Re: [PATCH v2] mm, sysctl: make VM stats configurable

[kemi]

On 2017å09æ25æ 09:36, Huang, Ying wrote:
> Kemi Wang <kemi.wang@xxxxxxxxx> writes:
> 
>> This is the second step which introduces a tunable interface that allow VM
>> stats configurable for optimizing zone_statistics(), as suggested by Dave
>> Hansen and Ying Huang.
>>
>> =======================================
>> When performance becomes a bottleneck and you can tolerate some possible
>> tool breakage and some decreased counter precision (e.g. numa counter), you
>> can do:
>> 	echo [C|c]oarse > /proc/sys/vm/vmstat_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 performance is not a bottleneck and you want all tooling to work,
>> you
> 
> When page allocation performance isn't a bottleneck ...
> 

OK, it's better.

>> can do:
>> 	echo [S|s]trict > /proc/sys/vm/vmstat_mode
>>
>> =======================================
>> We recommend automatic detection of virtual memory statistics by system,
>> this is also system default configuration, you can do:
>> 	echo [A|a]uto > /proc/sys/vm/vmstat_mode
>> In this case, automatic detection of VM statistics, numa counter update
>> is skipped unless it has been read by users at least once, e.g. cat
>> /proc/zoneinfo.
>>
>> Therefore, with different VM stats mode, numa counters update can operate
>> differently so that everybody can benefit.
>>
>> Many thanks to Michal Hocko and Dave Hansen for comments to help improve
>> the original patch.
>>
>> ChangeLog:
>>   Since 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@xxxxxxxxxx>
>> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
>> Suggested-by: Ying Huang <ying.huang@xxxxxxxxx>
>> Signed-off-by: Kemi Wang <kemi.wang@xxxxxxxxx>
>> ---
>>  Documentation/sysctl/vm.txt |  26 +++++++++
>>  drivers/base/node.c         |   2 +
>>  include/linux/vmstat.h      |  22 ++++++++
>>  init/main.c                 |   2 +
>>  kernel/sysctl.c             |   7 +++
>>  mm/page_alloc.c             |  14 +++++
>>  mm/vmstat.c                 | 126 ++++++++++++++++++++++++++++++++++++++++++++
>>  7 files changed, 199 insertions(+)
>>
>> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
>> index 9baf66a..6ab2843 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
>> +- vmstat_mode
>>  - watermark_scale_factor
>>  - zone_reclaim_mode
>>  
>> @@ -843,6 +844,31 @@ ten times more freeable objects than there are.
>>  
>>  =============================================================
>>  
>> +vmstat_mode
>> +
>> +This interface allows virtual memory statistics configurable.
>> +
>> +When performance becomes a bottleneck and you can tolerate some possible
>> +tool breakage and some decreased counter precision (e.g. numa counter), you
>> +can do:
>> +	echo [C|c]oarse > /proc/sys/vm/vmstat_mode
>> +ignorable statistics list:
>> +- numa counters
>> +
>> +When performance is not a bottleneck and you want all tooling to work, you
>> +can do:
>> +	echo [S|s]trict > /proc/sys/vm/vmstat_mode
>> +
>> +We recommend automatic detection of virtual memory statistics by system,
>> +this is also system default configuration, you can do:
>> +	echo [A|a]uto > /proc/sys/vm/vmstat_mode
>> +
>> +E.g. numa statistics does not affect system's decision and it is very
>> +rarely consumed. If set vmstat_mode = auto, numa counters update is skipped
>> +unless the counter is *read* by users at least once.
>> +
>> +==============================================================
>> +
>>  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..033c0c3 100644
>> --- a/drivers/base/node.c
>> +++ b/drivers/base/node.c
>> @@ -153,6 +153,7 @@ 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)
>>  {
>> +	disable_zone_statistics = false;
>>  	return sprintf(buf,
>>  		       "numa_hit %lu\n"
>>  		       "numa_miss %lu\n"
>> @@ -194,6 +195,7 @@ static ssize_t node_read_vmstat(struct device *dev,
>>  			     NR_VM_NUMA_STAT_ITEMS],
>>  			     node_page_state(pgdat, i));
>>  
>> +	disable_zone_statistics = false;
>>  	return n;
>>  }
>>  static DEVICE_ATTR(vmstat, S_IRUGO, node_read_vmstat, NULL);
>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
>> index ade7cb5..22670cf 100644
>> --- a/include/linux/vmstat.h
>> +++ b/include/linux/vmstat.h
>> @@ -6,9 +6,27 @@
>>  #include <linux/mmzone.h>
>>  #include <linux/vm_event_item.h>
>>  #include <linux/atomic.h>
>> +#include <linux/static_key.h>
>>  
>>  extern int sysctl_stat_interval;
>>  
>> +DECLARE_STATIC_KEY_FALSE(vmstat_mode_key);
>> +extern bool disable_zone_statistics;
>> +/*
>> + * vmstat_mode:
>> + * 0 = auto mode of vmstat, automatic detection of VM statistics.
>> + * 1 = strict mode of vmstat, keep all VM statistics.
>> + * 2 = coarse mode of vmstat, ignore unimportant VM statistics.
>> + */
>> +#define VMSTAT_AUTO_MODE 0
>> +#define VMSTAT_STRICT_MODE  1
>> +#define VMSTAT_COARSE_MODE  2
>> +#define VMSTAT_MODE_LEN 16
>> +extern int vmstat_mode;
>> +extern char sysctl_vmstat_mode[];
>> +extern int sysctl_vmstat_mode_handler(struct ctl_table *table, int write,
>> +		void __user *buffer, size_t *length, loff_t *ppos);
>> +
>>  #ifdef CONFIG_VM_EVENT_COUNTERS
>>  /*
>>   * Light weight per cpu counter implementation.
>> @@ -229,6 +247,10 @@ 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);
>>  #else
>>  #define sum_zone_node_page_state(node, item) global_zone_page_state(item)
>>  #define node_page_state(node, item) global_node_page_state(item)
>> diff --git a/init/main.c b/init/main.c
>> index 0ee9c686..940b71c 100644
>> --- a/init/main.c
>> +++ b/init/main.c
>> @@ -567,6 +567,8 @@ asmlinkage __visible void __init start_kernel(void)
>>  	sort_main_extable();
>>  	trap_init();
>>  	mm_init();
>> +	pr_info("vmstat: System detection of virtual memory statistics, NUMA\n"
>> +"counters update is skipped unless they are read by users at least once\n");
>>  
>>  	ftrace_init();
>>  
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index 6648fbb..f5b813b 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -1234,6 +1234,13 @@ static struct ctl_table kern_table[] = {
>>  
>>  static struct ctl_table vm_table[] = {
>>  	{
>> +		.procname	= "vmstat_mode",
>> +		.data		= &sysctl_vmstat_mode,
>> +		.maxlen		= VMSTAT_MODE_LEN,
>> +		.mode		= 0644,
>> +		.proc_handler	= sysctl_vmstat_mode_handler,
>> +	},
>> +	{
>>  		.procname	= "overcommit_memory",
>>  		.data		= &sysctl_overcommit_memory,
>>  		.maxlen		= sizeof(sysctl_overcommit_memory),
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index c841af8..46afc8a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -83,6 +83,9 @@ DEFINE_PER_CPU(int, numa_node);
>>  EXPORT_PER_CPU_SYMBOL(numa_node);
>>  #endif
>>  
>> +DEFINE_STATIC_KEY_FALSE(vmstat_mode_key);
>> +bool disable_zone_statistics = true;
>> +
>>  #ifdef CONFIG_HAVE_MEMORYLESS_NODES
>>  /*
>>   * N.B., Do NOT reference the '_numa_mem_' per cpu variable directly.
>> @@ -2743,6 +2746,17 @@ static inline void zone_statistics(struct zone *preferred_zone, struct zone *z)
>>  #ifdef CONFIG_NUMA
>>  	enum numa_stat_item local_stat = NUMA_LOCAL;
>>  
>> +	/*
>> +	 * skip zone_statistics() if vmstat mode is coarse or zone statistics
>> +	 * is inactive in auto mode
>> +	 */
>> +
>> +	if (static_branch_unlikely(&vmstat_mode_key)) {
>> +		if (vmstat_mode == VMSTAT_COARSE_MODE)
>> +			return;
>> +	} else if (disable_zone_statistics)
>> +		return;
> 
> I suspect this will not help performance.  I suggest to make the most
> common case (auto mode + disable_zone_statistics (true)?)
> unconditionally with jump label, and other combination conditionally.
> 

Thanks, Ying!
As inspired by your comment, I propose the solution below that can eliminate
the overhead of branch selection further in zone_statistics().

When key is set to true (strict mode, or active state in auto mode), jump to the branch
for numa counter update.
When key is set to false (coarse mode, or inactive state in auto mode), just return back.


In this way, we can keep simple in fast path and move overhead to slow path(more cases
need to be considered when choose branch to jump). It will not add extra overhead when
numa counters are really updated in zone_statistics(), and also keep the benefit of auto
mode at the same time.


> Best Regards,
> Huang, Ying
> 
>> +
>>  	if (z->node != numa_node_id())
>>  		local_stat = NUMA_OTHER;
>>  
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index 4bb13e7..d4ab53e 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -32,6 +32,94 @@
>>  
>>  #define NUMA_STATS_THRESHOLD (U16_MAX - 2)
>>  
>> +int vmstat_mode = VMSTAT_AUTO_MODE;
>> +char sysctl_vmstat_mode[VMSTAT_MODE_LEN] = "auto";
>> +static const char *vmstat_mode_name[3] = {"auto", "strict", "coarse"};
>> +static DEFINE_MUTEX(vmstat_mode_lock);
>> +
>> +
>> +static int __parse_vmstat_mode(char *s)
>> +{
>> +	const char *str = s;
>> +
>> +	if (strcmp(str, "auto") == 0 || strcmp(str, "Auto") == 0) {
>> +		vmstat_mode = VMSTAT_AUTO_MODE;
>> +		static_branch_disable(&vmstat_mode_key);
>> +	} else if (strcmp(str, "strict") == 0 || strcmp(str, "Strict") == 0) {
>> +		vmstat_mode = VMSTAT_STRICT_MODE;
>> +		static_branch_enable(&vmstat_mode_key);
>> +	} else if (strcmp(str, "coarse") == 0 || strcmp(str, "Coarse") == 0) {
>> +		vmstat_mode = VMSTAT_COARSE_MODE;
>> +		static_branch_enable(&vmstat_mode_key);
>> +	} else {
>> +		pr_warn("Ignoring invalid vmstat_mode value: %s\n", s);
>> +		return -EINVAL;
>> +	}
>> +	return 0;
>> +}
>> +
>> +int sysctl_vmstat_mode_handler(struct ctl_table *table, int write,
>> +		void __user *buffer, size_t *length, loff_t *ppos)
>> +{
>> +	char old_string[VMSTAT_MODE_LEN];
>> +	int ret, oldval;
>> +
>> +	mutex_lock(&vmstat_mode_lock);
>> +	if (write)
>> +		strncpy(old_string, (char *)table->data, VMSTAT_MODE_LEN);
>> +	ret = proc_dostring(table, write, buffer, length, ppos);
>> +	if (ret || !write) {
>> +		mutex_unlock(&vmstat_mode_lock);
>> +		return ret;
>> +	}
>> +
>> +	oldval = vmstat_mode;
>> +	if (__parse_vmstat_mode((char *)table->data)) {
>> +		/*
>> +		 * invalid sysctl_vmstat_mode value, restore saved string
>> +		 */
>> +		strncpy((char *)table->data, old_string, VMSTAT_MODE_LEN);
>> +		vmstat_mode = oldval;
>> +	} else {
>> +		/*
>> +		 * check whether vmstat mode changes or not
>> +		 */
>> +		if (vmstat_mode == oldval) {
>> +			/* no change */
>> +			mutex_unlock(&vmstat_mode_lock);
>> +			return 0;
>> +		} else if (vmstat_mode == VMSTAT_AUTO_MODE) {
>> +			pr_info("vmstat mode changes from %s to auto mode\n",
>> +					vmstat_mode_name[oldval]);
>> +			/*
>> +			 * Set default numa stats action when vmstat mode changes
>> +			 * from coarse to auto
>> +			 */
>> +			if (oldval == VMSTAT_COARSE_MODE)
>> +				disable_zone_statistics = true;
>> +		} else if (vmstat_mode == VMSTAT_STRICT_MODE)
>> +			pr_info("vmstat mode changes from %s to strict mode\n",
>> +					vmstat_mode_name[oldval]);
>> +		else if (vmstat_mode == VMSTAT_COARSE_MODE) {
>> +			pr_info("vmstat mode changes from %s to coarse mode\n",
>> +					vmstat_mode_name[oldval]);
>> +#ifdef CONFIG_NUMA
>> +			/*
>> +			 * Invalidate numa counters when vmstat mode is set to coarse
>> +			 * mode, because users can't tell the difference between the
>> +			 * dead state and when allocator activity is quiet once
>> +			 * zone_statistics() is turned off.
>> +			 */
>> +			invalid_numa_statistics();
>> +#endif
>> +		} else
>> +			pr_warn("invalid vmstat_mode:%d\n", vmstat_mode);
>> +	}
>> +
>> +	mutex_unlock(&vmstat_mode_lock);
>> +	return 0;
>> +}
>> +
>>  #ifdef CONFIG_VM_EVENT_COUNTERS
>>  DEFINE_PER_CPU(struct vm_event_state, vm_event_states) = {{0}};
>>  EXPORT_PER_CPU_SYMBOL(vm_event_states);
>> @@ -914,6 +1002,42 @@ unsigned long sum_zone_numa_state(int node,
>>  	return count;
>>  }
>>  
>> +/* zero numa counters within a zone */
>> +void zero_zone_numa_counters(struct zone *zone)
>> +{
>> +	int item, cpu;
>> +
>> +	for (item = 0; item < NR_VM_NUMA_STAT_ITEMS; item++) {
>> +		atomic_long_set(&zone->vm_numa_stat[item], 0);
>> +		for_each_online_cpu(cpu)
>> +			per_cpu_ptr(zone->pageset, cpu)->vm_numa_stat_diff[item] = 0;
>> +	}
>> +}
>> +
>> +/* zero numa counters of all the populated zones */
>> +void zero_zones_numa_counters(void)
>> +{
>> +	struct zone *zone;
>> +
>> +	for_each_populated_zone(zone)
>> +		zero_zone_numa_counters(zone);
>> +}
>> +
>> +/* zero global numa counters */
>> +void zero_global_numa_counters(void)
>> +{
>> +	int item;
>> +
>> +	for (item = 0; item < NR_VM_NUMA_STAT_ITEMS; item++)
>> +		atomic_long_set(&vm_numa_stat[item], 0);
>> +}
>> +
>> +void invalid_numa_statistics(void)
>> +{
>> +	zero_zones_numa_counters();
>> +	zero_global_numa_counters();
>> +}
>> +
>>  /*
>>   * Determine the per node value of a stat item.
>>   */
>> @@ -1582,6 +1706,7 @@ static int zoneinfo_show(struct seq_file *m, void *arg)
>>  {
>>  	pg_data_t *pgdat = (pg_data_t *)arg;
>>  	walk_zones_in_node(m, pgdat, false, false, zoneinfo_show_print);
>> +	disable_zone_statistics = false;
>>  	return 0;
>>  }
>>  
>> @@ -1678,6 +1803,7 @@ static int vmstat_show(struct seq_file *m, void *arg)
>>  
>>  static void vmstat_stop(struct seq_file *m, void *arg)
>>  {
>> +	disable_zone_statistics = false;
>>  	kfree(m->private);
>>  	m->private = NULL;
>>  }