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Message-ID: <7efa316c-d39b-59a5-bc52-62325127a917@google.com>
Date: Fri, 16 Apr 2021 14:31:28 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: chukaiping <chukaiping@...du.com>
cc: mcgrof@...nel.org, keescook@...omium.org, yzaikin@...gle.com,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm/compaction:let proactive compaction order
configurable
On Sat, 17 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.
>
Still not entirely clear on the use case beyond hugepages. In your
response from v1, you indicated you were not concerned with allocation
latency of hugepages but rather had a thundering herd problem where once
fragmentation got bad, many threads started compacting all at once.
I'm not sure that tuning the proactive compaction order is the right
solution. I think the proactive compaction order is more about starting
compaction when a known order of interest (like a hugepage) is fully
depleted and we want a page of that order so the idea is to start
recovering from that situation.
Is this not a userspace policy decision? I'm wondering if it would simply
be better to manually invoke compaction periodically or when the
fragmentation ratio has reached a certain level. You can manually invoke
compaction yourself through sysfs for each node on the system.
> Signed-off-by: chukaiping <chukaiping@...du.com>
> Reported-by: kernel test robot <lkp@...el.com>
> ---
>
> Changes in v2:
> - fix the compile error in ia64 and powerpc
> - change the hard coded max order number from 10 to MAX_ORDER - 1
>
> include/linux/compaction.h | 1 +
> kernel/sysctl.c | 11 +++++++++++
> mm/compaction.c | 14 +++++++++++---
> 3 files changed, 23 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..a607d4d 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -195,6 +195,8 @@ enum sysctl_writes_mode {
> #endif /* CONFIG_SMP */
> #endif /* CONFIG_SCHED_DEBUG */
>
> +static int max_buddy_zone = MAX_ORDER - 1;
> +
> #ifdef CONFIG_COMPACTION
> static int min_extfrag_threshold;
> static int max_extfrag_threshold = 1000;
> @@ -2871,6 +2873,15 @@ 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 = &max_buddy_zone,
> + },
> + {
> .procname = "extfrag_threshold",
> .data = &sysctl_extfrag_threshold,
> .maxlen = sizeof(int),
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e04f447..bfd1d5e 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;
>
> /*
> * This is the entry point for compacting all nodes via
> @@ -2958,6 +2959,13 @@ static int __init kcompactd_init(void)
> int nid;
> int ret;
>
> + /*
> + * move the initialization of sysctl_compaction_order to here to
> + * eliminate compile error in ia64 and powerpc architecture because
> + * COMPACTION_HPAGE_ORDER is a variable in this architecture
> + */
> + sysctl_compaction_order = COMPACTION_HPAGE_ORDER;
> +
> ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> "mm/compaction:online",
> kcompactd_cpu_online, NULL);
> --
> 1.7.1
>
>
>
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