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. > + *bit_off = pcpu_block_off_to_off(i, start); > return; > } > /* reset to satisfy the second predicate above */ @@ -632,12 > +662,57 @@ static void pcpu_block_update(struct pcpu_block_md *block, int > start, int end) > block->right_free = contig; > > if (contig > block->contig_hint) { > + /* promote the old contig_hint to be the new scan_hint */ > + if (start > block->contig_hint_start) { > + if (block->contig_hint > block->scan_hint) { > + block->scan_hint_start = > + block->contig_hint_start; > + block->scan_hint = block->contig_hint; > + } else if (start < block->scan_hint_start) { > + /* > + * The old contig_hint == scan_hint. But, the > + * new contig is larger so hold the invariant > + * scan_hint_start < contig_hint_start. > + */ > + block->scan_hint = 0; > + } > + } else { > + block->scan_hint = 0; > + } > block->contig_hint_start = start; > block->contig_hint = contig; > - } else if (block->contig_hint_start && contig == block->contig_hint && > - (!start || __ffs(start) > __ffs(block->contig_hint_start))) { > - /* use the start with the best alignment */ > - block->contig_hint_start = start; > + } else if (contig == block->contig_hint) { > + if (block->contig_hint_start && > + (!start || > + __ffs(start) > __ffs(block->contig_hint_start))) { > + /* start has a better alignment so use it */ > + block->contig_hint_start = start; > + if (start < block->scan_hint_start && > + block->contig_hint > block->scan_hint) > + block->scan_hint = 0; > + } else if (start > block->scan_hint_start || > + block->contig_hint > block->scan_hint) { > + /* > + * Knowing contig == contig_hint, update the scan_hint > + * if it is farther than or larger than the current > + * scan_hint. > + */ > + block->scan_hint_start = start; > + block->scan_hint = contig; > + } > + } else { > + /* > + * The region is smaller than the contig_hint. So only update > + * the scan_hint if it is larger than or equal and farther than > + * the current scan_hint. > + */ > + if ((start < block->contig_hint_start && > + (contig > block->scan_hint || > + (contig == block->scan_hint && > + start > block->scan_hint_start)))) { > + block->scan_hint_start = start; > + block->scan_hint = contig; > + } > } > } > > @@ -656,7 +731,7 @@ static void pcpu_block_refresh_hint(struct > pcpu_chunk *chunk, int index) > int rs, re; /* region start, region end */ > > /* clear hints */ > - block->contig_hint = 0; > + block->contig_hint = block->scan_hint = 0; > block->left_free = block->right_free = 0; > > /* iterate over free areas and update the contig hints */ @@ -713,6 > +788,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk > *chunk, int bit_off, > PCPU_BITMAP_BLOCK_BITS, > s_off + bits); > > + if (pcpu_region_overlap(s_block->scan_hint_start, > + s_block->scan_hint_start + s_block->scan_hint, > + s_off, > + s_off + bits)) > + s_block->scan_hint = 0; > + > if (pcpu_region_overlap(s_block->contig_hint_start, > s_block->contig_hint_start + > s_block->contig_hint, > @@ -749,6 +830,9 @@ static void pcpu_block_update_hint_alloc(struct > pcpu_chunk *chunk, int bit_off, > /* reset the block */ > e_block++; > } else { > + if (e_off > e_block->scan_hint_start) > + e_block->scan_hint = 0; > + > if (e_off > e_block->contig_hint_start) { > /* contig hint is broken - scan to fix it */ > pcpu_block_refresh_hint(chunk, e_index); @@ -763,6 > +847,7 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk > *chunk, int bit_off, > /* update in-between md_blocks */ > nr_empty_pages += (e_index - s_index - 1); > for (block = s_block + 1; block < e_block; block++) { > + block->scan_hint = 0; > block->contig_hint = 0; > block->left_free = 0; > block->right_free = 0; > @@ -873,6 +958,7 @@ static void pcpu_block_update_hint_free(struct > pcpu_chunk *chunk, int bit_off, > nr_empty_pages += (e_index - s_index - 1); > for (block = s_block + 1; block < e_block; block++) { > block->first_free = 0; > + block->scan_hint = 0; > block->contig_hint_start = 0; > block->contig_hint = PCPU_BITMAP_BLOCK_BITS; > block->left_free = PCPU_BITMAP_BLOCK_BITS; @@ -1084,6 > +1170,7 @@ static void pcpu_init_md_blocks(struct pcpu_chunk *chunk) > for (md_block = chunk->md_blocks; > md_block != chunk->md_blocks + pcpu_chunk_nr_blocks(chunk); > md_block++) { > + md_block->scan_hint = 0; > md_block->contig_hint = PCPU_BITMAP_BLOCK_BITS; > md_block->left_free = PCPU_BITMAP_BLOCK_BITS; > md_block->right_free = PCPU_BITMAP_BLOCK_BITS; Reviewed-by: Peng Fan <peng.fan@xxxxxxx> > -- > 2.17.1
