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@xxxxxxxxx>
>
> 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@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>