Re: [PATCH 4/6] mm, memory_hotplug: reorganize new pgdat initialization

Michal Hocko
2022-01-27T14:44:28Z

On Thu 27-01-22 13:46:53, David Hildenbrand wrote:
> On 27.01.22 09:53, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@suse.com>
> >
> > When a !node_online node is brought up it needs a hotplug specific
> > initialization because the node could be either uninitialized yet or it
> > could have been recycled after previous hotremove. hotadd_init_pgdat is
> > responsible for that.
> >
> > Internal pgdat state is initialized at two places currently
> > - hotadd_init_pgdat
> > - free_area_init_core_hotplug
> > There is no real clear cut what should go where but this patch's chosen to
> > move the whole internal state initialization into free_area_init_core_hotplug.
> > hotadd_init_pgdat is still responsible to pull all the parts together -
> > most notably to initialize zonelists because those depend on the overall topology.
> >
> > This patch doesn't introduce any functional change.
> >
> > Acked-by: Rafael Aquini <raquini@redhat.com>
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > ---
> > include/linux/memory_hotplug.h | 2 +-
> > mm/memory_hotplug.c | 28 +++-------------------------
> > mm/page_alloc.c | 25 +++++++++++++++++++++++--
> > 3 files changed, 27 insertions(+), 28 deletions(-)
> >
> > diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> > index 60f09d3ebb3d..76bf2de86def 100644
> > --- a/include/linux/memory_hotplug.h
> > +++ b/include/linux/memory_hotplug.h
> > @@ -319,7 +319,7 @@ extern void set_zone_contiguous(struct zone *zone);
> > extern void clear_zone_contiguous(struct zone *zone);
> >
> > #ifdef CONFIG_MEMORY_HOTPLUG
> > -extern void __ref free_area_init_core_hotplug(int nid);
> > +extern void __ref free_area_init_core_hotplug(struct pglist_data *pgdat);
> > extern int __add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
> > extern int add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags);
> > extern int add_memory_resource(int nid, struct resource *resource,
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 875cdc7ffa58..ddc62f8b591f 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1166,39 +1166,16 @@ static pg_data_t __ref *hotadd_init_pgdat(int nid)
> > {
> > struct pglist_data *pgdat;
> >
> > - pgdat = NODE_DATA(nid);
> > -
> > /*
> > * NODE_DATA is preallocated (free_area_init) but its internal
> > * state is not allocated completely. Add missing pieces.
> > * Completely offline nodes stay around and they just need
> > * reintialization.
> > */
> > - if (pgdat->per_cpu_nodestats == &boot_nodestats) {
> > - pgdat->per_cpu_nodestats =
> > - alloc_percpu(struct per_cpu_nodestat);
> > - } else {
> > - int cpu;
> > - /*
> > - * Reset the nr_zones, order and highest_zoneidx before reuse.
> > - * Note that kswapd will init kswapd_highest_zoneidx properly
> > - * when it starts in the near future.
> > - */
> > - pgdat->nr_zones = 0;
> > - pgdat->kswapd_order = 0;
> > - pgdat->kswapd_highest_zoneidx = 0;
> > - for_each_online_cpu(cpu) {
> > - struct per_cpu_nodestat *p;
> > -
> > - p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
> > - memset(p, 0, sizeof(*p));
> > - }
> > - }
> > -
> > - pgdat->node_start_pfn = 0;
> > + pgdat = NODE_DATA(nid);
> >
> > /* init node's zones as empty zones, we don't have any present pages.*/
> > - free_area_init_core_hotplug(nid);
> > + free_area_init_core_hotplug(pgdat);
> >
> > /*
> > * The node we allocated has no zone fallback lists. For avoiding
> > @@ -1210,6 +1187,7 @@ static pg_data_t __ref *hotadd_init_pgdat(int nid)
> > * When memory is hot-added, all the memory is in offline state. So
> > * clear all zones' present_pages because they will be updated in
> > * online_pages() and offline_pages().
> > + * TODO: should be in free_area_init_core_hotplug?
> > */
> > reset_node_managed_pages(pgdat);
> > reset_node_present_pages(pgdat);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 1a05669044d3..32d0189de4c5 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -7506,12 +7506,33 @@ static void __meminit zone_init_internals(struct zone *zone, enum zone_type idx,
> > * NOTE: this function is only called during memory hotplug
> > */
> > #ifdef CONFIG_MEMORY_HOTPLUG
> > -void __ref free_area_init_core_hotplug(int nid)
> > +void __ref free_area_init_core_hotplug(struct pglist_data *pgdat)
> > {
> > + int nid = pgdat->node_id;
> > enum zone_type z;
> > - pg_data_t *pgdat = NODE_DATA(nid);
> > + int cpu;
> >
> > pgdat_init_internals(pgdat);
> > +
> > + if (pgdat->per_cpu_nodestats == &boot_nodestats)
> > + pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
> > +
> > + /*
> > + * Reset the nr_zones, order and highest_zoneidx before reuse.
> > + * Note that kswapd will init kswapd_highest_zoneidx properly
> > + * when it starts in the near future.
> > + */
> > + pgdat->nr_zones = 0;
> > + pgdat->kswapd_order = 0;
> > + pgdat->kswapd_highest_zoneidx = 0;
> > + pgdat->node_start_pfn = 0;
> > + for_each_online_cpu(cpu) {
> > + struct per_cpu_nodestat *p;
> > +
> > + p = per_cpu_ptr(pgdat->per_cpu_nodestats, cpu);
> > + memset(p, 0, sizeof(*p));
> > + }
> > +
> > for (z = 0; z < MAX_NR_ZONES; z++)
> > zone_init_internals(&pgdat->node_zones[z], z, nid, 0);
> > }
>
> I feel like we should be initializing all of this only once, just after
> allocating the node. There should be no difference between a node we're
> reusing and a "fresh" node. IOW, memory offlining should be resetting
> all state accordingly when the last memory goes away.
>
> But I might be wrong and this patch looks like an improvement, as you
> say, without functional change

Yeah, I really wanted to have this simple and straightforward. To be
completely honest I am not even sure this is necessary. Something really
woth looking at.

> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!

--
Michal Hocko
SUSE Labs