linux-kernel - RE: [PATCH v2] net: netvsc: Update default VMBus channels

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Message-ID:
 <CH2PPF910B3338D030358D7ED43B94F261ECA802@CH2PPF910B3338D.namprd21.prod.outlook.com>
Date: Thu, 15 Aug 2024 14:50:58 +0000
From: Haiyang Zhang <haiyangz@...rosoft.com>
To: mhklinux <mhklinux@...look.com>, Erni Sri Satya Vennela
	<ernis@...ux.microsoft.com>, KY Srinivasan <kys@...rosoft.com>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	"davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: Erni Sri Satya Vennela <ernis@...rosoft.com>
Subject: RE: [PATCH v2] net: netvsc: Update default VMBus channels



> -----Original Message-----
> From: Michael Kelley <mhklinux@...look.com>
> Sent: Wednesday, August 14, 2024 7:57 PM
> To: Erni Sri Satya Vennela <ernis@...ux.microsoft.com>; KY Srinivasan
> <kys@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>;
> wei.liu@...nel.org; Dexuan Cui <decui@...rosoft.com>; davem@...emloft.net;
> edumazet@...gle.com; kuba@...nel.org; pabeni@...hat.com; linux-
> hyperv@...r.kernel.org; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Cc: Erni Sri Satya Vennela <ernis@...rosoft.com>
> Subject: RE: [PATCH v2] net: netvsc: Update default VMBus channels
> 
> From: Erni Sri Satya Vennela <ernis@...ux.microsoft.com> Sent: Wednesday,
> August 14, 2024 9:59 AM
> >
> > Change VMBus channels macro (VRSS_CHANNEL_DEFAULT) in
> > Linux netvsc from 8 to 16 to align with Azure Windows VM
> > and improve networking throughput.
> >
> > For VMs having less than 16 vCPUS, the channels depend
> > on number of vCPUs. Between 16 to 32 vCPUs, the channels
> > default to VRSS_CHANNEL_DEFAULT. For greater than 32 vCPUs,
> > set the channels to number of physical cores / 2 as a way
> > to optimize CPU resource utilization and scale for high-end
> > processors with many cores.
> > Maximum number of channels are by default set to 64.
> 
> Where in the code is this enforced? It's not part of this patch. It
> might be in rndis_set_subchannel(), where a value larger than
> 64 could be sent to the Hyper-V host, expecting that the Hyper-V
> host will limit it to 64. But netvsc driver code is declaring an array
> of size VRSS_CHANNEL_MAX, and there's nothing that guarantees
> that Hyper-V will always limit the channel count to 64. But maybe
> the netvsc driver enforces the limit of VRSS_CHANNEL_MAX in a
> place that I didn't immediately see in a quick look at the code.

Yes, netvsc driver limits the num_chn to be <=64:

#define VRSS_CHANNEL_MAX 64

        /* This guarantees that num_possible_rss_qs <= num_online_cpus */
        num_possible_rss_qs = min_t(u32, num_online_cpus(),
                                    rsscap.num_recv_que);

        net_device->max_chn = min_t(u32, VRSS_CHANNEL_MAX, num_possible_rss_qs);

        /* We will use the given number of channels if available. */
        net_device->num_chn = min(net_device->max_chn, device_info->num_chn);


Thanks,
- Haiyang