Re: [Xen-devel] [PATCH V4 2/3] xen-netback: switch to NAPI + kthread 1:1 model

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-08-06 at 14:33 +0100, Wei Liu wrote:
> I sent this patch right before Ian requested more comments in code. Now
> I've updated this one. Not going to resend the whole series as this is
> only changes in comments.
> 
> Sorry for my bad English, I don't know whether I've made them clear
> enough. Suggestions are welcomed.
> 
> 
> Wei.
> 
> ---8<---
> From bb33027fe4bafeea546352cd3e409466f8bd7aa4 Mon Sep 17 00:00:00 2001
> From: Wei Liu <wei.liu2@xxxxxxxxxx>
> Date: Tue, 6 Aug 2013 06:59:15 +0100
> Subject: [PATCH] xen-netback: switch to NAPI + kthread 1:1 model
> 
> This patch implements 1:1 model netback. NAPI and kthread are utilized
> to do the weight-lifting job:
> 
> - NAPI is used for guest side TX (host side RX)
> - kthread is used for guest side RX (host side TX)
> 
> Xenvif and xen_netbk are made into one structure to reduce code size.
> 
> This model provides better scheduling fairness among vifs. It is also
> prerequisite for implementing multiqueue for Xen netback.
> 
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> ---
>  drivers/net/xen-netback/common.h    |  128 +++++---
>  drivers/net/xen-netback/interface.c |  119 ++++---
>  drivers/net/xen-netback/netback.c   |  607 
> ++++++++++-------------------------
>  3 files changed, 347 insertions(+), 507 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/common.h 
> b/drivers/net/xen-netback/common.h
> index 8a4d77e..184ae0a 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -45,31 +45,105 @@
>  #include <xen/grant_table.h>
>  #include <xen/xenbus.h>
>  
> -struct xen_netbk;
> +typedef unsigned int pending_ring_idx_t;
> +#define INVALID_PENDING_RING_IDX (~0U)
> +
> +/* For the head field in pending_tx_info: It is used to indicate
> + * whether this tx info is the head of one or more coalesced requests.
> + *
> + * When head != INVALID_PENDING_RING_IDX, it means the start of a new
> + * tx requests queue and the end of previous queue.
> + *
> + * An example sequence of head fields (I = INVALID_PENDING_RING_IDX):
> + *
> + * ... 0 I I I 5 I 9 I I I ...
> + *
> + * After consming the first packet we have:

"consuming"

The first packet here is "0 I I I"? Consisting of the head (0) and 3
extra data slots (the 3xI)?

> + *
> + * ... V V V V 5 I 9 I I I ...
> + *
> + * where V stands for "valid pending ring index", any number other
> + * than INVALID_PENDING_RING_IDX is OK. In practice we use 0.

OK, this is where I get confused, because 0 is also valid in a different
state, this one:
> + * The non-INVALID_PENDING_RING_IDX (say 0, 5 and 9 in the above
> + * example) number is the index into pending_tx_info and mmap_pages
> + * arrays.

So what does V mean and how to you distinguish this from a
non-INVALID_PENDING_RING_IDX which happens to correspond to the 0 you
use in practice for V?

I'm also not sure how 0 is considered a "valid pending ring index". I
suppose it is not INVALID_PENDING_RING_IDX but it appears that it
doesn't have any actual meaning? So it's just an arbitrary valid but
meaningless number perhaps?

Ian.


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