RE: [PATCH 05/12] percpu: relegate chunks unusable when failing small allocations

[Peng Fan]

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 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@xxxxxxxxxx>
> ---
>  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@xxxxxxx>

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

Thanks,
Peng.


> --
> 2.17.1