Re: [RFC PATCH] perf/smmuv3: Fix shared interrupt handling
From: Robin Murphy
Date: Wed Jun 24 2020 - 09:08:36 EST
On 2020-06-24 13:50, Will Deacon wrote:
On Wed, Jun 24, 2020 at 12:48:14PM +0100, Robin Murphy wrote:
On 2020-04-08 17:49, Robin Murphy wrote:
IRQF_SHARED is dangerous, since it allows other agents to retarget the
IRQ's affinity without migrating PMU contexts to match, breaking the way
in which perf manages mutual exclusion for accessing events. Although
this means it's not realistically possible to support PMU IRQs being
shared with other drivers, we *can* handle sharing between multiple PMU
instances with some explicit affinity bookkeeping and manual interrupt
multiplexing.
RCU helps us handle interrupts efficiently without having to worry about
fine-grained locking for relatively-theoretical race conditions with the
probe/remove/CPU hotplug slow paths. The resulting machinery ends up
looking largely generic, so it should be feasible to factor out with a
"system PMU" base class for similar multi-instance drivers.
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
RFC because I don't have the means to test it, and if the general
approach passes muster then I'd want to tackle the aforementioned
factoring-out before merging anything anyway.
Any comments on whether it's worth pursuing this?
Sorry, I don't really get the problem that it's solving. Is there a crash
log somewhere I can look at? If all the users of the IRQ are managed by
this driver, why is IRQF_SHARED dangerous?
Because as-is, multiple PMU instances may make different choices about
which CPU they associate with, change the shared IRQ affinity behind
each others' backs, and break the "IRQ handler runs on event->cpu"
assumption that perf core relies on for correctness. I'm not sure how
likely it would be to actually crash rather than just lead to subtle
nastiness, but wither way it's not good, and since people seem to be
tempted to wire up system PMU instances this way we could do with a
general approach for dealing with it.
Robin.