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SubjectRe: [PATCH 05/12] percpu: relegate chunks unusable when failing small allocations
On Sat, Mar 02, 2019 at 01:55:54PM +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 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.
>

There isn't I just picked a small allocation size.

Thanks,
Dennis

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