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SubjectShould irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu?
Hi,

I'm reworking Xen's interrupt handling to isolate it a bit from the
workings of the apic-based code, as Eric suggested a while back.

As I've mentioned before, Xen represents interrupts as event channels.
There are two major classes of event channels: per-cpu and, erm, not
percpu. Per-cpu event channels are for things like timers and IPI
function calls which are inherently per-cpu; it's meaningless to
consider, for example, migrating them from cpu to cpu. I guess they're
analogous to the the local apic vectors.

(Non-percpu event channels can be bound to a particular cpu, and rebound
at will; I'm not worried about them here.)

Previously I allocated an irq per percpu event channel per cpu. This
was pretty wasteful, since I need about 5-6 of them per cpu, so the
number of interrupts increases quite quickly as cpus does. There's no
deep problem with that, but it gets fairly ugly in /proc/interrupts, and
there's some tricky corners to manage in suspend/resume.

This time around I'm allocating a single irq for each percpu interrupt
source (so one for timers, one for IPI, etc), and mapping each per-cpu
event channel to each. But I'm wondering what the correct behaviour of
irq_chip->mask/unmask should be in this case. Each event channel is
individually maskable, so when ->mask gets called, I can either mask all
the event channels associated with that irq, or just the one for this
cpu. The latter makes most sense for me, but I don't quite understand
the irq infrastructure enough to know if it will have bad effects globally.

When I request the irq, I pass IRQF_PERCPU in the flags, but aside from
preventing migration, this only seems to have an effect on __do_IRQ(),
which looks like a legacy path anyway. It seems to me that by setting
it that I'm giving the interrupt subsystem fair warning that ->mask() is
only going to disable the interrupt on this cpu.

Are there any other ill-effects of sharing an irq across all cpus like
this? I guess there's some risk of contention on the irq_desc lock.

Thanks,
J


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