Subject: Re: [PATCH v3 net-next 1/5] net: xdp: introduce bulking for xdp tx return path

From: Lorenzo Bianconi <[email-masked]>
Date: Wed, 4 Nov 2020 12:19:02 +0100

> On Wed,  4 Nov 2020 11:22:54 +0100
> Lorenzo Bianconi <[email-masked]> wrote:
> 

[...]

> > +/* XDP bulk APIs introduce a defer/flush mechanism to return
> > + * pages belonging to the same xdp_mem_allocator object
> > + * (identified via the mem.id field) in bulk to optimize
> > + * I-cache and D-cache.
> > + * The bulk queue size is set to 16 to be aligned to how
> > + * XDP_REDIRECT bulking works. The bulk is flushed when
> 
> If this is connected, then why have you not redefined DEV_MAP_BULK_SIZE?
> 
> Cc. DPAA2 maintainers as they use this define in their drivers.
> You want to make sure this driver is flexible enough for future changes.
> 
> Like:
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 3814fb631d52..44440a36f96f 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -245,6 +245,6 @@ bool xdp_attachment_flags_ok(struct xdp_attachment_info *info,
>  void xdp_attachment_setup(struct xdp_attachment_info *info,
>                           struct netdev_bpf *bpf);
>  
> -#define DEV_MAP_BULK_SIZE 16
> +#define DEV_MAP_BULK_SIZE XDP_BULK_QUEUE_SIZE

my idea was to address it in a separated patch, but if you prefer I can merge
this change in v4

>  
>  #endif /* __LINUX_NET_XDP_H__ */
> 
> 
> > + * it is full or when mem.id changes.
> > + * xdp_frame_bulk is usually stored/allocated on the function
> > + * call-stack to avoid locking penalties.
> > + */
> > +void xdp_flush_frame_bulk(struct xdp_frame_bulk *bq)
> > +{
> > +	struct xdp_mem_allocator *xa = bq->xa;
> > +	int i;
> > +
> > +	if (unlikely(!xa))
> > +		return;
> > +
> > +	for (i = 0; i < bq->count; i++) {
> > +		struct page *page = virt_to_head_page(bq->q[i]);
> > +
> > +		page_pool_put_full_page(xa->page_pool, page, false);
> > +	}
> > +	bq->count = 0;
> > +}
> > +EXPORT_SYMBOL_GPL(xdp_flush_frame_bulk);
> > +
> > +void xdp_return_frame_bulk(struct xdp_frame *xdpf,
> > +			   struct xdp_frame_bulk *bq)
> > +{
> > +	struct xdp_mem_info *mem = &xdpf->mem;
> > +	struct xdp_mem_allocator *xa;
> > +
> > +	if (mem->type != MEM_TYPE_PAGE_POOL) {
> > +		__xdp_return(xdpf->data, &xdpf->mem, false);
> > +		return;
> > +	}
> >
> 
> I cannot make up my mind: It would be a micro-optimization to move
> this if-statement to include/net/xdp.h, but it will make code harder to
> read/follow, and the call you replace xdp_return_frame() is also in
> xdp.c with same call to _xdp_return().  Let keep it as-is. (we can
> followup with micro-optimizations)

ack

Regards,
Lorenzo

> 
> 
> > +	rcu_read_lock();
> > +
> > +	xa = bq->xa;
> > +	if (unlikely(!xa)) {
> > +		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
> > +		bq->count = 0;
> > +		bq->xa = xa;
> > +	}
> > +
> > +	if (bq->count == XDP_BULK_QUEUE_SIZE)
> > +		xdp_flush_frame_bulk(bq);
> > +
> > +	if (mem->id != xa->mem.id) {
> > +		xdp_flush_frame_bulk(bq);
> > +		bq->xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
> > +	}
> > +
> > +	bq->q[bq->count++] = xdpf->data;
> > +
> > +	rcu_read_unlock();
> > +}
> > +EXPORT_SYMBOL_GPL(xdp_return_frame_bulk);
> > +
> >  void xdp_return_buff(struct xdp_buff *xdp)
> >  {
> >  	__xdp_return(xdp->data, &xdp->rxq->mem, true);
> 
> 
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>