> -----Original Message----- > From: Dennis Zhou [mailto:dennis@xxxxxxxxxx] > Sent: 2019年3月4日 4:23 > 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 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@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. Oops，I made a mistake, you are right, the iterator needs such logic. Thanks, Peng. > > Thanks, > Dennis
