On Tue, Apr 29, 2025 at 04:04:25PM -0700, Shakeel Butt wrote:
> The consume_stock() does not need to check gfp_mask for spinning and can
> simply trylock the local lock to decide to proceed or fail. No need to
> spin at all for local lock.
>
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> ---
> mm/memcontrol.c | 20 +++++++-------------
> 1 file changed, 7 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 650fe4314c39..40d0838d88bc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1804,16 +1804,14 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> * consume_stock: Try to consume stocked charge on this cpu.
> * @memcg: memcg to consume from.
> * @nr_pages: how many pages to charge.
> - * @gfp_mask: allocation mask.
> *
> - * The charges will only happen if @memcg matches the current cpu's memcg
> - * stock, and at least @nr_pages are available in that stock. Failure to
> - * service an allocation will refill the stock.
> + * Consume the cached charge if enough nr_pages are present otherwise return
> + * failure. Also return failure for charge request larger than
> + * MEMCG_CHARGE_BATCH or if the local lock is already taken.
> *
> * returns true if successful, false otherwise.
> */
> -static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
> - gfp_t gfp_mask)
> +static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> {
> struct memcg_stock_pcp *stock;
> uint8_t stock_pages;
> @@ -1821,12 +1819,8 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
> bool ret = false;
> int i;
>
> - if (nr_pages > MEMCG_CHARGE_BATCH)
> - return ret;
> -
> - if (gfpflags_allow_spinning(gfp_mask))
> - local_lock_irqsave(&memcg_stock.stock_lock, flags);
> - else if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags))
> + if (nr_pages > MEMCG_CHARGE_BATCH ||
> + !local_trylock_irqsave(&memcg_stock.stock_lock, flags))
I don't think it's a good idea.
spin_trylock() will fail often enough in PREEMPT_RT.
Even during normal boot I see preemption between tasks and they
contend on the same cpu for the same local_lock==spin_lock.
Making them take slow path is a significant behavior change
that needs to be carefully considered.
Also please cc bpf@vger in the future for these kind of changes.
