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Date:   Mon, 12 Apr 2021 11:26:27 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     chukaiping <chukaiping@baidu.com>
cc:     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
Subject: Re: [PATCH] mm/compaction:let proactive compaction order
 configurable
In-Reply-To: <1618218330-50591-1-git-send-email-chukaiping@baidu.com>
Message-ID: <e57c2db3-11f-4d1b-b5cc-8a9e112af34@google.com>
References: <1618218330-50591-1-git-send-email-chukaiping@baidu.com>
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On Mon, 12 Apr 2021, 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.
> 

I'm curious why you have proactive compaction enabled at all in this case?

The order-9 threshold is likely to optimize for hugepage availability, but 
in your setup it appears that's not a goal.

So what benefit does proactive compaction provide if only done for order-3 
or order-4?

> 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;
>  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,
> +		.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
> 
>