Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs

[Marcelo Tosatti]

On Thu, Feb 04, 2021 at 01:47:38PM -0500, Nitesh Narayan Lal wrote:
> 
> On 2/4/21 1:15 PM, Marcelo Tosatti wrote:
> > On Thu, Jan 28, 2021 at 09:01:37PM +0100, Thomas Gleixner wrote:
> >> On Thu, Jan 28 2021 at 13:59, Marcelo Tosatti wrote:
> >>>> The whole pile wants to be reverted. It's simply broken in several ways.
> >>> I was asking for your comments on interaction with CPU hotplug :-)
> >> Which I answered in an seperate mail :)
> >>
> >>> So housekeeping_cpumask has multiple meanings. In this case:
> >> ...
> >>
> >>> So as long as the meaning of the flags are respected, seems
> >>> alright.
> >> Yes. Stuff like the managed interrupts preference for housekeeping CPUs
> >> when a affinity mask spawns housekeeping and isolated is perfectly
> >> fine. It's well thought out and has no limitations.
> >>
> >>> Nitesh, is there anything preventing this from being fixed
> >>> in userspace ? (as Thomas suggested previously).
> >> Everything with is not managed can be steered by user space.
> > Yes, but it seems to be racy (that is, there is a window where the 
> > interrupt can be delivered to an isolated CPU).
> >
> > ethtool ->
> > xgbe_set_channels ->
> > xgbe_full_restart_dev ->
> > xgbe_alloc_memory ->
> > xgbe_alloc_channels ->
> > cpumask_local_spread
> >
> > Also ifconfig eth0 down / ifconfig eth0 up leads
> > to cpumask_spread_local.
> 
> There's always that possibility.

Then there is a window where isolation can be broken.

> We have to ensure that we move the IRQs by a tuned daemon or some other
> userspace script every time there is a net-dev change (eg. device comes up,
> creates VFs, etc).

Again, race window open can result in interrupt to isolated CPU.

> > How about adding a new flag for isolcpus instead?
> >
> 
> Do you mean a flag based on which we can switch the affinity mask to
> housekeeping for all the devices at the time of IRQ distribution?

Yes a new flag for isolcpus. HK_FLAG_IRQ_SPREAD or some better name.