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

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Date:   Tue, 26 Feb 2019 00:03:29 +0000
From:   Peng Fan <peng.fan@....com>
To:     "dennis@...nel.org" <dennis@...nel.org>
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]

Hi Dennis,

> -----Original Message-----
> From: dennis@...nel.org [mailto:dennis@...nel.org]
> Sent: 2019年2月25日 23:16
> To: Peng Fan <peng.fan@....com>
> Cc: tj@...nel.org; cl@...ux.com; linux-mm@...ck.org;
> linux-kernel@...r.kernel.org; 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.

That's ok to keep consistency, since you prefer that.

Thanks,
Peng.

> 
> Thanks,
> Dennis