RE: [PATCH 2/2] percpu: km: no need to consider pcpu_group_offsets[0]

[Peng Fan]

Hi Dennis,

> -----Original Message-----
> From: dennis@xxxxxxxxxx [mailto:dennis@xxxxxxxxxx]
> Sent: 2019年2月25日 23:16
> To: Peng Fan <peng.fan@xxxxxxx>
> Cc: tj@xxxxxxxxxx; cl@xxxxxxxxx; linux-mm@xxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; van.freenix@xxxxxxxxx
> 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@xxxxxxx>
> > ---
> >  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