Messages in this thread | | | Date | Thu, 31 Dec 2009 13:54:35 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 1/4] ksym_tracer: Fix to make the tracer work |
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K.Prasad wrote: > On Wed, Dec 30, 2009 at 02:22:22PM +0800, Li Zefan wrote: >> ksym tracer doesn't work: >> >> # echo tasklist_lock:rw- > ksym_trace_filter >> -bash: echo: write error: No such device >> >> It's because we pass to perf_event_create_kernel_counter() >> a cpu number which is not present. >> >> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com> >> --- >> kernel/hw_breakpoint.c | 10 +++++++--- >> kernel/trace/trace_ksym.c | 1 - >> 2 files changed, 7 insertions(+), 4 deletions(-) >> >> diff --git a/kernel/hw_breakpoint.c b/kernel/hw_breakpoint.c >> index dbcbf6a..50dbd59 100644 >> --- a/kernel/hw_breakpoint.c >> +++ b/kernel/hw_breakpoint.c >> @@ -40,6 +40,7 @@ >> #include <linux/percpu.h> >> #include <linux/sched.h> >> #include <linux/init.h> >> +#include <linux/cpu.h> >> #include <linux/smp.h> >> >> #include <linux/hw_breakpoint.h> >> @@ -388,7 +389,8 @@ register_wide_hw_breakpoint(struct perf_event_attr *attr, >> if (!cpu_events) >> return ERR_PTR(-ENOMEM); >> >> - for_each_possible_cpu(cpu) { >> + get_online_cpus(); >> + for_each_online_cpu(cpu) { >> pevent = per_cpu_ptr(cpu_events, cpu); >> bp = perf_event_create_kernel_counter(attr, cpu, -1, triggered); >> > > Frederic must have used for_each_possible_cpu() to account for CPUs that > are offline at the time of registration, but may eventually turn online. > Since register_wide_hw_breakpoint() interface is designed to deliver > system-wide breakpoints, the debug registers of a new online CPU will > should have the breakpoints populated to comprehensively notify all > memory accesses over target address. > > I'd rather wait to hear from Frederic to know why > perf_event_create_kernel_counter() returns an error when run for an > offline cpu and how it can be solved. >
See the comment in find_get_context() in kernel/perf_event.c:
/* * We could be clever and allow to attach a event to an * offline CPU and activate it when the CPU comes up, but * that's for later. */ if (!cpu_online(cpu)) return ERR_PTR(-ENODEV);
So I think we can use for_each_possible_cpu() in the future, but not now.
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