On Sun, Mar 03, 2019 at 06:01:42AM +0000, Peng Fan wrote: > Hi Dennis > > > -----Original Message----- > > From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On > > Behalf Of Dennis Zhou > > Sent: 2019年2月28日 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 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@xxxxxxxxxx> > > --- > > 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
