linux-kernel - Re: [PATCH v2 3/3] mm/vmstat: do not refresh stats for nohz

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Date:   Mon, 5 Jun 2023 09:59:57 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v2 3/3] mm/vmstat: do not refresh stats for nohz_full CPUs

On Fri 02-06-23 15:58:00, Marcelo Tosatti wrote:
> The interruption caused by queueing work on nohz_full CPUs 
> is undesirable for certain aplications.

This is not a proper changelog. I am not going to write a changelog for
you this time. Please explain why this is really needed and why this
approach is desired. E.g. why don't you prevent userspace from
refreshing stats if interference is not desirable. Also would it make
some sense to reduce flushing to cpumask of the calling process?
(certainly a daring thought but have you even considered it?)

> Fix by not refreshing per-CPU stats of nohz_full CPUs. 
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> ---
> v2: opencode schedule_on_each_cpu (Michal Hocko)
> 
> Index: linux-vmstat-remote/mm/vmstat.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmstat.c
> +++ linux-vmstat-remote/mm/vmstat.c
> @@ -1881,8 +1881,13 @@ int vmstat_refresh(struct ctl_table *tab
>  		   void *buffer, size_t *lenp, loff_t *ppos)
>  {
>  	long val;
> -	int err;
>  	int i;
> +	int cpu;
> +	struct work_struct __percpu *works;
> +
> +	works = alloc_percpu(struct work_struct);
> +	if (!works)
> +		return -ENOMEM;
>  
>  	/*
>  	 * The regular update, every sysctl_stat_interval, may come later
> @@ -1896,9 +1901,24 @@ int vmstat_refresh(struct ctl_table *tab
>  	 * 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;
> +	cpus_read_lock();
> +	for_each_online_cpu(cpu) {
> +		struct work_struct *work;
> +
> +		if (cpu_is_isolated(cpu))
> +			continue;
> +		work = per_cpu_ptr(works, cpu);
> +		INIT_WORK(work, refresh_vm_stats);
> +		schedule_work_on(cpu, work);
> +	}
> +
> +	for_each_online_cpu(cpu) {
> +		if (cpu_is_isolated(cpu))
> +			continue;
> +		flush_work(per_cpu_ptr(works, cpu));
> +	}
> +	cpus_read_unlock();
> +	free_percpu(works);
>  	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
>  		/*
>  		 * Skip checking stats known to go negative occasionally.
> 

-- 
Michal Hocko
SUSE Labs