Re: [PATCH v5 3/7] arm64: perf: Remove PMU locking

On Fri, Jun 19, 2020 at 01:29:59AM -0700, Stephen Boyd wrote:

Quoting Alexandru Elisei (2020-06-18 03:51:31)

quoted

The armv8pmu_{start,stop}() functions are called from the irq handler, so we're
safe from preemption in this case. They are also called via
pmu->pmu_{enable,disable} callbacks, and I didn't find an explicit contract
regarding preemption in include/linux/perf_event.h. I've checked the other call
sites, and I didn't find any instances where they are called with preemption
enabled, which makes sense as we don't want to disable the PMU on a another CPU by
accident.

If they're all callbacks then it's overkill to add this. Presumably it
is better to enforce this wherever the callbacks are called from so as
to not litter the callee with random cant_sleep() calls. Probably best
to ignore my suggestion.

quoted

I would be inclined to add cant_sleep() calls to armv8pmu_{start,stop}(). In the
previous iteration, there were WARN_ONs in these functions, and Will said [1] they
can be removed because they are per-CPU operations. Will, what do you think about
adding the lockdep assertions?

[1] https://www.spinics.net/lists/arm-kernel/msg745161.html

If I read it correctly Will is saying the same thing in that thread.

Right, in the cases where perf core already relies on things not being
preemptible, we don't need to add extra checks.

Will

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