Re: vmstat: make vmstat_updater deferrable again and shut down on idle

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 10 Dec 2015 14:45:02 -0600 (CST) Christoph Lameter <cl@linux.com> wrote:

> Currently the vmstat updater is not deferrable as a result of commit
> ba4877b9ca51f80b5d30f304a46762f0509e1635. This in turn can cause multiple
> interruptions of the applications because the vmstat updater may run at
> different times than tick processing. No good.
> 
> Make vmstate_update deferrable again and provide a function that
> folds the differentials when the processor is going to idle mode thus
> addressing the issue of the above commit in a clean way.
> 
> Note that the shepherd thread will continue scanning the differentials
> from another processor and will reenable the vmstat workers if it
> detects any changes.
> 
> Fixes: ba4877b9ca51f80b5d30f304a46762f0509e1635 (do not use deferrable delay)
> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> ...
>
>  /*
> + * Switch off vmstat processing and then fold all the remaining differentials
> + * until the diffs stay at zero. The function is used by NOHZ and can only be
> + * invoked when tick processing is not active.
> + */
> +void quiet_vmstat(void)
> +{
> +	do {
> +		if (!cpumask_test_and_set_cpu(smp_processor_id(), cpu_stat_off))
> +			cancel_delayed_work(this_cpu_ptr(&vmstat_work));
> +
> +	} while (refresh_cpu_vm_stats(false));
> +}

How do we know this will terminate in a reasonable amount of time if
other CPUs are pounding away?


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