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SubjectRe: [PATCH v3] mm, sysctl: make NUMA stats configurable
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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@redhat.com>
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Suggested-by: Ying Huang <ying.huang@intel.com>
> Signed-off-by: Kemi Wang <kemi.wang@intel.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?
Also, is it possible that with contemporary userspace/distros (systemd
etc.) there will always be something that will read one of those upon boot?

> +
> +==============================================================
> +
> 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.

...

> #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? 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...)

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