[PATCH] 9p/xen: increase XEN_9PFS_RING_ORDER

Stefano Stabellini posted 1 patch 5 days ago
Failed in applying to current master (apply log)
net/9p/trans_xen.c | 61 ++++++++++++++++++++++++++--------------------
1 file changed, 34 insertions(+), 27 deletions(-)

[PATCH] 9p/xen: increase XEN_9PFS_RING_ORDER

Posted by Stefano Stabellini 5 days ago
From: Stefano Stabellini <stefano.stabellini@xilinx.com>

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.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
---
 net/9p/trans_xen.c | 61 ++++++++++++++++++++++++++--------------------
 1 file changed, 34 insertions(+), 27 deletions(-)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index 086a4abdfa7c..cf5ea74be7cc 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -44,8 +44,8 @@
 #include <net/9p/transport.h>
 
 #define XEN_9PFS_NUM_RINGS 2
-#define XEN_9PFS_RING_ORDER 6
-#define XEN_9PFS_RING_SIZE  XEN_FLEX_RING_SIZE(XEN_9PFS_RING_ORDER)
+#define XEN_9PFS_RING_ORDER 9
+#define XEN_9PFS_RING_SIZE(ring)  XEN_FLEX_RING_SIZE(ring->intf->ring_order)
 
 struct xen_9pfs_header {
 	uint32_t size;
@@ -130,8 +130,8 @@ static bool p9_xen_write_todo(struct xen_9pfs_dataring *ring, RING_IDX size)
 	prod = ring->intf->out_prod;
 	virt_mb();
 
-	return XEN_9PFS_RING_SIZE -
-		xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE) >= size;
+	return XEN_9PFS_RING_SIZE(ring) -
+		xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE(ring)) >= size;
 }
 
 static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
@@ -165,17 +165,18 @@ static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
 	prod = ring->intf->out_prod;
 	virt_mb();
 
-	if (XEN_9PFS_RING_SIZE - xen_9pfs_queued(prod, cons,
-						 XEN_9PFS_RING_SIZE) < size) {
+	if (XEN_9PFS_RING_SIZE(ring) -
+	    xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE(ring)) < size) {
 		spin_unlock_irqrestore(&ring->lock, flags);
 		goto again;
 	}
 
-	masked_prod = xen_9pfs_mask(prod, XEN_9PFS_RING_SIZE);
-	masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE);
+	masked_prod = xen_9pfs_mask(prod, XEN_9PFS_RING_SIZE(ring));
+	masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE(ring));
 
 	xen_9pfs_write_packet(ring->data.out, p9_req->tc->sdata, size,
-			      &masked_prod, masked_cons, XEN_9PFS_RING_SIZE);
+			      &masked_prod, masked_cons,
+			      XEN_9PFS_RING_SIZE(ring));
 
 	p9_req->status = REQ_STATUS_SENT;
 	virt_wmb();			/* write ring before updating pointer */
@@ -204,19 +205,19 @@ static void p9_xen_response(struct work_struct *work)
 		prod = ring->intf->in_prod;
 		virt_rmb();
 
-		if (xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE) <
+		if (xen_9pfs_queued(prod, cons, XEN_9PFS_RING_SIZE(ring)) <
 		    sizeof(h)) {
 			notify_remote_via_irq(ring->irq);
 			return;
 		}
 
-		masked_prod = xen_9pfs_mask(prod, XEN_9PFS_RING_SIZE);
-		masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE);
+		masked_prod = xen_9pfs_mask(prod, XEN_9PFS_RING_SIZE(ring));
+		masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE(ring));
 
 		/* First, read just the header */
 		xen_9pfs_read_packet(&h, ring->data.in, sizeof(h),
 				     masked_prod, &masked_cons,
-				     XEN_9PFS_RING_SIZE);
+				     XEN_9PFS_RING_SIZE(ring));
 
 		req = p9_tag_lookup(priv->client, h.tag);
 		if (!req || req->status != REQ_STATUS_SENT) {
@@ -230,11 +231,11 @@ static void p9_xen_response(struct work_struct *work)
 		memcpy(req->rc, &h, sizeof(h));
 		req->rc->offset = 0;
 
-		masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE);
+		masked_cons = xen_9pfs_mask(cons, XEN_9PFS_RING_SIZE(ring));
 		/* Then, read the whole packet (including the header) */
 		xen_9pfs_read_packet(req->rc->sdata, ring->data.in, h.size,
 				     masked_prod, &masked_cons,
-				     XEN_9PFS_RING_SIZE);
+				     XEN_9PFS_RING_SIZE(ring));
 
 		virt_mb();
 		cons += h.size;
@@ -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,
@@ -292,14 +293,16 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
 		if (priv->rings[i].irq > 0)
 			unbind_from_irqhandler(priv->rings[i].irq, priv->dev);
 		if (priv->rings[i].data.in) {
-			for (j = 0; j < (1 << XEN_9PFS_RING_ORDER); j++) {
+			for (j = 0;
+			     j < (1 << priv->rings[i].intf->ring_order);
+			     j++) {
 				grant_ref_t ref;
 
 				ref = priv->rings[i].intf->ref[j];
 				gnttab_end_foreign_access(ref, 0, 0);
 			}
 			free_pages((unsigned long)priv->rings[i].data.in,
-				   XEN_9PFS_RING_ORDER -
+				   priv->rings[i].intf->ring_order -
 				   (PAGE_SHIFT - XEN_PAGE_SHIFT));
 		}
 		gnttab_end_foreign_access(priv->rings[i].ref, 0, 0);
@@ -320,7 +323,8 @@ static int xen_9pfs_front_remove(struct xenbus_device *dev)
 }
 
 static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
-					 struct xen_9pfs_dataring *ring)
+					 struct xen_9pfs_dataring *ring,
+					 unsigned int order)
 {
 	int i = 0;
 	int ret = -ENOMEM;
@@ -339,21 +343,21 @@ static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
 		goto out;
 	ring->ref = ret;
 	bytes = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
-			XEN_9PFS_RING_ORDER - (PAGE_SHIFT - XEN_PAGE_SHIFT));
+			order - (PAGE_SHIFT - XEN_PAGE_SHIFT));
 	if (!bytes) {
 		ret = -ENOMEM;
 		goto out;
 	}
-	for (; i < (1 << XEN_9PFS_RING_ORDER); i++) {
+	for (; i < (1 << order); i++) {
 		ret = gnttab_grant_foreign_access(
 				dev->otherend_id, virt_to_gfn(bytes) + i, 0);
 		if (ret < 0)
 			goto out;
 		ring->intf->ref[i] = ret;
 	}
-	ring->intf->ring_order = XEN_9PFS_RING_ORDER;
+	ring->intf->ring_order = order;
 	ring->data.in = bytes;
-	ring->data.out = bytes + XEN_9PFS_RING_SIZE;
+	ring->data.out = bytes + XEN_FLEX_RING_SIZE(order);
 
 	ret = xenbus_alloc_evtchn(dev, &ring->evtchn);
 	if (ret)
@@ -371,7 +375,7 @@ static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
 		for (i--; i >= 0; i--)
 			gnttab_end_foreign_access(ring->intf->ref[i], 0, 0);
 		free_pages((unsigned long)bytes,
-			   XEN_9PFS_RING_ORDER -
+			   ring->intf->ring_order -
 			   (PAGE_SHIFT - XEN_PAGE_SHIFT));
 	}
 	gnttab_end_foreign_access(ring->ref, 0, 0);
@@ -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 (p9_xen_trans.maxsize > XEN_FLEX_RING_SIZE(max_ring_order))
+		p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order);
 
 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
@@ -419,7 +425,8 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
 
 	for (i = 0; i < priv->num_rings; i++) {
 		priv->rings[i].priv = priv;
-		ret = xen_9pfs_front_alloc_dataring(dev, &priv->rings[i]);
+		ret = xen_9pfs_front_alloc_dataring(dev, &priv->rings[i],
+						    max_ring_order);
 		if (ret < 0)
 			goto error;
 	}
-- 
2.17.1


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

Posted by Dominique Martinet 5 days ago
Stefano Stabellini wrote on Wed, May 20, 2020:
> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> 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

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

Posted by Stefano Stabellini 5 days ago
On Wed, 20 May 2020, Dominique Martinet wrote:
> Stefano Stabellini wrote on Wed, May 20, 2020:
> > From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> > 
> > 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 :))

Your point is valid but the size calculation (XEN_FLEX_RING_SIZE) should
work correctly even with order 0:

    (1UL << ((0) + XEN_PAGE_SHIFT - 1)) = 1 << (12 - 1) = 2048

So I am thinking that the protocol should still work correctly, although
the performance might be undesirable.

FYI The smallest backend I know of has order 6.


> > +	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?

Yes, you are right, thanks for noticing this! I meant to do that here
too but somehow forgot. This should be:

   p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order) / 2;