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SubjectRe: [RFC][PATCH] perf: Remove superfluous perf_pmu_disable calls
On Wed, Jul 06, 2016 at 04:09:31PM +0200, Peter Zijlstra wrote:
>
> Hai,
>
> So pmu::add() and pmu::del() are guaranteed to be called with ctx->lock
> held, which implies that local IRQs are disabled.
>
> Furthermore, it is also guaranteed that perf_pmu_disable() is already
> called when we call these methods.

It's probably worth noting that the latter is true since commit
443772776c69ac92 ("perf: Disable all pmus on unthrottling and
rescheduling"), circa December 2013.

Much of this code was likely written before that, or cargo-culted from
existing code which was.

> The following patch removes all perf_pmu_{dis,en}able() calls (and
> local_irq_disable() where encountered) from pmu::{add,del}()
> implementations.
>
> pmu::{start,stop}() are a little bit tricky, since we can call them from
> the PMI handler, but we do guarantee local IRQs are disabled. PPC in
> particular seems to need perf_pmu_{dis,en}able() there to actually
> reprogram things, this is safe for them since they don't have actual
> NMIs I suppose.
>
> ---
> arch/alpha/kernel/perf_event.c | 22 ----------------------
> arch/mips/kernel/perf_event_mipsxx.c | 3 ---
> arch/powerpc/perf/core-book3s.c | 16 +++-------------
> arch/powerpc/perf/core-fsl-emb.c | 7 +++----
> arch/s390/kernel/perf_cpum_sf.c | 4 ----
> arch/sh/kernel/perf_event.c | 3 ---
> b/arch/blackfin/kernel/perf_event.c | 3 ---
> b/arch/metag/kernel/perf/perf_event.c | 3 ---
> b/arch/sparc/kernel/perf_event.c | 9 ---------
> drivers/bus/arm-cci.c | 3 ---
> drivers/perf/arm_pmu.c | 3 ---
> kernel/events/core.c | 6 ++++++
> 12 files changed, 12 insertions(+), 70 deletions(-)

[...]

> --- a/drivers/bus/arm-cci.c
> +++ b/drivers/bus/arm-cci.c
> @@ -1230,8 +1230,6 @@ static int cci_pmu_add(struct perf_event
> int idx;
> int err = 0;
>
> - perf_pmu_disable(event->pmu);
> -
> /* If we don't have a space for the counter then finish early. */
> idx = pmu_get_event_idx(hw_events, event);
> if (idx < 0) {
> @@ -1250,7 +1248,6 @@ static int cci_pmu_add(struct perf_event
> perf_event_update_userpage(event);
>
> out:
> - perf_pmu_enable(event->pmu);
> return err;
> }
>
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -240,8 +240,6 @@ armpmu_add(struct perf_event *event, int
> if (!cpumask_test_cpu(smp_processor_id(), &armpmu->supported_cpus))
> return -ENOENT;
>
> - perf_pmu_disable(event->pmu);
> -
> /* If we don't have a space for the counter then finish early. */
> idx = armpmu->get_event_idx(hw_events, event);
> if (idx < 0) {
> @@ -265,7 +263,6 @@ armpmu_add(struct perf_event *event, int
> perf_event_update_userpage(event);
>
> out:
> - perf_pmu_enable(event->pmu);
> return err;
> }

These both look right to me. The only caller of either is
event_sched_in(), which handles the disable/enable itself. That uses
event->pmu, so the heterogeneous case doesn't throw a spanner in the
works here.

> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8597,6 +8597,12 @@ int perf_pmu_register(struct pmu *pmu, c
> }
> }
>
> + /*
> + * Software events cannot have pmu_{en,dis}able() calls because that
> + * would make it 'hard' to put them in groups with hardware events.
> + */
> + WARN_ON_ONCE(pmu->task_ctx_nr == perf_sw_event && pmu->pmu_enable);
> +
> if (!pmu->pmu_enable) {
> pmu->pmu_enable = perf_pmu_nop_void;
> pmu->pmu_disable = perf_pmu_nop_void;
>

Looks sensible to me.

For the parts above:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.

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