Messages in this thread | | | Date | Tue, 12 Jun 2012 11:36:26 +0530 | From | "K.Prasad" <> | Subject | Re: [Patch][perf] Invoke __perf_event_disable without an IPI |
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On Mon, Jun 11, 2012 at 01:13:33PM +0200, Peter Zijlstra wrote: > On Mon, 2012-06-11 at 11:32 +0530, K.Prasad wrote: > > > 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?
Hardware breakpoints for user-space have traditionally operated in a one-shot mode i.e. breakpoint is disabled after the first hit by invoking perf_event_disable from hw_breakpoint_handler.
PowerPC server class processors follow 'trigger-before-execute', wherein the breakpoint exception is triggered before the instruction performing the memory access is executed. So the one-shot mode prevents the processor from entering an infinite loop and the debugging tools like GDB re-enable the breakpoints explicitly after a SIGTRAP.
Thanks, K.Prasad
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