Messages in this thread | | | Date | Tue, 23 Sep 2008 13:02:44 -0700 | From | Jeremy Fitzhardinge <> | Subject | Should irq_chip->mask disable percpu interrupts to all cpus, or just to this cpu? |
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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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