netdev - Re: [PATCH net-next 1/1] ice: Improve MSI-X vector enablement fallback logic

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Date:   Wed, 20 Jan 2021 02:13:36 +0000
From:   "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
To:     "kuba@...nel.org" <kuba@...nel.org>
CC:     "Brelinski, TonyX" <tonyx.brelinski@...el.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Creeley, Brett" <brett.creeley@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/1] ice: Improve MSI-X vector enablement
 fallback logic

On Tue, 2021-01-19 at 16:41 -0800, Jakub Kicinski wrote:
> On Wed, 20 Jan 2021 00:12:26 +0000 Venkataramanan, Anirudh wrote:
> > > > Attempt [0]: Enable the best-case scenario MSI-X vectors.
> > > > 
> > > > Attempt [1]: Enable MSI-X vectors with the number of pf-  
> > > > > num_lan_msix  
> > > > reduced by a factor of 2 from the previous attempt (i.e.
> > > > num_online_cpus() / 2).
> > > > 
> > > > Attempt [2]: Same as attempt [1], except reduce by a factor of
> > > > 4.
> > > > 
> > > > Attempt [3]: Enable the bare-minimum MSI-X vectors.
> > > > 
> > > > Also, if the adjusted_base_msix ever hits the minimum required
> > > > for
> > > > LAN,
> > > > then just set the needed MSI-X for that feature to the minimum
> > > > (similar to attempt [3]).  
> > > 
> > > I don't really get why you switch to this manual "exponential
> > > back-
> > > off"
> > > rather than continuing to use pci_enable_msix_range(), but fixing
> > > the
> > > capping to ICE_MIN_LAN_VECS.  
> > 
> > As per the current logic, if the driver does not get the number of
> > MSI-
> > X vectors it needs, it will immediately drop to "Do I have at least
> > two
> > (ICE_MIN_LAN_VECS) MSI-X vectors?". If yes, the driver will enable
> > a
> > single Tx/Rx traffic queue pair, bound to one of the two MSI-X
> > vectors.
> > 
> > This is a bit of an all-or-nothing type approach. There's a mid-
> > ground
> > that can allow more queues to be enabled (ex. driver asked for 300
> > vectors, but got 68 vectors, so enabled 64 data queues) and this
> > patch
> > implements the mid-ground logic. 
> > 
> > This mid-ground logic can also be implemented based on the return
> > value
> > of pci_enable_msix_range() but IMHO the implementation in this
> > patch
> > using pci_enable_msix_exact is better because it's always only
> > enabling/reserving as many MSI-X vectors as required, not more, not
> > less.
> 
> What do you mean by "required" in the last sentence? 

.. as "required" in that particular iteration of the loop.

> The driver
> requests num_online_cpus()-worth of IRQs, so it must work with any
> number of IRQs. Why is num_cpus() / 1,2,4,8 "required"?

Let me back up a bit here. 

Ultimately, the issue we are trying to solve here is "what happens when
the driver doesn't get as many MSI-X vectors as it needs, and how it's
interpreted by the end user"

Let's say there are these two systems, each with 256 cores but the
response to pci_enable_msix_range() is different:

System 1: 256 cores, pci_enable_msix_range returns 75 vectors
System 2: 256 cores, pci_enable_msix_range returns 220 vectors 

In this case, the number of queues the user would see enabled on each
of these systems would be very different (73 on system 1 and 218 on
system 2). This variabilty makes it difficult to define what the
expected behavior should be, because it's not exactly obvious to the
user how many free MSI-X vectors a given system has. Instead, if the
driver reduced it's demand for vectors in a well defined manner
(num_cpus() / 1,2,4,8), the user visible difference between the two
systems wouldn't be so drastic.

If this is plain wrong or if there's a preferred approach, I'd be happy
to discuss further.

Ani