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SubjectRe: [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.

Thanks,
Dennis

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