RE: [PATCH 04/12] percpu: manage chunks based on contig_bits instead of free_bytes

[Peng Fan]

> -----Original Message-----
> From: Dennis Zhou [mailto:dennis@xxxxxxxxxx]
> Sent: 2019å3æ3æ 6:32
> To: Peng Fan <peng.fan@xxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>; Christoph Lameter <cl@xxxxxxxxx>; Vlad
> Buslov <vladbu@xxxxxxxxxxxx>; kernel-team@xxxxxx; linux-mm@xxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 04/12] percpu: manage chunks based on contig_bits
> instead of free_bytes
> 
> On Sat, Mar 02, 2019 at 01:48:20PM +0000, Peng Fan wrote:
> >
> >
> > > -----Original Message-----
> > > From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx]
> On
> > > Behalf Of Dennis Zhou
> > > Sent: 2019å2æ28æ 10:19
> > > To: Dennis Zhou <dennis@xxxxxxxxxx>; Tejun Heo <tj@xxxxxxxxxx>;
> > > Christoph Lameter <cl@xxxxxxxxx>
> > > Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>; kernel-team@xxxxxx;
> > > linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > > Subject: [PATCH 04/12] percpu: manage chunks based on contig_bits
> > > instead of free_bytes
> > >
> > > When a chunk becomes fragmented, it can end up having a large number
> > > of small allocation areas free. The free_bytes sorting of chunks
> > > leads to unnecessary checking of chunks that cannot satisfy the allocation.
> > > Switch to contig_bits sorting to prevent scanning chunks that may
> > > not be able to service the allocation request.
> > >
> > > Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> > > ---
> > >  mm/percpu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/percpu.c b/mm/percpu.c index
> > > b40112b2fc59..c996bcffbb2a 100644
> > > --- a/mm/percpu.c
> > > +++ b/mm/percpu.c
> > > @@ -234,7 +234,7 @@ static int pcpu_chunk_slot(const struct
> > > pcpu_chunk
> > > *chunk)
> > >  	if (chunk->free_bytes < PCPU_MIN_ALLOC_SIZE ||
> chunk->contig_bits
> > > == 0)
> > >  		return 0;
> > >
> > > -	return pcpu_size_to_slot(chunk->free_bytes);
> > > +	return pcpu_size_to_slot(chunk->contig_bits *
> > > +PCPU_MIN_ALLOC_SIZE);
> > >  }
> > >
> > >  /* set the pointer to a chunk in a page struct */
> >
> > Reviewed-by: Peng Fan <peng.fan@xxxxxxx>
> >
> > Not relevant to this patch, another optimization to percpu might be
> > good to use per chunk spin_lock, not gobal pcpu_lock.
> >
> 
> Percpu memory itself is expensive and for the most part shouldn't be part of
> the critical path. Ideally, we don't have multiple chunks being allocated
> simultaneously because once an allocation is given out, the chunk is pinned
> until all allocations are freed.

Thanks for clarification, since not critical patch, use global lock should be ok.

Thanks,
Peng.

> 
> Thanks,
> Dennis