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

On Thu, Feb 04, 2021 at 01:47:38PM -0500, Nitesh Narayan Lal wrote:

On 2/4/21 1:15 PM, Marcelo Tosatti wrote:

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On Thu, Jan 28, 2021 at 09:01:37PM +0100, Thomas Gleixner wrote:

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On Thu, Jan 28 2021 at 13:59, Marcelo Tosatti wrote:

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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 :)

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So housekeeping_cpumask has multiple meanings. In this case:

...

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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.

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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.

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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.