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Date:   Mon, 20 Jul 2020 10:03:49 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2] mm: vmstat: fix /proc/sys/vm/stat_refresh generating
 false warnings
Message-ID: <20200720080349.GC18535@dhcp22.suse.cz>
References: <20200714173920.3319063-1-guro@fb.com>
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On Tue 14-07-20 10:39:20, Roman Gushchin wrote:
> I've noticed a number of warnings like "vmstat_refresh: nr_free_cma
> -5" or "vmstat_refresh: nr_zone_write_pending -11" on our production
> hosts. The numbers of these warnings were relatively low and stable,
> so it didn't look like we are systematically leaking the counters.
> The corresponding vmstat counters also looked sane.
> 
> These warnings are generated by the vmstat_refresh() function, which
> assumes that atomic zone and numa counters can't go below zero.
> However, on a SMP machine it's not quite right: due to per-cpu
> caching it can in theory be as low as -(zone threshold) * NR_CPUs.
> 
> For instance, let's say all cma pages are in use and NR_FREE_CMA_PAGES
> reached 0. Then we've reclaimed a small number of cma pages on each
> CPU except CPU0, so that most percpu NR_FREE_CMA_PAGES counters are
> slightly positive (the atomic counter is still 0). Then somebody on
> CPU0 consumes all these pages. The number of pages can easily exceed
> the threshold and a negative value will be committed to the atomic
> counter.
> 
> To fix the problem and avoid generating false warnings, let's just
> relax the condition and warn only if the value is less than minus
> the maximum theoretically possible drift value, which is 125 *
> number of online CPUs. It will still allow to catch systematic leaks,
> but will not generate bogus warnings.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Michal Hocko <mhocko@suse.com>

One minor nit which can be handled by a separate patch but now that you
are touching this code...

> ---
>  Documentation/admin-guide/sysctl/vm.rst |  4 ++--
>  mm/vmstat.c                             | 30 ++++++++++++++++---------
>  2 files changed, 21 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 4b9d2e8e9142..95fb80d0c606 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -822,8 +822,8 @@ e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo
>  
>  As a side-effect, it also checks for negative totals (elsewhere reported
>  as 0) and "fails" with EINVAL if any are found, with a warning in dmesg.
> -(At time of writing, a few stats are known sometimes to be found negative,
> -with no ill effects: errors and warnings on these stats are suppressed.)
> +(On a SMP machine some stats can temporarily become negative, with no ill
> +effects: errors and warnings on these stats are suppressed.)
>  
>  
>  numa_stat
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index a21140373edb..8f0ef8aaf8ee 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -169,6 +169,8 @@ EXPORT_SYMBOL(vm_node_stat);
>  
>  #ifdef CONFIG_SMP
>  
> +#define MAX_THRESHOLD 125

This would deserve a comment. 88f5acf88ae6a didn't really explain why
this specific value has been selected but the specific value shouldn't
really matter much. I would go with the following at least.
"
Maximum sync threshold for per-cpu vmstat counters. 
"
> +
>  int calculate_pressure_threshold(struct zone *zone)
>  {
>  	int threshold;
> @@ -186,11 +188,9 @@ int calculate_pressure_threshold(struct zone *zone)
>  	threshold = max(1, (int)(watermark_distance / num_online_cpus()));
>  
>  	/*
> -	 * Maximum threshold is 125
> +	 * Threshold is capped by MAX_THRESHOLD
>  	 */
> -	threshold = min(125, threshold);
> -
> -	return threshold;
> +	return min(MAX_THRESHOLD, threshold);
>  }
>  
>  int calculate_normal_threshold(struct zone *zone)
> @@ -610,6 +610,9 @@ void dec_node_page_state(struct page *page, enum node_stat_item item)
>  }
>  EXPORT_SYMBOL(dec_node_page_state);
>  #else
> +
> +#define MAX_THRESHOLD 0
> +
>  /*
>   * Use interrupt disable to serialize counter updates
>   */
> @@ -1810,7 +1813,7 @@ static void refresh_vm_stats(struct work_struct *work)
>  int vmstat_refresh(struct ctl_table *table, int write,
>  		   void *buffer, size_t *lenp, loff_t *ppos)
>  {
> -	long val;
> +	long val, max_drift;
>  	int err;
>  	int i;
>  
> @@ -1821,17 +1824,22 @@ int vmstat_refresh(struct ctl_table *table, int write,
>  	 * pages, immediately after running a test.  /proc/sys/vm/stat_refresh,
>  	 * which can equally be echo'ed to or cat'ted from (by root),
>  	 * can be used to update the stats just before reading them.
> -	 *
> -	 * Oh, and since global_zone_page_state() etc. are so careful to hide
> -	 * transiently negative values, report an error here if any of
> -	 * the stats is negative, so we know to go looking for imbalance.
>  	 */
>  	err = schedule_on_each_cpu(refresh_vm_stats);
>  	if (err)
>  		return err;
> +
> +	/*
> +	 * Since global_zone_page_state() etc. are so careful to hide
> +	 * transiently negative values, report an error here if any of
> +	 * the stats is negative and are less than the maximum drift value,
> +	 * so we know to go looking for imbalance.
> +	 */
> +	max_drift = num_online_cpus() * MAX_THRESHOLD;
> +
>  	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
>  		val = atomic_long_read(&vm_zone_stat[i]);
> -		if (val < 0) {
> +		if (val < -max_drift) {
>  			pr_warn("%s: %s %ld\n",
>  				__func__, zone_stat_name(i), val);
>  			err = -EINVAL;
> @@ -1840,7 +1848,7 @@ int vmstat_refresh(struct ctl_table *table, int write,
>  #ifdef CONFIG_NUMA
>  	for (i = 0; i < NR_VM_NUMA_STAT_ITEMS; i++) {
>  		val = atomic_long_read(&vm_numa_stat[i]);
> -		if (val < 0) {
> +		if (val < -max_drift) {
>  			pr_warn("%s: %s %ld\n",
>  				__func__, numa_stat_name(i), val);
>  			err = -EINVAL;
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs