Documentation: Add managed interrupts

I stumbled upon "isolcpus=managed_irq" which is the last piece which
can only be handled by isolcpus= and has no runtime knob. I knew roughly
what managed interrupts should do but I lacked some details how it is
used and what the managed_irq sub parameter means in practise.

This documents what we have as of today and how it works. I added some
examples how the parameter affects the configuration. Did I miss
something?

Given that the spreading as computed group_cpus_evenly() does not take
the mask of isolated CPUs into account I'm not sure how relevant the
managed_irq argument is. The virtio_scsi driver has no way to limit the
interrupts and I don't see this for the nvme. Even if the number of
queues can be reduced to two (as in the example) it is still spread
evenly in the system instead and the isolated CPUs are not taken into
account.
To make this worse, you can even argue further whether or not the
application on the isolated CPU wants to receive the interrupt directly
or would prefer not to.

Given all this, I am not sure if it makes sense to add 'io_queue' to the
mix or if it could be incorporated into 'managed_irq'.

One more point: Given that isolcpus= is marked deprecated as of commit
   b0d40d2b22fe4 ("sched/isolation: Document isolcpus= boot parameter flags, mark it deprecated")

and the 'managed_irq' is evaluated at device's probe time it would
require additional callbacks to re-evaluate the situation. Probably for
'io_queue', too. Does is make sense or should we simply drop the
"deprecation" notice and allowing using it long term?
Dynamic partitions work with cpusets, there this (managed_irq)
limitation but is it really? And if static partition is the use case why
bother.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Acked-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260401110232.ET5RxZfl@linutronix.de>

0 files changed, 0 insertions, 0 deletions