Re: [V9fs-developer] [PATCH] 9p/xen: increase XEN_9PFS_RING_ORDER

[Dominique Martinet <asmadeus@xxxxxxxxxxxxx>]

Stefano Stabellini wrote on Wed, May 20, 2020:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> 
> Increase XEN_9PFS_RING_ORDER to 9 for performance reason. Order 9 is the
> max allowed by the protocol.
> 
> We can't assume that all backends will support order 9. The xenstore
> property max-ring-page-order specifies the max order supported by the
> backend. We'll use max-ring-page-order for the size of the ring.
> 
> This means that the size of the ring is not static
> (XEN_FLEX_RING_SIZE(9)) anymore. Change XEN_9PFS_RING_SIZE to take an
> argument and base the calculation on the order chosen at setup time.
> 
> 
> Finally, modify p9_xen_trans.maxsize to be divided by 4 compared to the
> original value. We need to divide it by 2 because we have two rings
> coming off the same order allocation: the in and out rings. This was a
> mistake in the original code. Also divide it further by 2 because we
> don't want a single request/reply to fill up the entire ring. There can
> be multiple requests/replies outstanding at any given time and if we use
> the full ring with one, we risk forcing the backend to wait for the
> client to read back more replies before continuing, which is not
> performant.

Sounds good to me overall. A couple of comments inline.
Also worth noting I need to rebuild myself a test setup so might take a
bit of time to actually run tests, but I might just trust you on this
one for now if it builds with no new warning... Looks like it would
probably work :p

> [...]
> @@ -264,7 +265,7 @@ static irqreturn_t xen_9pfs_front_event_handler(int irq, 
> void *r)
>  
>  static struct p9_trans_module p9_xen_trans = {
>       .name = "xen",
> -     .maxsize = 1 << (XEN_9PFS_RING_ORDER + XEN_PAGE_SHIFT),
> +     .maxsize = 1 << (XEN_9PFS_RING_ORDER + XEN_PAGE_SHIFT - 2),
>       .def = 1,
>       .create = p9_xen_create,
>       .close = p9_xen_close,
> [...]
> @@ -401,8 +405,10 @@ static int xen_9pfs_front_probe(struct xenbus_device 
> *dev,
>               return -EINVAL;
>       max_ring_order = xenbus_read_unsigned(dev->otherend,
>                                             "max-ring-page-order", 0);
> -     if (max_ring_order < XEN_9PFS_RING_ORDER)
> -             return -EINVAL;
> +     if (max_ring_order > XEN_9PFS_RING_ORDER)
> +             max_ring_order = XEN_9PFS_RING_ORDER;

(If there are backends with very small max_ring_orders, we no longer
error out when we encounter one, it might make sense to add a min
define? Although to be honest 9p works with pretty small maxsizes so I
don't see much reason to error out, and even order 0 will be one page
worth.. I hope there is no xenbus that small though :))

> +     if (p9_xen_trans.maxsize > XEN_FLEX_RING_SIZE(max_ring_order))
> +             p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order);

So base maxsize initial value is 1 << (order + page_shift - 2) ; but
this is 1 << (order + page_shift - 1) -- I agree with the logic you gave
in commit message so would think this needs to be shifted down one more
like the base value as well.
What do you think?

-- 
Dominique