Re: [PATCH bpf-next v5 4/7] memcg: Use trylock to access memcg stock_lock.

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Jan 14, 2025 at 06:17:43PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Teach memcg to operate under trylock conditions when spinning locks
> cannot be used.
> 
> local_trylock might fail and this would lead to charge cache bypass if
> the calling context doesn't allow spinning (gfpflags_allow_spinning).
> In those cases charge the memcg counter directly and fail early if
> that is not possible. This might cause a pre-mature charge failing
> but it will allow an opportunistic charging that is safe from
> try_alloc_pages path.
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

> @@ -1851,7 +1856,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
>  {
>  	unsigned long flags;
>  
> -	local_lock_irqsave(&memcg_stock.stock_lock, flags);
> +	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
> +		/*
> +		 * In case of unlikely failure to lock percpu stock_lock
> +		 * uncharge memcg directly.
> +		 */
> +		mem_cgroup_cancel_charge(memcg, nr_pages);

mem_cgroup_cancel_charge() has been removed by a patch in mm-tree. Maybe
we can either revive mem_cgroup_cancel_charge() or simply inline it
here.

> +		return;
> +	}
>  	__refill_stock(memcg, nr_pages);
>  	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>  }