Subject: Re: [PATCH 07/12] use a dynamic pool of sk_buffs to keep up with fast targets

From: Andrew Morton <[email-masked]>
Date: Mon, 2 Jul 2007 21:36:36 -0700

On Tue, 26 Jun 2007 14:50:11 -0400 "Ed L. Cashin" <[email-masked]> wrote:

> Use a dynamic pool of sk_buffs to keep up with fast targets.

That's far too skimpy a description of what this patch is doing, what it is
for, what makes AOE need this functionality, etc.

My initial thought is that if there is a legitimate need for this new capability
then it should be made available to other parts of the kernel rather than being
private to the AEO driver.

But 12 words is not enough information for us to make that judgement.

I have one lower-level comment way down below:

> Signed-off-by: Ed L. Cashin <[email-masked]>
> ---
>  drivers/block/aoe/aoe.h    |    5 ++
>  drivers/block/aoe/aoecmd.c |  129 +++++++++++++++++++++++++++++---------------
>  drivers/block/aoe/aoedev.c |   51 +++++++++++++++---
>  3 files changed, 134 insertions(+), 51 deletions(-)
> 
> diff --git a/drivers/block/aoe/aoe.h b/drivers/block/aoe/aoe.h
> index 7fa86dd..55c2f08 100644
> --- a/drivers/block/aoe/aoe.h
> +++ b/drivers/block/aoe/aoe.h
> @@ -98,6 +98,7 @@ enum {
>  	MIN_BUFS = 16,
>  	NTARGETS = 8,
>  	NAOEIFS = 8,
> +	NSKBPOOLMAX = 128,
>  
>  	TIMERTICK = HZ / 10,
>  	MINTIMER = HZ >> 2,
> @@ -147,6 +148,7 @@ struct aoetgt {
>  	u16 useme;
>  	ulong lastwadj;		/* last window adjustment */
>  int wpkts, rpkts;
> +int dataref;
>  };
>  
>  struct aoedev {
> @@ -168,6 +170,9 @@ struct aoedev {
>  	spinlock_t lock;
>  	struct sk_buff *sendq_hd; /* packets needing to be sent, list head */
>  	struct sk_buff *sendq_tl;
> +	struct sk_buff *skbpool_hd;
> +	struct sk_buff *skbpool_tl;
> +	int nskbpool;
>  	mempool_t *bufpool;	/* for deadlock-free Buf allocation */
>  	struct list_head bufq;	/* queue of bios to work on */
>  	struct buf *inprocess;	/* the one we're currently working on */
> diff --git a/drivers/block/aoe/aoecmd.c b/drivers/block/aoe/aoecmd.c
> index 62ba58c..89df9de 100644
> --- a/drivers/block/aoe/aoecmd.c
> +++ b/drivers/block/aoe/aoecmd.c
> @@ -105,43 +105,102 @@ ifrotate(struct aoetgt *t)
>  	}
>  }
>  
> +static void
> +skb_pool_put(struct aoedev *d, struct sk_buff *skb)
> +{
> +	if (!d->skbpool_hd)
> +		d->skbpool_hd = skb;
> +	else
> +		d->skbpool_tl->next = skb;
> +	d->skbpool_tl = skb;
> +}
> +
> +static struct sk_buff *
> +skb_pool_get(struct aoedev *d)
> +{
> +	struct sk_buff *skb;
> +
> +	skb = d->skbpool_hd;
> +	if (skb)
> +	if (atomic_read(&skb_shinfo(skb)->dataref) == 1) {
> +		d->skbpool_hd = skb->next;
> +		skb->next = NULL;
> +		return skb;
> +	}
> +	if (d->nskbpool < NSKBPOOLMAX)
> +	if ((skb = new_skb(ETH_ZLEN))) {
> +		d->nskbpool++;
> +		return skb;
> +	}
> +	return NULL;
> +}
> +
> +/* freeframe is where we do our load balancing so it's a little hairy. */
>  static struct frame *
>  freeframe(struct aoedev *d)
>  {
> -	struct frame *f, *e;
> +	struct frame *f, *e, *rf;
>  	struct aoetgt **t;
> -	ulong n;
> +	struct sk_buff *skb;
>  
>  	if (d->targets[0] == NULL) {	/* shouldn't happen, but I'm paranoid */
>  		printk(KERN_ERR "aoe: NULL TARGETS!\n");
>  		return NULL;
>  	}
> -	t = d->targets;
> -	do {
> +	t = d->tgt;
> +	t++;
> +	if (t >= &d->targets[NTARGETS] || !*t)
> +		t = d->targets;
> +	for (;;) {
> +		if ((*t)->nout < (*t)->maxout)
>  		if (t != d->htgt)
> -		if ((*t)->ifp->nd)
> -		if ((*t)->nout < (*t)->maxout) {
> -			n = (*t)->nframes;
> +		if ((*t)->ifp->nd) {
> +			rf = NULL;
>  			f = (*t)->frames;
> -			e = f + n;
> +			e = f + (*t)->nframes;
>  			for (; f<e; f++) {
>  				if (f->tag != FREETAG)
>  					continue;
> -				if (atomic_read(&skb_shinfo(f->skb)->dataref) != 1) {
> -					n--;
> +				skb = f->skb;
> +				if (!skb)
> +				if (!(f->skb = skb = new_skb(ETH_ZLEN)))
> +					continue;
> +				if (atomic_read(&skb_shinfo(skb)->dataref) != 1) {
> +					if (!rf)
> +						rf = f;
>  					continue;
>  				}
> -				skb_shinfo(f->skb)->nr_frags = f->skb->data_len = 0;
> -				skb_trim(f->skb, 0);
> +gotone:				skb_shinfo(skb)->nr_frags = skb->data_len = 0;
> +				skb_trim(skb, 0);
>  				d->tgt = t;
>  				ifrotate(*t);
>  				return f;
>  			}
> -			if (n == 0)	/* slow polling network card */
> +			/* Work can be done, but the network layer is
> +			   holding our precious packets.  Try to grab
> +			   one from the pool. */
> +			f = rf;
> +			if (f == NULL) {	/* more paranoia */
> +				printk(KERN_ERR "aoe: freeframe: unexpected null rf.\n");
> +				d->flags |= DEVFL_KICKME;
> +				return NULL;
> +			}
> +			skb = skb_pool_get(d);
> +			if (skb) {
> +				skb_pool_put(d, f->skb);
> +				f->skb = skb;
> +				goto gotone;
> +			}
> +			(*t)->dataref++;
> +			if ((*t)->nout == 0)
>  				d->flags |= DEVFL_KICKME;
>  		}
> +		if (t == d->tgt)	/* we've looped and found nada */
> +			break;
>  		t++;
> -	} while (t < &d->targets[NTARGETS] && *t);
> +		if (t >= &d->targets[NTARGETS] || !*t)
> +			t = d->targets;
> +	}
>  	return NULL;
>  }
>  
> @@ -870,44 +929,26 @@ addtgt(struct aoedev *d, char *addr, ulong nframes)
>  
>  	tt = d->targets;
>  	te = tt + NTARGETS;
> -	for (; tt<te; tt++) {
> -		if (*tt == NULL)
> -			break;
> -		else if (memcmp((*tt)->addr, addr, 6) > 0) {
> -			memmove(tt+1, tt, (void *)te-(void *)(tt+1));
> -			break;
> -		}
> -	}
> +	for (; tt<te && *tt; tt++)
> +		;
> +
>  	if (tt == te)
>  		return NULL;
>  
>  	t = kcalloc(1, sizeof *t, GFP_ATOMIC);
> +	if (!t)
> +		return NULL;
>  	f = kcalloc(nframes, sizeof *f, GFP_ATOMIC);
> -	switch (!t || !f) {
> -	case 0:
> -		t->nframes = nframes;
> -		t->frames = f;
> -		e = f + nframes;
> -		for (; f<e; f++) {
> -			f->tag = FREETAG;
> -			f->skb = new_skb(ETH_ZLEN);
> -			if (!f->skb)
> -				break;
> -		}
> -		if (f == e)
> -			break;
> -		while (f > t->frames) {
> -			f--;
> -			dev_kfree_skb(f->skb);
> -		}
> -	default:
> -		if (f)
> -			kfree(f);
> -		if (t)
> -			kfree(t);
> +	if (!f) {
> +		kfree(t);
>  		return NULL;
>  	}
>  
> +	t->nframes = nframes;
> +	t->frames = f;
> +	e = f + nframes;
> +	for (; f<e; f++)
> +		f->tag = FREETAG;
>  	memcpy(t->addr, addr, sizeof t->addr);
>  	t->ifp = t->ifs;
>  	t->maxout = t->nframes;
> diff --git a/drivers/block/aoe/aoedev.c b/drivers/block/aoe/aoedev.c
> index 8659796..cf4e58d 100644
> --- a/drivers/block/aoe/aoedev.c
> +++ b/drivers/block/aoe/aoedev.c
> @@ -7,11 +7,13 @@
>  #include <linux/hdreg.h>
>  #include <linux/blkdev.h>
>  #include <linux/netdevice.h>
> +#include <linux/delay.h>
>  #include "aoe.h"
>  
>  static void dummy_timer(ulong);
>  static void aoedev_freedev(struct aoedev *);
> -static void freetgt(struct aoetgt *t);
> +static void freetgt(struct aoedev *d, struct aoetgt *t);
> +static void skbpoolfree(struct aoedev *d);
>  
>  static struct aoedev *devlist;
>  static spinlock_t devlist_lock;
> @@ -125,9 +127,10 @@ aoedev_freedev(struct aoedev *d)
>  	t = d->targets;
>  	e = t + NTARGETS;
>  	for (; t<e && *t; t++)
> -		freetgt(*t);
> +		freetgt(d, *t);
>  	if (d->bufpool)
>  		mempool_destroy(d->bufpool);
> +	skbpoolfree(d);
>  	kfree(d);
>  }
>  
> @@ -176,6 +179,42 @@ aoedev_flush(const char __user *str, size_t cnt)
>  	return 0;
>  }
>  
> +/* I'm not really sure that this is a realistic problem, but if the
> +network driver goes gonzo let's just leak memory after complaining. */
> +static void
> +skbfree(struct sk_buff *skb)
> +{
> +	enum { Sms= 100, Tms= 3*1000};
> +	int i = Tms / Sms;
> +
> +	if (skb == NULL)
> +		return;
> +	while (atomic_read(&skb_shinfo(skb)->dataref) != 1 && i-- > 0)
> +		msleep_interruptible(Sms);

If the calling process has signal_pending(), this will turn into a busy
loop.  On non-preempt uniproc it'll lock the box.

The only way this code is safe is if it's called from a kernel thread.  Is
it?

> +	if (i <= 0) {
> +		printk(KERN_ERR
> +			"aoe: %s holds ref: cannot free skb -- memory leaked.\n",
> +			skb->dev ? skb->dev->name : "netif");
> +		return;
> +	}
> +	skb_shinfo(skb)->nr_frags = skb->data_len = 0;
> +	skb_trim(skb, 0);
> +	dev_kfree_skb(skb);
> +}
> +
> +static void
> +skbpoolfree(struct aoedev *d)
> +{
> +	struct sk_buff *skb;
> +
> +	while ((skb = d->skbpool_hd)) {
> +		d->skbpool_hd = skb->next;
> +		skb->next = NULL;
> +		skbfree(skb);
> +	}
> +	d->skbpool_tl = NULL;
> +}
> +
>  /* find it or malloc it */
>  struct aoedev *
>  aoedev_by_sysminor_m(ulong sysminor)
> @@ -214,16 +253,14 @@ aoedev_by_sysminor_m(ulong sysminor)
>  }
>  
>  static void
> -freetgt(struct aoetgt *t)
> +freetgt(struct aoedev *d, struct aoetgt *t)
>  {
>  	struct frame *f, *e;
>  
>  	f = t->frames;
>  	e = f + t->nframes;
> -	for (; f<e; f++) {
> -		skb_shinfo(f->skb)->nr_frags = 0;
> -		dev_kfree_skb(f->skb);
> -	}
> +	for (; f<e; f++)
> +		skbfree(f->skb);
>  	kfree(t->frames);
>  	kfree(t);
>  }
> -- 
> 1.5.2.1
> 
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