Re: [PATCH] mm: simplify zone_idx()

[Barry Song <21cnbao@gmail.com>]

On Sat, Apr 12, 2025 at 10:06 PM gaoxu <gaoxu2@honor.com> wrote:
>
> >
> > On Sat, Apr 12, 2025 at 8:34 PM gaoxu <gaoxu2@honor.com> wrote:
> > >
> > > >
> > > > On Fri, Apr 11, 2025 at 2:42 AM Mike Rapoport <rppt@kernel.org> 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/m
> > > m/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.
> >
> > The struct zone contains many CONFIG_ options to include or exclude certain
> > fields.
> > If we're confident that our new zone_idx doesn't increase cacheline usage for all
> > those cases, this seems like a worthwhile cleanup. Have you checked the
> > numbers?
>
> The zone info obtained through T32 in the Android 15-6.6 system(arm64):
> (struct zone) struct (1664 bytes,
>                 [0] unsigned long [4] _watermark,
>                 [32] unsigned long watermark_boost,
>                 [40] unsigned long nr_reserved_highatomic,
>                 [48] long [5] lowmem_reserve,
>                 [88] struct pglist_data * zone_pgdat,
>                 [96] struct per_cpu_pages * per_cpu_pageset,
>                 [104] struct per_cpu_zonestat * per_cpu_zonestats,
>                 [112] int pageset_high,
>                 [116] int pageset_batch,
>                 [120] unsigned long zone_start_pfn,
>                 [128] atomic_long_t managed_pages,
>                 [136] unsigned long spanned_pages,
>                 [144] unsigned long present_pages,
>                 [152] unsigned long present_early_pages,
>                 [160] unsigned long cma_pages,
>                 [168] const char * name,
>                 [176] unsigned long nr_isolate_pageblock,
>                 [184] seqlock_t span_seqlock,
>                 [192] int order,
>                 [196] int initialized,
>                 [256] struct cacheline_padding _pad1_,
>                 [256] struct free_area [11] free_area,
>                 [1400] unsigned long flags,
>                 [1408] spinlock_t lock,
>                 [1472] struct cacheline_padding _pad2_,
>                 [1472] unsigned long percpu_drift_mark,
>                 [1480] unsigned long compact_cached_free_pfn,
>                 [1488] unsigned long [2] compact_cached_migrate_pfn,
>                 [1504] unsigned long compact_init_migrate_pfn,
>                 [1512] unsigned long compact_init_free_pfn,
>                 [1520] unsigned int compact_considered,
>                 [1524] unsigned int compact_defer_shift,
>                 [1528] int compact_order_failed,
>                 [1532] bool compact_blockskip_flush,
>                 [1533] bool contiguous,
>                 [1536] struct cacheline_padding _pad3_,
>                 [1536] atomic_long_t [11] vm_stat,
>                 [1624] atomic_long_t [0] vm_numa_event)
>
> 1) It can be observed that there are 56B of free space in CACHELINE_PADDING(pad1)；
> 2) Before the variables in CACHELINE_PADDING(pad1), there are two CONFIGs that are not enabled in Android 15-6.6:
>         #ifdef CONFIG_NUMA
>                 int node;
>         #endif
>
>         #ifndef CONFIG_SPARSEMEM
>                 unsigned long *pageblock_flags;
>         #endif /* CONFIG_SPARSEMEM */
>         These two CONFIGs occupy 16B.
> 3) Compared to the latest Linux code, two new variables, unsigned long nr_free_highatomic and int pageset_high_max, occupy an additional 16B;
> Based on the above analysis, there are still 24B of free space before CACHELINE_PADDING(pad1).
> (If I misunderstand, please point it out in a timely manner. Thank you!)
>
> It would be more appropriate to place the newly added variable zone_idx before initialized.

I don't have a strong opinion on whether we need `zone_idx`—I'm okay
with having it or not. If you'd like to add it, feel free to send out
a v2 noting that it doesn't increase the struct size. If no one
objects, it might be a nice cleanup.

> >
> > > >
> > > > +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@google.co
> > > m/) 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@google.co
> > > m/) has not yet merged into the Linux mainline; it is already included
> > > in Android 15-6.6.
> > > >
> > >
> > > > >
> > > > > > Signed-off-by: gao xu <gaoxu2@honor.com>
> > > > > > ---
> > > > > >  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