Re: [RFC] per-CPU usage in perf core-book3s

[maddy <maddy@linux.ibm.com>]

On 1/27/20 8:36 PM, Sebastian Andrzej Siewior wrote:
> I've been looking at usage of per-CPU variable cpu_hw_events in
> arch/powerpc/perf/core-book3s.c.
>
> power_pmu_enable() and power_pmu_disable() (pmu::pmu_enable() and
> pmu::pmu_disable()) are accessing the variable and the callbacks are
> invoked always with disabled interrupts.
>
> power_pmu_event_init() (pmu::event_init()) is invoked from preemptible
> context and uses get_cpu_var() to obtain a stable pointer (by disabling
> preemption).
>
> pmu::pmu_enable() and pmu::pmu_disable() can be invoked via a hrtimer
> (perf_mux_hrtimer_handler()) and it invokes pmu::pmu_enable() and
> pmu::pmu_disable() as part of the callback.
>
> Is there anything that prevents the timer callback to interrupt
> pmu::event_init() while it is accessing per-CPU data?

Sorry for the delayed response.

Yes, currently we dont have anything that prevents the timer
callback to interrupt pmu::event_init. Nice catch. Thanks for
pointing this out.

Looking at the code, per-cpu variable access are made to
check for constraints and for Branch Stack (BHRB). So could
wrap this block of  pmu::event_init with local_irq_save/restore.
Will send a patch to fix it.


Maddy

>
> Sebastian