On Mon, 18 Mar 2024 21:07:36 +0100 Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
> When the complete PCP is drained a much larger number of pages
> than the usual batch size might be freed at once,
How much larger? Please include the numbers here.
> causing large
> IRQ and preemption latency spikes, as they are all freed while
> holding the pcp and zone spinlocks.
How large are these spikes?
> To avoid those latency spikes, limit the number of pages freed
> in a single bulk operation to common batch limits.
>
And how large are they after this?
> ---
> mm/page_alloc.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a663202045dc..64a6f9823c8c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2215,12 +2215,15 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
> */
> static void drain_pages_zone(unsigned int cpu, struct zone *zone)
> {
> - struct per_cpu_pages *pcp;
> + struct per_cpu_pages *pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
> + int count = READ_ONCE(pcp->count);
> +
> + while (count) {
> + int to_drain = min(count, pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX);
> + count -= to_drain;
>
> - pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
> - if (pcp->count) {
> spin_lock(&pcp->lock);
> - free_pcppages_bulk(zone, pcp->count, pcp, 0);
> + free_pcppages_bulk(zone, to_drain, pcp, 0);
> spin_unlock(&pcp->lock);
> }
I'm not seeing what prevents two CPUs from trying to free the same
pages simultaneously?
