Re: [RFC 2/2] x86_64: expand kernel stack to 16K

[Rusty Russell <rusty@rustcorp.com.au>]

Minchan Kim <minchan@kernel.org> writes:
> On Wed, May 28, 2014 at 12:04:09PM +0300, Michael S. Tsirkin wrote:
>> On Wed, May 28, 2014 at 03:53:59PM +0900, Minchan Kim wrote:
>> > [ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:   9)     6456      80   __kmalloc+0x1cb/0x200
>> > [ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  10)     6376     376   vring_add_indirect+0x36/0x200
>> > [ 1065.604404] kworker/-5766    0d..2 1071625993us : stack_trace_call:  11)     6000     144   virtqueue_add_sgs+0x2e2/0x320

Hmm, we can actually skip the vring_add_indirect if we're hurting for
stack.  It just means the request will try to fit linearly in the ring,
rather than using indirect.

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1e443629f76d..496e727cefc8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -184,6 +184,13 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
 	return head;
 }
 
+/* The Morton Technique */
+static noinline bool stack_trouble(void)
+{
+	unsigned long sp = (unsigned long)&sp;
+	return sp - (sp & ~(THREAD_SIZE - 1)) < 3000;
+}
+
 static inline int virtqueue_add(struct virtqueue *_vq,
 				struct scatterlist *sgs[],
 				struct scatterlist *(*next)
@@ -226,7 +233,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
 	/* If the host supports indirect descriptor tables, and we have multiple
 	 * buffers, then go indirect. FIXME: tune this threshold */
-	if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
+	if (vq->indirect && total_sg > 1 && vq->vq.num_free && !stack_trouble()) {
 		head = vring_add_indirect(vq, sgs, next, total_sg, total_out,
 					  total_in,
 					  out_sgs, in_sgs, gfp);

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