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SubjectRe: [PATCH 7/7] mm: memcontrol: consolidate lruvec stat flushing
On Tue 02-02-21 13:47:46, Johannes Weiner wrote:
> There are two functions to flush the per-cpu data of an lruvec into
> the rest of the cgroup tree: when the cgroup is being freed, and when
> a CPU disappears during hotplug. The difference is whether all CPUs or
> just one is being collected, but the rest of the flushing code is the
> same. Merge them into one function and share the common code.

IIUC the only reason for the cpu == -1 special case is to avoid
zeroying, right? Is this optimization worth the special case? The code
would be slightly easier to follow without this.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Anyway the above is not really a fundamental objection. It is more important
to unify the flushing.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/memcontrol.c | 88 +++++++++++++++++++++++--------------------------
> 1 file changed, 42 insertions(+), 46 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b205b2413186..88e8afc49a46 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2410,39 +2410,56 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> mutex_unlock(&percpu_charge_mutex);
> }
>
> -static int memcg_hotplug_cpu_dead(unsigned int cpu)
> +static void memcg_flush_lruvec_page_state(struct mem_cgroup *memcg, int cpu)
> {
> - struct memcg_stock_pcp *stock;
> - struct mem_cgroup *memcg;
> -
> - stock = &per_cpu(memcg_stock, cpu);
> - drain_stock(stock);
> + int nid;
>
> - for_each_mem_cgroup(memcg) {
> + for_each_node(nid) {
> + struct mem_cgroup_per_node *pn = memcg->nodeinfo[nid];
> + unsigned long stat[NR_VM_NODE_STAT_ITEMS] = { 0, };
> + struct batched_lruvec_stat *lstatc;
> int i;
>
> - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
> - int nid;
> -
> - for_each_node(nid) {
> - struct batched_lruvec_stat *lstatc;
> - struct mem_cgroup_per_node *pn;
> - long x;
> -
> - pn = memcg->nodeinfo[nid];
> + if (cpu == -1) {
> + int cpui;
> + /*
> + * The memcg is about to be freed, collect all
> + * CPUs, no need to zero anything out.
> + */
> + for_each_online_cpu(cpui) {
> + lstatc = per_cpu_ptr(pn->lruvec_stat_cpu, cpui);
> + for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> + stat[i] += lstatc->count[i];
> + }
> + } else {
> + /*
> + * The CPU has gone away, collect and zero out
> + * its stats, it may come back later.
> + */
> + for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
> lstatc = per_cpu_ptr(pn->lruvec_stat_cpu, cpu);
> -
> - x = lstatc->count[i];
> + stat[i] = lstatc->count[i];
> lstatc->count[i] = 0;
> -
> - if (x) {
> - do {
> - atomic_long_add(x, &pn->lruvec_stat[i]);
> - } while ((pn = parent_nodeinfo(pn, nid)));
> - }
> }
> }
> +
> + do {
> + for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> + atomic_long_add(stat[i], &pn->lruvec_stat[i]);
> + } while ((pn = parent_nodeinfo(pn, nid)));
> }
> +}
> +
> +static int memcg_hotplug_cpu_dead(unsigned int cpu)
> +{
> + struct memcg_stock_pcp *stock;
> + struct mem_cgroup *memcg;
> +
> + stock = &per_cpu(memcg_stock, cpu);
> + drain_stock(stock);
> +
> + for_each_mem_cgroup(memcg)
> + memcg_flush_lruvec_page_state(memcg, cpu);
>
> return 0;
> }
> @@ -3636,27 +3653,6 @@ static u64 mem_cgroup_read_u64(struct cgroup_subsys_state *css,
> }
> }
>
> -static void memcg_flush_lruvec_page_state(struct mem_cgroup *memcg)
> -{
> - int node;
> -
> - for_each_node(node) {
> - struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];
> - unsigned long stat[NR_VM_NODE_STAT_ITEMS] = {0, };
> - struct mem_cgroup_per_node *pi;
> - int cpu, i;
> -
> - for_each_online_cpu(cpu)
> - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> - stat[i] += per_cpu(
> - pn->lruvec_stat_cpu->count[i], cpu);
> -
> - for (pi = pn; pi; pi = parent_nodeinfo(pi, node))
> - for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> - atomic_long_add(stat[i], &pi->lruvec_stat[i]);
> - }
> -}
> -
> #ifdef CONFIG_MEMCG_KMEM
> static int memcg_online_kmem(struct mem_cgroup *memcg)
> {
> @@ -5197,7 +5193,7 @@ static void mem_cgroup_free(struct mem_cgroup *memcg)
> * Flush percpu lruvec stats to guarantee the value
> * correctness on parent's and all ancestor levels.
> */
> - memcg_flush_lruvec_page_state(memcg);
> + memcg_flush_lruvec_page_state(memcg, -1);
> __mem_cgroup_free(memcg);
> }
>
> --
> 2.30.0
>

--
Michal Hocko
SUSE Labs

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