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SubjectRE: [PATCH 05/12] percpu: relegate chunks unusable when failing small allocations
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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 05/12] percpu: relegate chunks unusable when failing small
> allocations
>
> In certain cases, requestors of percpu memory may want specific alignments.
> However, it is possible to end up in situations where the contig_hint matches,
> but the alignment does not. This causes excess scanning of chunks that will fail.
> To prevent this, if a small allocation fails (< 32B), the chunk is moved to the
> empty list. Once an allocation is freed from that chunk, it is placed back into
> rotation.
>
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> ---
> mm/percpu.c | 35 ++++++++++++++++++++++++++---------
> 1 file changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index c996bcffbb2a..3d7deece9556 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -94,6 +94,8 @@
>
> /* the slots are sorted by free bytes left, 1-31 bytes share the same slot */
> #define PCPU_SLOT_BASE_SHIFT 5
> +/* chunks in slots below this are subject to being sidelined on failed alloc */
> +#define PCPU_SLOT_FAIL_THRESHOLD 3
>
> #define PCPU_EMPTY_POP_PAGES_LOW 2
> #define PCPU_EMPTY_POP_PAGES_HIGH 4
> @@ -488,6 +490,22 @@ static void pcpu_mem_free(void *ptr)
> kvfree(ptr);
> }
>
> +static void __pcpu_chunk_move(struct pcpu_chunk *chunk, int slot,
> + bool move_front)
> +{
> + if (chunk != pcpu_reserved_chunk) {
> + if (move_front)
> + list_move(&chunk->list, &pcpu_slot[slot]);
> + else
> + list_move_tail(&chunk->list, &pcpu_slot[slot]);
> + }
> +}
> +
> +static void pcpu_chunk_move(struct pcpu_chunk *chunk, int slot) {
> + __pcpu_chunk_move(chunk, slot, true);
> +}
> +
> /**
> * pcpu_chunk_relocate - put chunk in the appropriate chunk slot
> * @chunk: chunk of interest
> @@ -505,12 +523,8 @@ static void pcpu_chunk_relocate(struct pcpu_chunk
> *chunk, int oslot) {
> int nslot = pcpu_chunk_slot(chunk);
>
> - if (chunk != pcpu_reserved_chunk && oslot != nslot) {
> - if (oslot < nslot)
> - list_move(&chunk->list, &pcpu_slot[nslot]);
> - else
> - list_move_tail(&chunk->list, &pcpu_slot[nslot]);
> - }
> + if (oslot != nslot)
> + __pcpu_chunk_move(chunk, nslot, oslot < nslot);
> }
>
> /**
> @@ -1381,7 +1395,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t
> align, bool reserved,
> bool is_atomic = (gfp & GFP_KERNEL) != GFP_KERNEL;
> bool do_warn = !(gfp & __GFP_NOWARN);
> static int warn_limit = 10;
> - struct pcpu_chunk *chunk;
> + struct pcpu_chunk *chunk, *next;
> const char *err;
> int slot, off, cpu, ret;
> unsigned long flags;
> @@ -1443,11 +1457,14 @@ static void __percpu *pcpu_alloc(size_t size,
> size_t align, bool reserved,
> restart:
> /* search through normal chunks */
> for (slot = pcpu_size_to_slot(size); slot < pcpu_nr_slots; slot++) {
> - list_for_each_entry(chunk, &pcpu_slot[slot], list) {
> + list_for_each_entry_safe(chunk, next, &pcpu_slot[slot], list) {
> off = pcpu_find_block_fit(chunk, bits, bit_align,
> is_atomic);
> - if (off < 0)
> + if (off < 0) {
> + if (slot < PCPU_SLOT_FAIL_THRESHOLD)
> + pcpu_chunk_move(chunk, 0);
> continue;
> + }
>
> off = pcpu_alloc_area(chunk, bits, bit_align, off);
> if (off >= 0)

For the code: Reviewed-by: Peng Fan <peng.fan@nxp.com>

But I did not understand well why choose 32B? If there are
more information, better put in commit log.

Thanks,
Peng.


> --
> 2.17.1

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