4月13日 0:58
收件人: Chu,Kaiping <chukaiping@baidu.com>
抄送: mcgrof@kernel.org; keescook@chromium.org; yzaikin@google.com; akpm@linux-foundation.org; linux-kernel@vger.kernel.org; linux-fsdevel@vger.kernel.org; linux-mm@kvack.org
主题: Re: [PATCH] mm/compaction:let proactive compaction order configurable

Hello.

On Mon, Apr 12, 2021 at 05:05:30PM +0800, chukaiping wrote:
> Currently the proactive compaction order is fixed to 
> COMPACTION_HPAGE_ORDER(9), it's OK in most machines with lots of 
> normal 4KB memory, but it's too high for the machines with small 
> normal memory, for example the machines with most memory configured as 
> 1GB hugetlbfs huge pages. In these machines the max order of free 
> pages is often below 9, and it's always below 9 even with hard 
> compaction. This will lead to proactive compaction be triggered very 
> frequently. In these machines we only care about order of 3 or 4.
> This patch export the oder to proc and let it configurable by user, 
> and the default value is still COMPACTION_HPAGE_ORDER.
> 
> Signed-off-by: chukaiping <chukaiping@baidu.com>
> ---
>  include/linux/compaction.h |    1 +
>  kernel/sysctl.c            |   10 ++++++++++
>  mm/compaction.c            |    7 ++++---
>  3 files changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h 
> index ed4070e..151ccd1 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -83,6 +83,7 @@ static inline unsigned long compact_gap(unsigned int 
> order)  #ifdef CONFIG_COMPACTION  extern int sysctl_compact_memory;  
> extern unsigned int sysctl_compaction_proactiveness;
> +extern unsigned int sysctl_compaction_order;
>  extern int sysctl_compaction_handler(struct ctl_table *table, int write,
>  			void *buffer, size_t *length, loff_t *ppos);  extern int 
> sysctl_extfrag_threshold; diff --git a/kernel/sysctl.c 
> b/kernel/sysctl.c index 62fbd09..277df31 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -114,6 +114,7 @@
>  static int __maybe_unused neg_one = -1;  static int __maybe_unused 
> two = 2;  static int __maybe_unused four = 4;
> +static int __maybe_unused ten = 10;

^^ does the upper limit have to be hard-coded like this?
--> the max order of buddy is defined by MAX_ORDER, I will change it to MAX_ORDER is next patch.

>  static unsigned long zero_ul;
>  static unsigned long one_ul = 1;
>  static unsigned long long_max = LONG_MAX;
> @@ -2871,6 +2872,15 @@ int proc_do_static_key(struct ctl_table *table, int write,
>  		.extra2		= &one_hundred,
>  	},
>  	{
> +		.procname       = "compaction_order",
> +		.data           = &sysctl_compaction_order,
> +		.maxlen         = sizeof(sysctl_compaction_order),
> +		.mode           = 0644,
> +		.proc_handler   = proc_dointvec_minmax,
> +		.extra1         = SYSCTL_ZERO,

I wonder what happens if this knob is set to 0. Have you tested such a
corner case?
--> in theory, 0 is also a configurable value, but the fragment index of order 0 is always 0, so it won't do any proactive compaction. I have had a test, if set order to 0, there is no any error, but proactive compaction won't happen.

> +		.extra2         = &ten,
> +	},
> +	{
>  		.procname	= "extfrag_threshold",
>  		.data		= &sysctl_extfrag_threshold,
>  		.maxlen		= sizeof(int),
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e04f447..a192996 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1925,16 +1925,16 @@ static bool kswapd_is_running(pg_data_t *pgdat)
>  
>  /*
>   * A zone's fragmentation score is the external fragmentation wrt to the
> - * COMPACTION_HPAGE_ORDER. It returns a value in the range [0, 100].
> + * sysctl_compaction_order. It returns a value in the range [0, 100].
>   */
>  static unsigned int fragmentation_score_zone(struct zone *zone)
>  {
> -	return extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> +	return extfrag_for_order(zone, sysctl_compaction_order);
>  }
>  
>  /*
>   * A weighted zone's fragmentation score is the external fragmentation
> - * wrt to the COMPACTION_HPAGE_ORDER scaled by the zone's size. It
> + * wrt to the sysctl_compaction_order scaled by the zone's size. It
>   * returns a value in the range [0, 100].
>   *
>   * The scaling factor ensures that proactive compaction focuses on larger
> @@ -2666,6 +2666,7 @@ static void compact_nodes(void)
>   * background. It takes values in the range [0, 100].
>   */
>  unsigned int __read_mostly sysctl_compaction_proactiveness = 20;
> +unsigned int __read_mostly sysctl_compaction_order = COMPACTION_HPAGE_ORDER;
>  
>  /*
>   * This is the entry point for compacting all nodes via
> -- 
> 1.7.1
> 

-- 
  Oleksandr Natalenko (post-factum)