linux-kernel - Re: [PATCH 1/4] memcg: simplify consume

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Message-ID: <dvyyqubghf67b3qsuoreegqk4qnuuqfkk7plpfhhrck5yeeuic@xbn4c6c7yc42>
Date: Tue, 29 Apr 2025 16:51:03 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Meta kernel team <kernel-team@...a.com>, bpf@...r.kernel.org
Subject: Re: [PATCH 1/4] memcg: simplify consume_stock

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@...ux.dev>
> ---
>  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@...r in the future for these kind of changes.