Re: [PATCH 4/4] perf/x86/intel: Throttle PEBS events only from pmi

[Peter Zijlstra]

On Wed, Dec 28, 2016 at 02:31:06PM +0100, Jiri Olsa wrote:
> This patch fixes following WARNING:
> 
>   WARNING: CPU: 15 PID: 15768 at arch/x86/events/core.c:1256 x86_pmu_start+0x1b3/0x1c0
>   ...
>   Call Trace:
>    <IRQ>
>    dump_stack+0x86/0xc3
>    __warn+0xcb/0xf0
>    warn_slowpath_null+0x1d/0x20
>    x86_pmu_start+0x1b3/0x1c0
>    perf_event_task_tick+0x342/0x3f0
>    scheduler_tick+0x75/0xd0
>    update_process_times+0x47/0x60
>    tick_sched_handle.isra.19+0x25/0x60
>    tick_sched_timer+0x3d/0x70
>    __hrtimer_run_queues+0xfb/0x510
>    hrtimer_interrupt+0x9d/0x1a0
>    local_apic_timer_interrupt+0x38/0x60
>    smp_trace_apic_timer_interrupt+0x56/0x25a
>    trace_apic_timer_interrupt+0x9d/0xb0
>    ...
> 
> which happens AFAICS under following conditions:
> (we have PEBS events configured)
> 
>   - x86_pmu_enable reconfigures counters and calls:
>        - x86_pmu_stop on PEBS event
>        - x86_pmu_stop drains the PEBS buffer, crosses
>          the throttle limit and sets:
>            event->hw.interrupts = MAX_INTERRUPTS
>        - following x86_pmu_start call starts the event
>   - perf_event_task_tick is triggered
>     - perf_adjust_freq_unthr_context sees event with
>       MAX_INTERRUPTS set and calls x86_pmu_start on already
>       started event, which triggers the warning
> 
> My first attempt to fix this was to unthrottle the event
> before starting it in x86_pmu_enable. But I think that
> omitting the throttling completely when we are not in the
> PMI is better.

Hurm,.. good catch that. Makes a bit of a mess of things though. Then
again, I cannot quickly think of something better either :/