Re: [patch 1/2] vmstat: Optimize refresh_cpu_vmstat()

[Michal Hocko <mhocko@suse.cz>]

On Mon 22-02-16 12:10:41, Christoph Lameter wrote:
> Create a new function zone_needs_update() that uses a memchr to check
> all diffs for being nonzero first.
> 
> If we use this function in refresh_cpu_vm_stat() then we can avoid the
> this_cpu_xchg() loop over all differentials. This becomes in particular
> important as the number of counters keeps on increasing.
> 
> This also avoids modifying the cachelines with the differentials
> unnecessarily.
> 
> Also add some likely()s to ensure that the icache requirements
> are low when we do not have any updates to process.

Do you have any numbers? Can you actually measure an interference of
refresh_cpu_vmstat?

> Signed-off-by: Christoph Lameter <cl@linux.com>
> 
> Index: linux/mm/vmstat.c
> ===================================================================
> --- linux.orig/mm/vmstat.c	2016-02-22 11:54:02.179095030 -0600
> +++ linux/mm/vmstat.c	2016-02-22 11:54:24.338528277 -0600
> @@ -444,6 +444,18 @@ static int fold_diff(int *diff)
>  	return changes;
>  }
>  
> +bool zone_needs_update(struct per_cpu_pageset *p)

static bool ....

> +{
> +
> +	BUILD_BUG_ON(sizeof(p->vm_stat_diff[0]) != 1);
> +	/*
> +	 * The fast way of checking if there are any vmstat diffs.
> +	 * This works because the diffs are byte sized items.
> +	 */
> +	return memchr_inv(p->vm_stat_diff, 0,
> +			NR_VM_ZONE_STAT_ITEMS) != NULL;
> +}
> +
>  /*
>   * Update the zone counters for the current cpu.
>   *
> @@ -470,18 +482,20 @@ static int refresh_cpu_vm_stats(bool do_
>  	for_each_populated_zone(zone) {
>  		struct per_cpu_pageset __percpu *p = zone->pageset;
>  
> -		for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
> -			int v;
> +		if (unlikely(zone_needs_update(this_cpu_ptr(p)))) {

why unlikely? The generated code looks exactly same with or without it
(same for the other likely annotation added by this patch).

[...]
-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>