回复: [PATCH] mm: simplify zone_idx()

[gaoxu <gaoxu2@honor.com>]

> 
> 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.
> 
> > >
> > > +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