Re: [PATCH v2] vsock: replace virtio_vsock_pkt with sk_buff

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Sat, Oct 15, 2022 at 12:49:59PM -0700, Cong Wang wrote:
> On Mon, Oct 03, 2022 at 12:11:39AM +0000, Bobby Eshleman wrote:
> > On Thu, Oct 06, 2022 at 09:34:10AM +0200, Stefano Garzarella wrote:
> > > On Thu, Oct 06, 2022 at 03:08:12AM -0400, Michael S. Tsirkin wrote:
> > > > On Wed, Oct 05, 2022 at 06:19:44PM -0700, Bobby Eshleman wrote:
> > > > > This patch replaces the struct virtio_vsock_pkt with struct sk_buff.
> > > > > 
> > > > > Using sk_buff in vsock benefits it by a) allowing vsock to be extended
> > > > > for socket-related features like sockmap, b) vsock may in the future
> > > > > use other sk_buff-dependent kernel capabilities, and c) vsock shares
> > > > > commonality with other socket types.
> > > > > 
> > > > > This patch is taken from the original series found here:
> > > > > https://lore.kernel.org/all/cover.1660362668.git.bobby.eshleman@bytedance.com/
> > > > > 
> > > > > Small-sized packet throughput improved by ~5% (from 18.53 Mb/s to 19.51
> > > > > Mb/s). Tested using uperf, 16B payloads, 64 threads, 100s, averaged from
> > > > > 10 test runs (n=10). This improvement is likely due to packet merging.
> > > > > 
> > > > > Large-sized packet throughput decreases ~9% (from 27.25 Gb/s to 25.04
> > > > > Gb/s). Tested using uperf, 64KB payloads, 64 threads, 100s, averaged
> > > > > from 10 test runs (n=10).
> > > > > 
> > > > > Medium-sized packet throughput decreases ~5% (from 4.0 Gb/s to 3.81
> > > > > Gb/s). Tested using uperf, 4k to 8k payload sizes picked randomly
> > > > > according to normal distribution, 64 threads, 100s, averaged from 10
> > > > > test runs (n=10).
> > > > 
> > > > It is surprizing to me that the original vsock code managed to outperform
> > > > the new one, given that to my knowledge we did not focus on optimizing it.
> > > 
> > > Yeah mee to.
> > > 
> > 
> > Indeed.
> > 
> > > From this numbers maybe the allocation cost has been reduced as it performs
> > > better with small packets. But with medium to large packets we perform
> > > worse, perhaps because previously we were allocating a contiguous buffer up
> > > to 64k?
> > > Instead alloc_skb() could allocate non-contiguous pages ? (which would solve
> > > the problems we saw a few days ago)
> > > 
> > 
> > I think this would be the case with alloc_skb_with_frags(), but
> > internally alloc_skb() uses kmalloc() for the payload and sk_buff_head
> > slab allocations for the sk_buff itself (all the more confusing to me,
> > as the prior allocator also uses two separate allocations per packet).
> 
> I think it is related to your implementation of
> virtio_transport_add_to_queue(), where you introduced much more
> complicated logic than before:
> 
> -	spin_lock_bh(&vsock->send_pkt_list_lock);
> -	list_add_tail(&pkt->list, &vsock->send_pkt_list);
> -	spin_unlock_bh(&vsock->send_pkt_list_lock);
> -
> +	virtio_transport_add_to_queue(&vsock->send_pkt_queue, skb);
> 

I wish it were that easy, but I included this change because it actually
boosts performance.

For 16B payloads, this change improves throughput from 16 Mb/s to 20Mb/s
in my test harness, and reduces the memory usage of the kmalloc-512 and
skbuff_head_cache slab caches by ~50MB at cache size peak (total slab
cache size from ~540MB to ~390MB), but typically (not at peak) the slab
cache size when this merging is used keeps the memory slab caches closer
to ~150MB smaller. Tests done using uperf.

For payloads greater than GOOD_COPY_LEN I don't see any any notable
difference between the skb code with merging and the skb code without
merging in terms of throughput. I assume this is because the skb->len
comparison with GOOD_COPY_LEN should short circuit the expression and
the other memory operations should not occur.

> A simple list_add_tail() is definitely faster than your
> virtio_transport_skbs_can_merge() check. So, why do you have to merge
> skb while we don't merge virtio_vsock_pkt?
> 

sk_buff is over twice the size of virtio_vsock_pkt (96B vs 232B). It
seems wise to reduce the footprint in other ways to try and keep it
comparable.

> _If_ you are trying to mimic TCP, I think you are doing it wrong, it can
> be much more efficient if you could do the merge in sendmsg() before skb
> is even allocated, see tcp_sendmsg_locked().

I'll definitely give it a read, merging before allocating an skb sounds
better.

Best,
Bobby