linux-kernel - Re: [PATCH] mm/mm_init.c: add zidcache to the init_reserved

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Message-Id: <20240904134102.a7171fb6676dd95dc3b8ede7@linux-foundation.org>
Date: Wed, 4 Sep 2024 13:41:02 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Qiang Liu <liuq131@...natelecom.cn>
Cc: rppt@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mm_init.c: add zidcache to the init_reserved_page
 function

On Wed,  4 Sep 2024 19:55:41 +0800 Qiang Liu <liuq131@...natelecom.cn> wrote:

> Each call to the init_reserved_page function will look up the
> corresponding zid for the given pfn parameter. Even if subsequent
> calls have the same zid for the pfn as the current one, the lookup
> will be repeated.
> 
> During system initialization, the memmap_init_reserved_pages function
> calls init_reserved_page for each contiguous memory region in mem_region.
> Implementing a cache for zid can significantly improve performance.
> Tests have shown that adding a zid cache reduces the execution time of
> the memmap_init_reserved_pages function by over 7%.
> 

OK, but how much speedup do we see overall?  In other words, is
memmap_init_reserved_pages() a significant consumer of execution time?

I'd be surprised if it makes much difference at all - MAX_NR_ZONES is a
small number.  Maybe we call init_reserved_page() a lot.


> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -710,19 +710,25 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
>  {
>  	pg_data_t *pgdat;
>  	int zid;
> +	struct zone *zone;
> +	static int zidcache;

What locking protects zidcache?  lock_device_hotplug() and/or
__init-time serialization?  This might be worth mentioning?

>  
>  	if (early_page_initialised(pfn, nid))
>  		return;
>  
>  	pgdat = NODE_DATA(nid);
>  
> -	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> -		struct zone *zone = &pgdat->node_zones[zid];
> +	zone = &pgdat->node_zones[zidcache];

OK, but if init_reserved_page() was previously called against a
different node, `zidcache' will refer to a zone in a different node. 
The code will work OK, but it's worth mentioning somewhere I guess.


> +	if (unlikely(zone_spans_pfn(zone, pfn)))

Isn't this wrong?  We need to redo the search if !zone_spans_pfn(...)?

> +		for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> +			zone = &pgdat->node_zones[zid];
>  
> -		if (zone_spans_pfn(zone, pfn))
> -			break;
> -	}
> -	__init_single_page(pfn_to_page(pfn), pfn, zid, nid);
> +			if (zone_spans_pfn(zone, pfn)) {
> +				zidcache = zid;
> +				break;
> +			}
> +		}
> +	__init_single_page(pfn_to_page(pfn), pfn, zidcache, nid);
>  }
>  #else
>  static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
> -- 
> 2.27.0