回复: [PATCH] mm: simplify zone_idx()

[gaoxu]

> 
> On Fri, Apr 11, 2025 at 2:42 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Thu, Apr 10, 2025 at 12:03:00PM +0000, gaoxu wrote:
> > > store zone_idx directly in struct zone to simplify and optimize zone_idx()
> >
> > Do you see an actual speed up somewhere?
Almost negligible. my simple code tests showed the patch provides an average improvement of ~0.02%.
Thus, in the Android 15-6.6 kernel, I confidently retained the original zone_idx function.
(https://android-review.googlesource.com/c/kernel/common/+/3578322/2/mm/page_alloc.c#770)

This patch only eliminates 2-3 assembly instructions, making it challenging to
observe measurable performance benefits.
However, since the zone struct includes CACHELINE_PADDING (reserving unused space),
adding a new member variable does not alter the size of zone. This makes the patch
effectively zero-cost while achieving a cleaner implementation of zone_idx.
> 
> +1. Curious if there's data indicating zone_idx is a hot path.
There are several functions in the memory management code that are frequently
executed and will call zone_idx:
rmqueue()->wakeup_kswapd()->zone_idx()
alloc_pages_bulk_noprof()->__count_zid_vm_events()->zone_idx()

The patch (https://lore.kernel.org/all/20240229183436.4110845-2-yuzhao@xxxxxxxxxx/)
will add new hotspot paths, with the details as follows:
__zone_watermark_ok()->zone_is_suitable()->zone_idx()
zone_watermark_fast()->zone_is_suitable()->zone_idx()
get_page_from_freelist()->zone_is_suitable()->zone_idx()
__free_one_page()->zone_max_order()->zone_idx()

Although The patch (https://lore.kernel.org/all/20240229183436.4110845-2-yuzhao@xxxxxxxxxx/)
has not yet merged into the Linux mainline; it is already included in Android 15-6.6.
> 

> >
> > > Signed-off-by: gao xu <gaoxu2@xxxxxxxxx>
> > > ---
> > >  include/linux/mmzone.h | 3 ++-
> > >  mm/mm_init.c           | 1 +
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index 4c95fcc9e..7b14f577d 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -941,6 +941,7 @@ struct zone {
> > >  #endif
> > >
> > >       const char              *name;
> > > +     enum zone_type  zone_idx;
> > >
> > >  #ifdef CONFIG_MEMORY_ISOLATION
> > >       /*
> > > @@ -1536,7 +1537,7 @@ static inline int local_memory_node(int node_id)
> { return node_id; };
> > >  /*
> > >   * zone_idx() returns 0 for the ZONE_DMA zone, 1 for the ZONE_NORMAL
> zone, etc.
> > >   */
> > > -#define zone_idx(zone)               ((zone) -
> (zone)->zone_pgdat->node_zones)
> > > +#define zone_idx(zone)               ((zone)->zone_idx)
> > >
> > >  #ifdef CONFIG_ZONE_DEVICE
> > >  static inline bool zone_is_zone_device(struct zone *zone)
> > > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > > index 9659689b8..a7f7264f1 100644
> > > --- a/mm/mm_init.c
> > > +++ b/mm/mm_init.c
> > > @@ -1425,6 +1425,7 @@ static void __meminit zone_init_internals(struct
> zone *zone, enum zone_type idx,
> > >       atomic_long_set(&zone->managed_pages, remaining_pages);
> > >       zone_set_nid(zone, nid);
> > >       zone->name = zone_names[idx];
> > > +     zone->zone_idx = idx;
> > >       zone->zone_pgdat = NODE_DATA(nid);
> > >       spin_lock_init(&zone->lock);
> > >       zone_seqlock_init(zone);
> > > --
> > > 2.17.1
> >
> > --
> > Sincerely yours,
> > Mike.
> 
> Thanks
> Barry