linux-kernel - Re: [PATCH v2 1/2] mm: make vmstat

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Message-ID: <0000013f61e7609b-a8d1907b-8169-4f77-ab83-a624a8d0ab4a-000000@email.amazonses.com>
Date:	Thu, 20 Jun 2013 14:05:20 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2 1/2] mm: make vmstat_update periodic run conditional

On Wed, 19 Jun 2013, Gilad Ben-Yossef wrote:

> +static void vmstat_update(struct work_struct *w)
> +{
> +	int cpu, this_cpu = smp_processor_id();
> +
> +	if (unlikely(this_cpu == vmstat_monitor_cpu))
> +		for_each_cpu_not(cpu, &vmstat_cpus)
> +			if (need_vmstat(cpu))
> +				start_cpu_timer(cpu);
> +
> +	if (likely(refresh_cpu_vm_stats(this_cpu) || (this_cpu == vmstat_monitor_cpu)))
> +		schedule_delayed_work(&__get_cpu_var(vmstat_work),
> +				round_jiffies_relative(sysctl_stat_interval));
> +	else
> +		cpumask_clear_cpu(this_cpu, &vmstat_cpus);

The clearing of vmstat_cpus could be avoided if this processor is not
running tickless. Frequent updates to vmstat_cpus could become an issue.

>  	case CPU_DOWN_PREPARE_FROZEN:
> -		cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> -		per_cpu(vmstat_work, cpu).work.func = NULL;
> +		if (cpumask_test_cpu(cpu, &vmstat_cpus)) {
> +			cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> +			per_cpu(vmstat_work, cpu).work.func = NULL;
> +			if(cpu == vmstat_monitor_cpu) {
> +				int this_cpu = smp_processor_id();
> +				vmstat_monitor_cpu = this_cpu;
> +				if (!cpumask_test_cpu(this_cpu, &vmstat_cpus))
> +					start_cpu_timer(this_cpu);
> +			}
> +		}
>  		break;

If the disabling of vmstat is tied into the nohz logic then these portions
are no longer necessary.

> @@ -1237,8 +1299,10 @@ static int __init setup_vmstat(void)
>
>  	register_cpu_notifier(&vmstat_notifier);
>
> +	vmstat_monitor_cpu = smp_processor_id();
> +

Drop the vmstat_monitor_cpu and use the dynticks processor.

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