Subject: Re: [RFC PATCH 1/2] net: use cmpxchg instead of spinlock in ptr rings

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 15 Nov 2016 01:01:25 +0200

On Thu, Nov 10, 2016 at 08:44:08PM -0800, John Fastabend wrote:
> 
> ---
>  include/linux/ptr_ring_ll.h |  136 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/skb_array.h   |   25 ++++++++
>  2 files changed, 161 insertions(+)
>  create mode 100644 include/linux/ptr_ring_ll.h
> 
> diff --git a/include/linux/ptr_ring_ll.h b/include/linux/ptr_ring_ll.h
> new file mode 100644
> index 0000000..bcb11f3
> --- /dev/null
> +++ b/include/linux/ptr_ring_ll.h
> @@ -0,0 +1,136 @@
> +/*
> + *	Definitions for the 'struct ptr_ring_ll' datastructure.
> + *
> + *	Author:
> + *		John Fastabend <[email-masked]>
> + *
> + *	Copyright (C) 2016 Intel Corp.
> + *
> + *	This program is free software; you can redistribute it and/or modify it
> + *	under the terms of the GNU General Public License as published by the
> + *	Free Software Foundation; either version 2 of the License, or (at your
> + *	option) any later version.
> + *
> + *	This is a limited-size FIFO maintaining pointers in FIFO order, with
> + *	one CPU producing entries and another consuming entries from a FIFO.
> + *	extended from ptr_ring_ll to use cmpxchg over spin lock.

So when is each one (ptr-ring/ptr-ring-ll) a win? _ll suffix seems to
imply this gives a better latency, OTOH for a ping/pong I suspect
ptr-ring would be better as it avoids index cache line bounces.

> + */
> +
> +#ifndef _LINUX_PTR_RING_LL_H
> +#define _LINUX_PTR_RING_LL_H 1
> +
> +#ifdef __KERNEL__
> +#include <linux/spinlock.h>
> +#include <linux/cache.h>
> +#include <linux/types.h>
> +#include <linux/compiler.h>
> +#include <linux/cache.h>
> +#include <linux/slab.h>
> +#include <asm/errno.h>
> +#endif
> +
> +struct ptr_ring_ll {
> +	u32 prod_size;
> +	u32 prod_mask;
> +	u32 prod_head;
> +	u32 prod_tail;
> +	u32 cons_size;
> +	u32 cons_mask;
> +	u32 cons_head;
> +	u32 cons_tail;
> +
> +	void **queue;
> +};
> +
> +/* Note: callers invoking this in a loop must use a compiler barrier,
> + * for example cpu_relax(). Callers must hold producer_lock.
> + */
> +static inline int __ptr_ring_ll_produce(struct ptr_ring_ll *r, void *ptr)
> +{
> +	u32 ret, head, tail, next, slots, mask;
> +
> +	do {
> +		head = READ_ONCE(r->prod_head);
> +		mask = READ_ONCE(r->prod_mask);
> +		tail = READ_ONCE(r->cons_tail);
> +
> +		slots = mask + tail - head;
> +		if (slots < 1)
> +			return -ENOMEM;
> +
> +		next = head + 1;
> +		ret = cmpxchg(&r->prod_head, head, next);
> +	} while (ret != head);

So why is this preferable to a lock?

I suspect it's nothing else than the qspinlock fairness
and polling code complexity. It's all not very useful if you
1. are just doing a couple of instructions under the lock
and
2. use a finite FIFO which is unfair anyway

How about this hack (lifted from virt_spin_lock):

static inline void quick_spin_lock(struct qspinlock *lock)
{
        do {
                while (atomic_read(&lock->val) != 0)
                        cpu_relax();
        } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
}

Or maybe we should even drop the atomic_read in the middle -
worth profiling and comparing:

static inline void quick_spin_lock(struct qspinlock *lock)
{
        while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
		cpu_relax();
}

Then, use quick_spin_lock instead of spin_lock everywhere in
ptr_ring - will that make it more efficient?

> +
> +	r->queue[head & mask] = ptr;
> +	smp_wmb();
> +
> +	while (r->prod_tail != head)
> +		cpu_relax();
> +
> +	r->prod_tail = next;
> +	return 0;
> +}
> +
> +static inline void *__ptr_ring_ll_consume(struct ptr_ring_ll *r)
> +{
> +	u32 ret, head, tail, next, slots, mask;
> +	void *ptr;
> +
> +	do {
> +		head = READ_ONCE(r->cons_head);
> +		mask = READ_ONCE(r->cons_mask);
> +		tail = READ_ONCE(r->prod_tail);
> +
> +		slots = tail - head;
> +		if (slots < 1)
> +			return ERR_PTR(-ENOMEM);
> +
> +		next = head + 1;
> +		ret = cmpxchg(&r->cons_head, head, next);
> +	} while (ret != head);
> +
> +	ptr = r->queue[head & mask];
> +	smp_rmb();
> +
> +	while (r->cons_tail != head)
> +		cpu_relax();
> +
> +	r->cons_tail = next;
> +	return ptr;
> +}
> +
> +static inline void **__ptr_ring_ll_init_queue_alloc(int size, gfp_t gfp)
> +{
> +	return kzalloc(ALIGN(size * sizeof(void *), SMP_CACHE_BYTES), gfp);
> +}
> +
> +static inline int ptr_ring_ll_init(struct ptr_ring_ll *r, int size, gfp_t gfp)
> +{
> +	r->queue = __ptr_ring_init_queue_alloc(size, gfp);
> +	if (!r->queue)
> +		return -ENOMEM;
> +
> +	r->prod_size = r->cons_size = size;
> +	r->prod_mask = r->cons_mask = size - 1;
> +	r->prod_tail = r->prod_head = 0;
> +	r->cons_tail = r->prod_tail = 0;
> +
> +	return 0;
> +}
> +
> +static inline void ptr_ring_ll_cleanup(struct ptr_ring_ll *r, void (*destroy)(void *))
> +{
> +	if (destroy) {
> +		void *ptr;
> +
> +		ptr = __ptr_ring_ll_consume(r);
> +		while (!IS_ERR_OR_NULL(ptr)) {
> +			destroy(ptr);
> +			ptr = __ptr_ring_ll_consume(r);
> +		}
> +	}
> +	kfree(r->queue);
> +}
> +
> +#endif /* _LINUX_PTR_RING_LL_H  */
> diff --git a/include/linux/skb_array.h b/include/linux/skb_array.h
> index f4dfade..9b43dfd 100644
> --- a/include/linux/skb_array.h
> +++ b/include/linux/skb_array.h
> @@ -22,6 +22,7 @@
>  
>  #ifdef __KERNEL__
>  #include <linux/ptr_ring.h>
> +#include <linux/ptr_ring_ll.h>
>  #include <linux/skbuff.h>
>  #include <linux/if_vlan.h>
>  #endif
> @@ -30,6 +31,10 @@ struct skb_array {
>  	struct ptr_ring ring;
>  };
>  
> +struct skb_array_ll {
> +	struct ptr_ring_ll ring;
> +};
> +
>  /* Might be slightly faster than skb_array_full below, but callers invoking
>   * this in a loop must use a compiler barrier, for example cpu_relax().
>   */
> @@ -43,6 +48,11 @@ static inline bool skb_array_full(struct skb_array *a)
>  	return ptr_ring_full(&a->ring);
>  }
>  
> +static inline int skb_array_ll_produce(struct skb_array_ll *a, struct sk_buff *skb)
> +{
> +	return __ptr_ring_ll_produce(&a->ring, skb);
> +}
> +
>  static inline int skb_array_produce(struct skb_array *a, struct sk_buff *skb)
>  {
>  	return ptr_ring_produce(&a->ring, skb);
> @@ -92,6 +102,11 @@ static inline bool skb_array_empty_any(struct skb_array *a)
>  	return ptr_ring_empty_any(&a->ring);
>  }
>  
> +static inline struct sk_buff *skb_array_ll_consume(struct skb_array_ll *a)
> +{
> +	return __ptr_ring_ll_consume(&a->ring);
> +}
> +
>  static inline struct sk_buff *skb_array_consume(struct skb_array *a)
>  {
>  	return ptr_ring_consume(&a->ring);
> @@ -146,6 +161,11 @@ static inline int skb_array_peek_len_any(struct skb_array *a)
>  	return PTR_RING_PEEK_CALL_ANY(&a->ring, __skb_array_len_with_tag);
>  }
>  
> +static inline int skb_array_ll_init(struct skb_array_ll *a, int size, gfp_t gfp)
> +{
> +	return ptr_ring_ll_init(&a->ring, size, gfp);
> +}
> +
>  static inline int skb_array_init(struct skb_array *a, int size, gfp_t gfp)
>  {
>  	return ptr_ring_init(&a->ring, size, gfp);
> @@ -170,6 +190,11 @@ static inline int skb_array_resize_multiple(struct skb_array **rings,
>  					__skb_array_destroy_skb);
>  }
>  
> +static inline void skb_array_ll_cleanup(struct skb_array_ll *a)
> +{
> +	ptr_ring_ll_cleanup(&a->ring, __skb_array_destroy_skb);
> +}
> +
>  static inline void skb_array_cleanup(struct skb_array *a)
>  {
>  	ptr_ring_cleanup(&a->ring, __skb_array_destroy_skb);