Date:   Mon, 9 Oct 2017 10:20:24 +0800
From:   kemi <kemi.wang@...el.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Christopher Lameter <cl@...ux.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Dave <dave.hansen@...ux.intel.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ying Huang <ying.huang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>,
        Proc sysctl <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm, sysctl: make NUMA stats configurable



On 2017年09月29日 15:03, Vlastimil Babka wrote:
> On 09/28/2017 08:11 AM, Kemi Wang wrote:
>> This is the second step which introduces a tunable interface that allow
>> numa stats configurable for optimizing zone_statistics(), as suggested by
>> Dave Hansen and Ying Huang.
>>
>> =========================================================================
>> When page allocation performance becomes a bottleneck and you can tolerate
>> some possible tool breakage and decreased numa counter precision, you can
>> do:
>> 	echo [C|c]oarse > /proc/sys/vm/numa_stats_mode
>> In this case, numa counter update is ignored. We can see about
>> *4.8%*(185->176) drop of cpu cycles per single page allocation and reclaim
>> on Jesper's page_bench01 (single thread) and *8.1%*(343->315) drop of cpu
>> cycles per single page allocation and reclaim on Jesper's page_bench03 (88
>> threads) running on a 2-Socket Broadwell-based server (88 threads, 126G
>> memory).
>>
>> Benchmark link provided by Jesper D Brouer(increase loop times to
>> 10000000):
>> https://github.com/netoptimizer/prototype-kernel/tree/master/kernel/mm/
>> bench
>>
>> =========================================================================
>> When page allocation performance is not a bottleneck and you want all
>> tooling to work, you can do:
>> 	echo [S|s]trict > /proc/sys/vm/numa_stats_mode
>>
>> =========================================================================
>> We recommend automatic detection of numa statistics by system, this is also
>> system default configuration, you can do:
>> 	echo [A|a]uto > /proc/sys/vm/numa_stats_mode
>> In this case, numa counter update is skipped unless it has been read by
>> users at least once, e.g. cat /proc/zoneinfo.
>>
>> Branch target selection with jump label:
>> a) When numa_stats_mode is changed to *strict*, jump to the branch for numa
>> counters update.
>> b) When numa_stats_mode is changed to *coarse*, return back directly.
>> c) When numa_stats_mode is changed to *auto*, the branch target used in
>> last time is kept, and the branch target is changed to the branch for numa
>> counters update once numa counters are *read* by users.
>>
>> Therefore, with the help of jump label, the page allocation performance is
>> hardly affected when numa counters are updated with a call in
>> zone_statistics(). Meanwhile, the auto mode can give people benefit without
>> manual tuning.
>>
>> Many thanks to Michal Hocko, Dave Hansen and Ying Huang for comments to
>> help improve the original patch.
>>
>> ChangeLog:
>>   V2->V3:
>>   a) Propose a better way to use jump label to eliminate the overhead of
>>   branch selection in zone_statistics(), as inspired by Ying Huang;
>>   b) Add a paragraph in commit log to describe the way for branch target
>>   selection;
>>   c) Use a more descriptive name numa_stats_mode instead of vmstat_mode,
>>   and change the description accordingly, as suggested by Michal Hocko;
>>   d) Make this functionality NUMA-specific via ifdef
>>
>>   V1->V2:
>>   a) Merge to one patch;
>>   b) Use jump label to eliminate the overhead of branch selection;
>>   c) Add a single-time log message at boot time to help tell users what
>>   happened.
>>
>> Reported-by: Jesper Dangaard Brouer <brouer@...hat.com>
>> Suggested-by: Dave Hansen <dave.hansen@...el.com>
>> Suggested-by: Ying Huang <ying.huang@...el.com>
>> Signed-off-by: Kemi Wang <kemi.wang@...el.com>
>> ---
>>  Documentation/sysctl/vm.txt |  24 +++++++++
>>  drivers/base/node.c         |   4 ++
>>  include/linux/vmstat.h      |  23 ++++++++
>>  init/main.c                 |   3 ++
>>  kernel/sysctl.c             |   7 +++
>>  mm/page_alloc.c             |  10 ++++
>>  mm/vmstat.c                 | 129 ++++++++++++++++++++++++++++++++++++++++++++
>>  7 files changed, 200 insertions(+)
>>
>> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
>> index 9baf66a..e310e69 100644
>> --- a/Documentation/sysctl/vm.txt
>> +++ b/Documentation/sysctl/vm.txt
>> @@ -61,6 +61,7 @@ Currently, these files are in /proc/sys/vm:
>>  - swappiness
>>  - user_reserve_kbytes
>>  - vfs_cache_pressure
>> +- numa_stats_mode
>>  - watermark_scale_factor
>>  - zone_reclaim_mode
>>  
>> @@ -843,6 +844,29 @@ ten times more freeable objects than there are.
>>  
>>  =============================================================
>>  
>> +numa_stats_mode
>> +
>> +This interface allows numa statistics configurable.
>> +
>> +When page allocation performance becomes a bottleneck and you can tolerate
>> +some possible tool breakage and decreased numa counter precision, you can
>> +do:
>> +	echo [C|c]oarse > /proc/sys/vm/numa_stats_mode
>> +
>> +When page allocation performance is not a bottleneck and you want all
>> +tooling to work, you can do:
>> +	echo [S|s]trict > /proc/sys/vm/numa_stat_mode
>> +
>> +We recommend automatic detection of numa statistics by system, because numa
>> +statistics does not affect system's decision and it is very rarely
>> +consumed. you can do:
>> +	echo [A|a]uto > /proc/sys/vm/numa_stats_mode
>> +This is also system default configuration, with this default setting, numa
>> +counters update is skipped unless the counter is *read* by users at least
>> +once.
> 
> It says "the counter", but it seems multiple files in /proc and /sys are
> triggering this, so perhaps list them?

Exactly, four files use it.
/proc/zoneinfo
/proc/vmstat
/sys/devices/system/node/node*/vmstat
/sys/devices/system/node/node*/numastat
Well, I am not sure that it is worthy to list here.

> Also, is it possible that with contemporary userspace/distros (systemd
> etc.) there will always be something that will read one of those upon boot?
> 
It depends on the tools used in userspace. If some tool really read it,
the active state in auto mode will be triggered.

>> +
>> +==============================================================
>> +
>>  watermark_scale_factor:
>>  
>>  This factor controls the aggressiveness of kswapd. It defines the
>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>> index 3855902..b57b5622 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -153,6 +153,8 @@ static DEVICE_ATTR(meminfo, S_IRUGO, node_read_meminfo, NULL);
>>  static ssize_t node_read_numastat(struct device *dev,
>>  				struct device_attribute *attr, char *buf)
>>  {
>> +	if (vm_numa_stats_mode == VM_NUMA_STAT_AUTO_MODE)
>> +		static_branch_enable(&vm_numa_stats_mode_key);
>>  	return sprintf(buf,
>>  		       "numa_hit %lu\n"
>>  		       "numa_miss %lu\n"
>> @@ -186,6 +188,8 @@ static ssize_t node_read_vmstat(struct device *dev,
>>  		n += sprintf(buf+n, "%s %lu\n",
>>  			     vmstat_text[i + NR_VM_ZONE_STAT_ITEMS],
>>  			     sum_zone_numa_state(nid, i));
>> +	if (vm_numa_stats_mode == VM_NUMA_STAT_AUTO_MODE)
>> +		static_branch_enable(&vm_numa_stats_mode_key);
>>  #endif
>>  
>>  	for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
>> index ade7cb5..d52e882 100644
>> --- a/include/linux/vmstat.h
>> +++ b/include/linux/vmstat.h
>> @@ -6,9 +6,28 @@
>>  #include <linux/mmzone.h>
>>  #include <linux/vm_event_item.h>
>>  #include <linux/atomic.h>
>> +#include <linux/static_key.h>
>>  
>>  extern int sysctl_stat_interval;
>>  
>> +#ifdef CONFIG_NUMA
>> +DECLARE_STATIC_KEY_FALSE(vm_numa_stats_mode_key);
>> +/*
>> + * vm_numa_stats_mode:
>> + * 0 = auto mode of NUMA stats, automatic detection of NUMA statistics.
>> + * 1 = strict mode of NUMA stats, keep NUMA statistics.
>> + * 2 = coarse mode of NUMA stats, ignore NUMA statistics.
>> + */
>> +#define VM_NUMA_STAT_AUTO_MODE 0
>> +#define VM_NUMA_STAT_STRICT_MODE  1
>> +#define VM_NUMA_STAT_COARSE_MODE  2
>> +#define VM_NUMA_STAT_MODE_LEN 16
>> +extern int vm_numa_stats_mode;
>> +extern char sysctl_vm_numa_stats_mode[];
>> +extern int sysctl_vm_numa_stats_mode_handler(struct ctl_table *table, int write,
>> +		void __user *buffer, size_t *length, loff_t *ppos);
>> +#endif
>> +
>>  #ifdef CONFIG_VM_EVENT_COUNTERS
>>  /*
>>   * Light weight per cpu counter implementation.
>> @@ -229,6 +248,10 @@ extern unsigned long sum_zone_node_page_state(int node,
>>  extern unsigned long sum_zone_numa_state(int node, enum numa_stat_item item);
>>  extern unsigned long node_page_state(struct pglist_data *pgdat,
>>  						enum node_stat_item item);
>> +extern void zero_zone_numa_counters(struct zone *zone);
>> +extern void zero_zones_numa_counters(void);
>> +extern void zero_global_numa_counters(void);
>> +extern void invalid_numa_statistics(void);
> 
> These seem to be called only from within mm/vmstat.c where they live, so
> I'd suggest removing these extern declarations, and making them static
> in vmstat.c.
> 

Agree. Thanks for catching it.

> ...
> 
>>  #define NUMA_STATS_THRESHOLD (U16_MAX - 2)
>>  
>> +#ifdef CONFIG_NUMA
>> +int vm_numa_stats_mode = VM_NUMA_STAT_AUTO_MODE;
>> +char sysctl_vm_numa_stats_mode[VM_NUMA_STAT_MODE_LEN] = "auto";
>> +static const char *vm_numa_stats_mode_name[3] = {"auto", "strict", "coarse"};
>> +static DEFINE_MUTEX(vm_numa_stats_mode_lock);
>> +
>> +static int __parse_vm_numa_stats_mode(char *s)
>> +{
>> +	const char *str = s;
>> +
>> +	if (strcmp(str, "auto") == 0 || strcmp(str, "Auto") == 0)
>> +		vm_numa_stats_mode = VM_NUMA_STAT_AUTO_MODE;
>> +	else if (strcmp(str, "strict") == 0 || strcmp(str, "Strict") == 0)
>> +		vm_numa_stats_mode = VM_NUMA_STAT_STRICT_MODE;
>> +	else if (strcmp(str, "coarse") == 0 || strcmp(str, "Coarse") == 0)
>> +		vm_numa_stats_mode = VM_NUMA_STAT_COARSE_MODE;
>> +	else {
>> +		pr_warn("Ignoring invalid vm_numa_stats_mode value: %s\n", s);
>> +		return -EINVAL;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +int sysctl_vm_numa_stats_mode_handler(struct ctl_table *table, int write,
>> +		void __user *buffer, size_t *length, loff_t *ppos)
>> +{
>> +	char old_string[VM_NUMA_STAT_MODE_LEN];
>> +	int ret, oldval;
>> +
>> +	mutex_lock(&vm_numa_stats_mode_lock);
>> +	if (write)
>> +		strncpy(old_string, (char *)table->data, VM_NUMA_STAT_MODE_LEN);
>> +	ret = proc_dostring(table, write, buffer, length, ppos);
>> +	if (ret || !write) {
>> +		mutex_unlock(&vm_numa_stats_mode_lock);
>> +		return ret;
>> +	}
>> +
>> +	oldval = vm_numa_stats_mode;
>> +	if (__parse_vm_numa_stats_mode((char *)table->data)) {
>> +		/*
>> +		 * invalid sysctl_vm_numa_stats_mode value, restore saved string
>> +		 */
>> +		strncpy((char *)table->data, old_string, VM_NUMA_STAT_MODE_LEN);
>> +		vm_numa_stats_mode = oldval;
> 
> Do we need to restore vm_numa_stats_mode? 

Not necessary. 

AFAICS it didn't change. Also,
> should the EINVAL be returned also to userspace? (not sure what's the
> API here, hmm man 2 sysctl doesn't list EINVAL...)
> 

I don't think so. __parse is only be called in sysctl handler and returns
an invalid value to help restore back.

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