linux-kernel - Re: [PATCH 2/2] percpu: km: no need to consider pcpu_group

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Date:   Mon, 25 Feb 2019 10:16:16 -0500
From:   "dennis@...nel.org" <dennis@...nel.org>
To:     Peng Fan <peng.fan@....com>
Cc:     "tj@...nel.org" <tj@...nel.org>, "cl@...ux.com" <cl@...ux.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "van.freenix@...il.com" <van.freenix@...il.com>
Subject: Re: [PATCH 2/2] percpu: km: no need to consider pcpu_group_offsets[0]

On Sun, Feb 24, 2019 at 01:13:50PM +0000, Peng Fan wrote:
> percpu-km is used on UP systems which only has one group,
> so the group offset will be always 0, there is no need
> to subtract pcpu_group_offsets[0] when assigning chunk->base_addr
> 
> Signed-off-by: Peng Fan <peng.fan@....com>
> ---
>  mm/percpu-km.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/percpu-km.c b/mm/percpu-km.c
> index 66e5598be876..8872c21a487b 100644
> --- a/mm/percpu-km.c
> +++ b/mm/percpu-km.c
> @@ -67,7 +67,7 @@ static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp)
>  		pcpu_set_page_chunk(nth_page(pages, i), chunk);
>  
>  	chunk->data = pages;
> -	chunk->base_addr = page_address(pages) - pcpu_group_offsets[0];
> +	chunk->base_addr = page_address(pages);
>  
>  	spin_lock_irqsave(&pcpu_lock, flags);
>  	pcpu_chunk_populated(chunk, 0, nr_pages, false);
> -- 
> 2.16.4
> 

While I do think you're right, creating a chunk is not a part of the
critical path and subtracting 0 is incredibly minor overhead. So I'd
rather keep the code as is to maintain consistency between percpu-vm.c
and percpu-km.c.

Thanks,
Dennis