netdev - RE: [PATCH net-next] virtio_net: Add TX stop and wake counters

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Message-ID:
 <CH0PR12MB85807A30B1F42A4E354516A1C9502@CH0PR12MB8580.namprd12.prod.outlook.com>
Date: Tue, 20 Feb 2024 18:02:46 +0000
From: Dan Jurgens <danielj@...dia.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
CC: Jason Wang <jasowang@...hat.com>, Jakub Kicinski <kuba@...nel.org>, Jason
 Xing <kerneljasonxing@...il.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "xuanzhuo@...ux.alibaba.com"
	<xuanzhuo@...ux.alibaba.com>, "virtualization@...ts.linux.dev"
	<virtualization@...ts.linux.dev>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"abeni@...hat.com" <abeni@...hat.com>, Parav Pandit <parav@...dia.com>
Subject: RE: [PATCH net-next] virtio_net: Add TX stop and wake counters

> From: Daniel Jurgens <danielj@...dia.com>
> Sent: Wednesday, February 7, 2024 2:59 PM
> To: Michael S. Tsirkin <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>; Jakub Kicinski
> <kuba@...nel.org>; Jason Xing <kerneljasonxing@...il.com>;
> netdev@...r.kernel.org; xuanzhuo@...ux.alibaba.com;
> virtualization@...ts.linux.dev; davem@...emloft.net;
> edumazet@...gle.com; abeni@...hat.com; Parav Pandit
> <parav@...dia.com>
> Subject: RE: [PATCH net-next] virtio_net: Add TX stop and wake counters
> 
> 
> > From: Michael S. Tsirkin <mst@...hat.com>
> > Sent: Wednesday, February 7, 2024 2:19 PM
> > To: Daniel Jurgens <danielj@...dia.com>
> > Subject: Re: [PATCH net-next] virtio_net: Add TX stop and wake
> > counters
> >
> > On Wed, Feb 07, 2024 at 07:38:16PM +0000, Daniel Jurgens wrote:
> > > > From: Michael S. Tsirkin <mst@...hat.com>
> > > > Sent: Sunday, February 4, 2024 6:40 AM
> > > > To: Jason Wang <jasowang@...hat.com>
> > > > Cc: Jakub Kicinski <kuba@...nel.org>; Jason Xing
> > > > <kerneljasonxing@...il.com>; Daniel Jurgens <danielj@...dia.com>;
> > > > netdev@...r.kernel.org; xuanzhuo@...ux.alibaba.com;
> > > > virtualization@...ts.linux.dev; davem@...emloft.net;
> > > > edumazet@...gle.com; abeni@...hat.com; Parav Pandit
> > > > <parav@...dia.com>
> > > > Subject: Re: [PATCH net-next] virtio_net: Add TX stop and wake
> > > > counters
> > > >
> > > > On Sun, Feb 04, 2024 at 09:20:18AM +0800, Jason Wang wrote:
> > > > > On Sat, Feb 3, 2024 at 12:01 AM Jakub Kicinski <kuba@...nel.org>
> > wrote:
> > > > > >
> > > > > > On Fri, 2 Feb 2024 14:52:59 +0800 Jason Xing wrote:
> > > > > > > > Can you say more? I'm curious what's your use case.
> > > > > > >
> > > > > > > I'm not working at Nvidia, so my point of view may differ
> > > > > > > from
> > theirs.
> > > > > > > From what I can tell is that those two counters help me
> > > > > > > narrow down the range if I have to diagnose/debug some issues.
> > > > > >
> > > > > > right, i'm asking to collect useful debugging tricks, nothing
> > > > > > against the patch itself :)
> > > > > >
> > > > > > > 1) I sometimes notice that if some irq is held too long
> > > > > > > (say, one simple case: output of printk printed to the
> > > > > > > console), those two counters can reflect the issue.
> > > > > > > 2) Similarly in virtio net, recently I traced such counters
> > > > > > > the current kernel does not have and it turned out that one
> > > > > > > of the output queues in the backend behaves badly.
> > > > > > > ...
> > > > > > >
> > > > > > > Stop/wake queue counters may not show directly the root
> > > > > > > cause of the issue, but help us 'guess' to some extent.
> > > > > >
> > > > > > I'm surprised you say you can detect stall-related issues with this.
> > > > > > I guess virtio doesn't have BQL support, which makes it special.
> > > > >
> > > > > Yes, virtio-net has a legacy orphan mode, this is something that
> > > > > needs to be dropped in the future. This would make BQL much more
> > > > > easier to be implemented.
> > > >
> > > >
> > > > It's not that we can't implement BQL, it's that it does not seem
> > > > to be benefitial - has been discussed many times.
> > > >
> > > > > > Normal HW drivers with BQL almost never stop the queue by
> > themselves.
> > > > > > I mean - if they do, and BQL is active, then the system is
> > > > > > probably misconfigured (queue is too short). This is what we
> > > > > > use at Meta to detect stalls in drivers with BQL:
> > > > > >
> > > > > > https://lore.kernel.org/all/20240131102150.728960-3-leitao@deb
> > > > > > ia
> > > > > > n.or
> > > > > > g/
> > > > > >
> > > > > > Daniel, I think this may be a good enough excuse to add
> > > > > > per-queue stats to the netdev genl family, if you're up for
> > > > > > that. LMK if you want more info, otherwise I guess ethtool -S
> > > > > > is fine
> > for now.
> > > > > >
> > > > >
> > > > > Thanks
> > >
> > > Michael,
> > > 	Are you OK with this patch? Unless I missed it I didn't see a
> > > response
> > from you in our conversation the day I sent it.
> > >
> >
> > I thought what is proposed is adding some support for these stats to core?
> > Did I misunderstood?
> >
> 
> That's a much bigger change and going that route I think still need to count
> them at the driver level. I said I could potentially take that on as a background
> project. But would prefer to go with ethtool -S for now.

Michael, are you a NACK on this? Jakub seemed OK with it, Jason also thinks it's useful, and it's low risk. 

> 
> > --
> > MST