virtio_ring stack usage.

Rusty Russell
2014-05-29T04:12:09Z

"Michael S. Tsirkin" <mst@redhat.com> writes:
> 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
>> [ 1065.604404] kworker/-5766 0d..2 1071625993us : stack_trace_call: 12) 5856 288 __virtblk_add_req+0xda/0x1b0
>> [ 1065.604404] kworker/-5766 0d..2 1071625993us : stack_trace_call: 13) 5568 96 virtio_queue_rq+0xd3/0x1d0
>
> virtio stack usage seems very high.
> Here is virtio_ring.su generated using -fstack-usage flag for gcc 4.8.2.
>
> virtio_ring.c:107:35:sg_next_arr 16 static
...
> <--- this is a surprise, I really expected it to be inlined
> same for sg_next_chained.
> <--- Rusty: should we force compiler to inline it?

Extra cc's dropped.

Weird, works here (gcc 4.8.2, 32 bit). Hmm, same with 64 bit:

gcc -Wp,-MD,drivers/virtio/.virtio_ring.o.d -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/home/rusty/devel/kernel/linux/arch/x86/include -Iarch/x86/include/generated -Iinclude -I/home/rusty/devel/kernel/linux/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/rusty/devel/kernel/linux/include/uapi -Iinclude/generated/uapi -include /home/rusty/devel/kernel/linux/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m64 -mno-mmx -mno-sse -mno-80387 -mno-fp-ret-in-387 -mpreferred-stack-boundary=3 -march=core2 -mno-red-zone -mcmodel=kernel -funit-at-a-time -maccumulate-outgoing-args -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_FXSAVEQ=1 -DCONFIG_AS_CRC32=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -Wframe-larger-than=1024 -fno-stack-protector -Wno-unused-but-set-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -DCC_HAVE_ASM_GOTO -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(virtio_ring)" -D"KBUILD_MODNAME=KBUILD_STR(virtio_ring)" -c -o drivers/virtio/virtio_ring.o drivers/virtio/virtio_ring.c

$ objdump -dr drivers/virtio/virtio_ring.o | grep sg_next
988: R_X86_64_PC32 sg_next-0x4
9d8: R_X86_64_PC32 sg_next-0x4
ae9: R_X86_64_PC32 sg_next-0x4
b99: R_X86_64_PC32 sg_next-0x4
d31: R_X86_64_PC32 sg_next-0x4
df1: R_X86_64_PC32 sg_next-0x4
$

It's worth noting that older GCCs would sometimes successfully inline
the indirect function (ie. sg_next_chained and sg_next_ar) but still
emit an unused copy. Is that happening for you too?

I added a hack to actually measure how much stack we're using (x86-64):

gcc 4.8.4:
[ 3.261826] virtio_blk: stack used = 408

gcc 4.6:
[ 3.276449] virtio_blk: stack depth = 448

Here's the hack I used:

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 6d8a87f252de..bcd6336e3561 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -151,15 +151,19 @@ static void virtblk_done(struct virtqueue *vq)
blk_mq_start_stopped_hw_queues(vblk->disk->queue);
}

+extern struct task_struct *record_stack;
+extern unsigned long stack_top;
+
static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
{
+ unsigned long stack_bottom = (unsigned long)&stack_bottom;
struct virtio_blk *vblk = hctx->queue->queuedata;
struct virtblk_req *vbr = req->special;
unsigned long flags;
unsigned int num;
const bool last = (req->cmd_flags & REQ_END) != 0;
int err;
-
+
BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);

vbr->req = req;
@@ -199,7 +203,10 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
}

spin_lock_irqsave(&vblk->vq_lock, flags);
+ record_stack = current;
err = __virtblk_add_req(vblk->vq, vbr, vbr->sg, num);
+ record_stack = NULL;
+ printk("virtio_blk: stack used = %lu\n", stack_bottom - stack_top);
if (err) {
virtqueue_kick(vblk->vq);
spin_unlock_irqrestore(&vblk->vq_lock, flags);
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1e443629f76d..39158d6079a9 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -113,6 +113,14 @@ static inline struct scatterlist *sg_next_arr(struct scatterlist *sg,
return sg + 1;
}

+extern struct task_struct *record_stack;
+struct task_struct *record_stack;
+EXPORT_SYMBOL(record_stack);
+
+extern unsigned long stack_top;
+unsigned long stack_top;
+EXPORT_SYMBOL(stack_top);
+
/* Set up an indirect table of descriptors and add it to the queue. */
static inline int vring_add_indirect(struct vring_virtqueue *vq,
struct scatterlist *sgs[],
@@ -141,6 +149,9 @@ static inline int vring_add_indirect(struct vring_virtqueue *vq,
if (!desc)
return -ENOMEM;

+ if (record_stack == current)
+ stack_top = (unsigned long)&desc;
+
/* Transfer entries from the sg lists into the indirect page */
i = 0;
for (n = 0; n < out_sgs; n++) {