linux-kernel - Re: [PATCH V6 1/2] percpu_ref: reduce memory footprint of percpu

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Date:   Wed, 30 Sep 2020 12:00:15 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Veronika Kabatova <vkabatov@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>, Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH V6 1/2] percpu_ref: reduce memory footprint of percpu_ref
 in fast path

On Wed, Sep 30, 2020 at 04:26:56PM +0800, Ming Lei wrote:
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index 87d8a38bdea1..1d6ed9ca23dd 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -92,18 +92,23 @@ enum {
>  	PERCPU_REF_ALLOW_REINIT	= 1 << 2,
>  };
>  
> -struct percpu_ref {
> +struct percpu_ref_data {
>  	atomic_long_t		count;
> -	/*
> -	 * The low bit of the pointer indicates whether the ref is in percpu
> -	 * mode; if set, then get/put will manipulate the atomic_t.
> -	 */
> -	unsigned long		percpu_count_ptr;
>  	percpu_ref_func_t	*release;
>  	percpu_ref_func_t	*confirm_switch;
>  	bool			force_atomic:1;
>  	bool			allow_reinit:1;
>  	struct rcu_head		rcu;
> +	struct percpu_ref	*ref;
> +};
> +
> +struct percpu_ref {
> +	/*
> +	 * The low bit of the pointer indicates whether the ref is in percpu
> +	 * mode; if set, then get/put will manipulate the atomic_t.
> +	 */
> +	unsigned long		percpu_count_ptr;
> +	struct percpu_ref_data  *data;
>  };

Can you please add a comment explaining why the two structs are split?

Thanks.

-- 
tejun