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SubjectRe: [Patch][perf] Invoke __perf_event_disable without an IPI
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On Mon, 2012-06-11 at 11:32 +0530, K.Prasad wrote:

> While debugging a warning message on PowerPC while using hardware
> breakpoints, it was discovered that when perf_event_disable is invoked
> through hw_breakpoint_handler function with interrupts disabled, a
> subsequent IPI in the code path would trigger a WARN_ON_ONCE message in
> smp_call_function_single function.
>
> This patch avoids the use of an IPI to invoke __perf_event_disable when
> it is safe to do so i.e. when the process associated with the perf-event
> would run on the current CPU and interrupts are disabled on the system.
> Since interrupts are always disabled before hw_breakpoint_handler in
> PowerPC, the warning message will no longer be seen.
>
> Reported-by: Edjunior Barbosa Machado <emachado@linux.vnet.ibm.com>
> Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
> ---
> kernel/events/core.c | 12 ++++++++++++
> 1 files changed, 12 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index fd126f8..0e2c1eb 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1302,6 +1302,7 @@ static int __perf_event_disable(void *info)
> */
> void perf_event_disable(struct perf_event *event)
> {
> + int ret;
> struct perf_event_context *ctx = event->ctx;
> struct task_struct *task = ctx->task;
>
> @@ -1314,6 +1315,17 @@ void perf_event_disable(struct perf_event *event)
> }
>
> retry:
> + /*
> + * perf_event_disable may be called when interrupts are disabled.
> + * For e.g. hw_breakpoint_handler exception in PowerPC. Hence using
> + * IPIs to invoke __perf_event_disable is not always suitable. When
> + * possible invoke __perf_event_disable directly.
> + */
> + if ((task_cpu(task) == smp_processor_id()) && irqs_disabled()) {

Urgh..

So what's the callchain for the ppc->hw_bp->perf that triggers this?


> + ret = __perf_event_disable(event);
> + if (!ret)
> + return;
> + }
> if (!task_function_call(task, __perf_event_disable, event))
> return;
>
>



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