linux-kernel - Re: [PATCH rdma-next v1 2/7] RDMA/mlx5: Replace cache list with Xarray

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Message-ID: <20220107232217.GA3108306@nvidia.com>
Date:   Fri, 7 Jan 2022 19:22:17 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Aharon Landau <aharonl@...dia.com>, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH rdma-next v1 2/7] RDMA/mlx5: Replace cache list with
 Xarray

On Thu, Dec 30, 2021 at 01:23:19PM +0200, Leon Romanovsky wrote:

> diff --git a/drivers/infiniband/hw/mlx5/mr.c b/drivers/infiniband/hw/mlx5/mr.c
> index 2cba55bb7825..8936b504ff99 100644
> +++ b/drivers/infiniband/hw/mlx5/mr.c
> @@ -147,14 +147,17 @@ static void create_mkey_callback(int status, struct mlx5_async_work *context)
>  	struct mlx5_cache_ent *ent = mr->cache_ent;
>  	struct mlx5_ib_dev *dev = ent->dev;
>  	unsigned long flags;
> +	void *old;
>  
>  	if (status) {
>  		mlx5_ib_warn(dev, "async reg mr failed. status %d\n", status);
>  		kfree(mr);
> -		spin_lock_irqsave(&ent->lock, flags);
> -		ent->pending--;
> +		xa_lock_irqsave(&ent->mkeys, flags);
> +		ent->reserved--;
> +		old = __xa_erase(&ent->mkeys, ent->reserved);
> +		WARN_ON(old != NULL);
>  		WRITE_ONCE(dev->fill_delay, 1);
> -		spin_unlock_irqrestore(&ent->lock, flags);
> +		xa_unlock_irqrestore(&ent->mkeys, flags);
>  		mod_timer(&dev->delay_timer, jiffies + HZ);
>  		return;
>  	}

Since push_reserve_mkey was put in a function these stack
manipulation should be too, especially since  it is open coded again
in the err_undo_reserve label below

> @@ -166,14 +169,14 @@ static void create_mkey_callback(int status, struct mlx5_async_work *context)
>  
>  	WRITE_ONCE(dev->cache.last_add, jiffies);
>  
> -	spin_lock_irqsave(&ent->lock, flags);
> -	list_add_tail(&mr->list, &ent->head);
> -	ent->available_mrs++;
> +	xa_lock_irqsave(&ent->mkeys, flags);
> +	old = __xa_store(&ent->mkeys, ent->stored, mr, GFP_ATOMIC);
> +	WARN_ON(old != NULL);
> +	ent->stored++;
>  	ent->total_mrs++;
>  	/* If we are doing fill_to_high_water then keep going. */
>  	queue_adjust_cache_locked(ent);
> -	ent->pending--;
> -	spin_unlock_irqrestore(&ent->lock, flags);
> +	xa_unlock_irqrestore(&ent->mkeys, flags);
>  }
>  
>  static struct mlx5_ib_mr *alloc_cache_mr(struct mlx5_cache_ent *ent, void *mkc)
> @@ -196,12 +199,48 @@ static struct mlx5_ib_mr *alloc_cache_mr(struct mlx5_cache_ent *ent, void *mkc)
>  	return mr;
>  }
>  
> +static int _push_reserve_mkey(struct mlx5_cache_ent *ent)
> +{
> +	unsigned long to_reserve;
> +	void *old;
> +
> +	while (true) {
> +		to_reserve = ent->reserved;
> +		old = __xa_cmpxchg(&ent->mkeys, to_reserve, NULL, XA_ZERO_ENTRY,
> +				  GFP_KERNEL);
> +
> +		if (xa_is_err(old))
> +			return xa_err(old);

Comment that this below is needed because xa_cmpxchg could drop the
lock and thus ent->reserved can change.

> +		if (to_reserve != ent->reserved || old != NULL) {

WARN_ON(old != NULL) ?  This can't happen right?

> +static int push_reserve_mkey(struct mlx5_cache_ent *ent)
> +{
> +	int ret;
> +
> +	xa_lock_irq(&ent->mkeys);
> +	ret = _push_reserve_mkey(ent);
> +	xa_unlock_irq(&ent->mkeys);
> +
> +	return ret;
> +}

Why not just use one function and a simple goto unwind?

> +
> +		xa_lock_irq(&ent->mkeys);
> +		err = _push_reserve_mkey(ent);
> +		if (err)
> +			goto err_unlock;
> +		if ((ent->reserved - ent->stored) > MAX_PENDING_REG_MR) {
>  			err = -EAGAIN;

Maybe this check should be inside push_reserve_mkey before it does any
xarray operation?

> @@ -286,40 +335,42 @@ static struct mlx5_ib_mr *create_cache_mr(struct mlx5_cache_ent *ent)
>  static void remove_cache_mr_locked(struct mlx5_cache_ent *ent)
>  {
>  	struct mlx5_ib_mr *mr;
> +	void *old;
>  
> +	if (!ent->stored)
>  		return;
> +	ent->stored--;
> +	mr = __xa_store(&ent->mkeys, ent->stored, XA_ZERO_ENTRY, GFP_KERNEL);
> +	WARN_ON(mr == NULL || xa_is_err(mr));
> +	ent->reserved--;
> +	old = __xa_erase(&ent->mkeys, ent->reserved);
> +	WARN_ON(old != NULL);
>  	ent->total_mrs--;
> +	xa_unlock_irq(&ent->mkeys);

This should avoid the double XA operations if reserved == stored.


> +		mr = __xa_store(&ent->mkeys, ent->stored, XA_ZERO_ENTRY,
> +				GFP_KERNEL);
> +		WARN_ON(mr == NULL || xa_is_err(mr));
> +		ent->reserved--;
> +		old = __xa_erase(&ent->mkeys, ent->reserved);
> +		WARN_ON(old != NULL);

'pop' should really be its own function too

> @@ -601,41 +655,35 @@ struct mlx5_ib_mr *mlx5_mr_cache_alloc(struct mlx5_ib_dev *dev,
>  static void mlx5_mr_cache_free(struct mlx5_ib_dev *dev, struct mlx5_ib_mr *mr)
>  {
>  	struct mlx5_cache_ent *ent = mr->cache_ent;
> +	void *old;
>  
> -	spin_lock_irq(&ent->lock);
> -	list_add_tail(&mr->list, &ent->head);
> -	ent->available_mrs++;
> +	xa_lock_irq(&ent->mkeys);
> +	old = __xa_store(&ent->mkeys, ent->stored, mr, 0);
> +	WARN_ON(old != NULL);

This allocates memory, errors now have to be handled by falling back to dereg_mr.

Shouldn't this adjust reserved too?

Jason