RE: [PATCH 07/12] percpu: add block level scan_hint

[Peng Fan <peng.fan@nxp.com>]

> -----Original Message-----
> From: Dennis Zhou [mailto:dennis@kernel.org]
> Sent: 2019年3月4日 4:23
> To: Peng Fan <peng.fan@nxp.com>
> Cc: Tejun Heo <tj@kernel.org>; Christoph Lameter <cl@linux.com>; Vlad
> Buslov <vladbu@mellanox.com>; kernel-team@fb.com; linux-mm@kvack.org;
> linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 07/12] percpu: add block level scan_hint
> 
> On Sun, Mar 03, 2019 at 06:01:42AM +0000, Peng Fan wrote:
> > Hi Dennis
> >
> > > -----Original Message-----
> > > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org]
> On
> > > Behalf Of Dennis Zhou
> > > Sent: 2019年2月28日 10:19
> > > To: Dennis Zhou <dennis@kernel.org>; Tejun Heo <tj@kernel.org>;
> > > Christoph Lameter <cl@linux.com>
> > > Cc: Vlad Buslov <vladbu@mellanox.com>; kernel-team@fb.com;
> > > linux-mm@kvack.org; linux-kernel@vger.kernel.org
> > > Subject: [PATCH 07/12] percpu: add block level scan_hint
> > >
> > > Fragmentation can cause both blocks and chunks to have an early
> > > first_firee bit available, but only able to satisfy allocations much
> > > later on. This patch introduces a scan_hint to help mitigate some
> unnecessary scanning.
> > >
> > > The scan_hint remembers the largest area prior to the contig_hint.
> > > If the contig_hint == scan_hint, then scan_hint_start > contig_hint_start.
> > > This is necessary for scan_hint discovery when refreshing a block.
> > >
> > > Signed-off-by: Dennis Zhou <dennis@kernel.org>
> > > ---
> > >  mm/percpu-internal.h |   9 ++++
> > >  mm/percpu.c          | 101
> > > ++++++++++++++++++++++++++++++++++++++++---
> > >  2 files changed, 103 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h index
> > > b1739dc06b73..ec58b244545d 100644
> > > --- a/mm/percpu-internal.h
> > > +++ b/mm/percpu-internal.h
> > > @@ -9,8 +9,17 @@
> > >   * pcpu_block_md is the metadata block struct.
> > >   * Each chunk's bitmap is split into a number of full blocks.
> > >   * All units are in terms of bits.
> > > + *
> > > + * The scan hint is the largest known contiguous area before the contig
> hint.
> > > + * It is not necessarily the actual largest contig hint though.
> > > + There is an
> > > + * invariant that the scan_hint_start > contig_hint_start iff
> > > + * scan_hint == contig_hint.  This is necessary because when
> > > + scanning forward,
> > > + * we don't know if a new contig hint would be better than the current
> one.
> > >   */
> > >  struct pcpu_block_md {
> > > +	int			scan_hint;	/* scan hint for block */
> > > +	int			scan_hint_start; /* block relative starting
> > > +						    position of the scan hint */
> > >  	int                     contig_hint;    /* contig hint for block
> */
> > >  	int                     contig_hint_start; /* block relative
> starting
> > >  						      position of the contig hint */ diff
> --git a/mm/percpu.c
> > > b/mm/percpu.c index 967c9cc3a928..df1aacf58ac8 100644
> > > --- a/mm/percpu.c
> > > +++ b/mm/percpu.c
> > > @@ -320,6 +320,34 @@ static unsigned long pcpu_block_off_to_off(int
> > > index, int off)
> > >  	return index * PCPU_BITMAP_BLOCK_BITS + off;  }
> > >
> > > +/*
> > > + * pcpu_next_hint - determine which hint to use
> > > + * @block: block of interest
> > > + * @alloc_bits: size of allocation
> > > + *
> > > + * This determines if we should scan based on the scan_hint or first_free.
> > > + * In general, we want to scan from first_free to fulfill
> > > +allocations by
> > > + * first fit.  However, if we know a scan_hint at position
> > > +scan_hint_start
> > > + * cannot fulfill an allocation, we can begin scanning from there
> > > +knowing
> > > + * the contig_hint will be our fallback.
> > > + */
> > > +static int pcpu_next_hint(struct pcpu_block_md *block, int
> > > +alloc_bits) {
> > > +	/*
> > > +	 * The three conditions below determine if we can skip past the
> > > +	 * scan_hint.  First, does the scan hint exist.  Second, is the
> > > +	 * contig_hint after the scan_hint (possibly not true iff
> > > +	 * contig_hint == scan_hint).  Third, is the allocation request
> > > +	 * larger than the scan_hint.
> > > +	 */
> > > +	if (block->scan_hint &&
> > > +	    block->contig_hint_start > block->scan_hint_start &&
> > > +	    alloc_bits > block->scan_hint)
> > > +		return block->scan_hint_start + block->scan_hint;
> > > +
> > > +	return block->first_free;
> > > +}
> > > +
> > >  /**
> > >   * pcpu_next_md_free_region - finds the next hint free area
> > >   * @chunk: chunk of interest
> > > @@ -415,9 +443,11 @@ static void pcpu_next_fit_region(struct
> > > pcpu_chunk *chunk, int alloc_bits,
> > >  		if (block->contig_hint &&
> > >  		    block->contig_hint_start >= block_off &&
> > >  		    block->contig_hint >= *bits + alloc_bits) {
> > > +			int start = pcpu_next_hint(block, alloc_bits);
> > > +
> > >  			*bits += alloc_bits + block->contig_hint_start -
> > > -				 block->first_free;
> > > -			*bit_off = pcpu_block_off_to_off(i, block->first_free);
> > > +				 start;
> >
> > This might not relevant to this patch.
> > Not sure it is intended or not.
> > For `alloc_bits + block->contig_hink_start - [block->first_free or
> > start]` If the reason is to let pcpu_is_populated return a proper
> > next_off when pcpu_is_populated fail, it makes sense. If not, why not just
> use *bits += alloc_bits.
> >
> 
> This is how the iterator works. Without it, it doesn't.

Oops，I made a mistake, you are right, the iterator needs such logic.

Thanks,
Peng.

> 
> Thanks,
> Dennis