On 2021/2/10 下午5:14, Michael S. Tsirkin wrote:
On Tue, Feb 09, 2021 at 10:00:22AM -0800, Wei Wang wrote:
On Tue, Feb 9, 2021 at 6:58 AM Willem de Bruijn
<willemdebruijn.ker...@gmail.com> wrote:
I have no preference. Just curious, especially if it complicates the patch.
My understanding is that. It's probably ok for net. But we probably need
to document the assumptions to make sure it was not abused in other drivers.
Introduce new parameters for find_vqs() can help to eliminate the subtle
stuffs but I agree it looks like a overkill.
(Btw, I forget the numbers but wonder how much difference if we simple
remove the free_old_xmits() from the rx NAPI path?)
The committed patchset did not record those numbers, but I found them
in an earlier iteration:
[PATCH net-next 0/3] virtio-net tx napi
https://lists.openwall.net/netdev/2017/04/02/55
It did seem to significantly reduce compute cycles ("Gcyc") at the
time. For instance:
TCP_RR Latency (us):
1x:
p50 24 24 21
p99 27 27 27
Gcycles 299 432 308
I'm concerned that removing it now may cause a regression report in a
few months. That is higher risk than the spurious interrupt warning
that was only reported after years of use.
Right.
So if Michael is fine with this approach, I'm ok with it. But we
probably need to a TODO to invent the interrupt handlers that can be
used for more than one virtqueues. When MSI-X is enabled, the interrupt
handler (vring_interrup()) assumes the interrupt is used by a single
virtqueue.
Thanks.
The approach to schedule tx-napi from virtnet_poll_cleantx instead of
cleaning directly in this rx-napi function was not effective at
suppressing the warning, I understand.
Correct. I tried the approach to schedule tx napi instead of directly
do free_old_xmit_skbs() in virtnet_poll_cleantx(). But the warning
still happens.
Two questions here: is the device using packed or split vqs?
And is event index enabled?
I think one issue is that at the moment with split and event index we
don't actually disable events at all.
Do we really have a way to disable that? (We don't have a flag like
packed virtqueue)
Or you mean the trick [1] when I post tx interrupt RFC?
Thanks
[1] https://lkml.org/lkml/2015/2/9/113
static void virtqueue_disable_cb_split(struct virtqueue *_vq)
{
struct vring_virtqueue *vq = to_vvq(_vq);
if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
if (!vq->event)
vq->split.vring.avail->flags =
cpu_to_virtio16(_vq->vdev,
vq->split.avail_flags_shadow);
}
}
Can you try your napi patch + disable event index?
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