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

[Jakub Kicinski <kuba@kernel.org>]

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? 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"?