perf: Pass appropriate frame pointer to dump_trace()

Pass the frame pointer from the regs of the interrupted path
to dump_trace() while processing the stack trace.

Currently, dump_trace() takes the current bp and starts the
callchain from dump_trace() itself. This is wasteful because
we need to walk through the entire NMI/DEBUG stack before
retrieving the interrupted point.

We can fix that by just using the frame pointer from the
captured regs. It points exactly where we want to start.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1262235183-5320-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>

arch/x86/kernel/cpu/perf_event.c

@@ -2347,7 +2347,7 @@ perf_callchain_kernel(struct pt_regs *regs, struct perf_callchain_entry *entry)
 	callchain_store(entry, PERF_CONTEXT_KERNEL);
 	callchain_store(entry, regs->ip);
 
-	dump_trace(NULL, regs, NULL, 0, &backtrace_ops, entry);
+	dump_trace(NULL, regs, NULL, regs->bp, &backtrace_ops, entry);
 }
 
 /*