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SubjectRe: [RFC] tracepoint: Run tracepoints even after CPU is offline
On Sun,  5 Aug 2018 20:44:37 -0700
"Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:

> Commit f37755490fe9 ("tracepoints: Do not trace when cpu is offline")
> causes a problem for lockdep using tracepoint code. Once a CPU is
> offline, tracepoints donot get called, however this causes a big problem
> for lockdep probes that need to run so that IRQ annotations are marked
> correctly.
>
> An issue is possible where while the CPU is going offline, an interrupt
> can come in and then a lockdep assert causes an annotation warning:
>
> [ 106.551354] IRQs not enabled as expected
> [ 106.551785] WARNING: CPU: 1 PID: 0 at kernel/time/tick-sched.c:982
> tick_nohz_idle_enter+0x99/0xb0
> [ 106.552964] Modules linked in:
> [ 106.553299] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G W
>
> We need tracepoints to run as late as possible. This commit tries to fix
> the issue by removing the cpu_online check in tracepoint code that was
> introduced in the mentioned commit, however now we run the risk of
> running dereferencing probes that aren't RCU protected, which gives an
> RCU warning like so on boot up:
> [ 0.030159] x86: Booting SMP configuration:
> [ 0.030169] .... node #0, CPUs: #1
> [ 0.001000]
> [ 0.001000] =============================
> [ 0.001000] WARNING: suspicious RCU usage
> [ 0.001000] 4.18.0-rc6+ #42 Not tainted
> [ 0.001000] -----------------------------
> [ 0.001000] ./include/trace/events/timer.h:38 suspicious
> rcu_dereference_check() usage!
> [ 0.001000]
> [ 0.001000] other info that might help us debug this:
> [ 0.001000]
> [ 0.001000]
> [ 0.001000] RCU used illegally from offline CPU!
> [ 0.001000] rcu_scheduler_active = 1, debug_locks = 1
> [ 0.001000] no locks held by swapper/1/0.
> [ 0.001000]
>
> Any ideas on how we can fix this? Basically we need RCU to work here
> even after !cpu_online. I thought of just using SRCU for all tracepoints
> however that may mean we can't use tracepoints from NMI..
>
> Tries-to-Fix: c3bc8fd637a9 ("tracing: Centralize preemptirq tracepoints and
> unify their usage")
> Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> include/linux/tracepoint.h | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index d9a084c72541..020885714a0f 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -365,19 +365,17 @@ extern void syscall_unregfunc(void);
> * "void *__data, proto" as the callback prototype.
> */
> #define DECLARE_TRACE_NOARGS(name) \
> - __DECLARE_TRACE(name, void, , \
> - cpu_online(raw_smp_processor_id()), \
> + __DECLARE_TRACE(name, void, , 1, \
> void *__data, __data)
>
> #define DECLARE_TRACE(name, proto, args) \
> - __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), \
> - cpu_online(raw_smp_processor_id()), \
> + __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), 1, \
> PARAMS(void *__data, proto), \
> PARAMS(__data, args))
>
> #define DECLARE_TRACE_CONDITION(name, proto, args, cond) \
> __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), \
> - cpu_online(raw_smp_processor_id()) && (PARAMS(cond)), \
> + PARAMS(cond), \
> PARAMS(void *__data, proto), \
> PARAMS(__data, args))
>

Actually, I took a look at it too, and came up with this patch. Can you
test it out? The difference is that it doesn't stop the _rcuidle
version of the trace events to not be called by "cpu_online".

Which brings up another question. Can srcu work on cpu offlined CPUs?

-- Steve

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index d9a084c72541..4aba6c807d28 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -211,8 +211,11 @@ extern void syscall_unregfunc(void);
__DO_TRACE(&__tracepoint_##name, \
TP_PROTO(data_proto), \
TP_ARGS(data_args), \
+ cpu_online(raw_smp_processor_id()) && \
TP_CONDITION(cond), 0); \
- if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) { \
+ if (IS_ENABLED(CONFIG_LOCKDEP) && \
+ cpu_online(raw_smp_processor_id()) && \
+ (cond)) { \
rcu_read_lock_sched_notrace(); \
rcu_dereference_sched(__tracepoint_##name.funcs);\
rcu_read_unlock_sched_notrace(); \
@@ -365,19 +368,17 @@ extern void syscall_unregfunc(void);
* "void *__data, proto" as the callback prototype.
*/
#define DECLARE_TRACE_NOARGS(name) \
- __DECLARE_TRACE(name, void, , \
- cpu_online(raw_smp_processor_id()), \
+ __DECLARE_TRACE(name, void, , 1, \
void *__data, __data)

#define DECLARE_TRACE(name, proto, args) \
- __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), \
- cpu_online(raw_smp_processor_id()), \
+ __DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), 1, \
PARAMS(void *__data, proto), \
PARAMS(__data, args))

#define DECLARE_TRACE_CONDITION(name, proto, args, cond) \
__DECLARE_TRACE(name, PARAMS(proto), PARAMS(args), \
- cpu_online(raw_smp_processor_id()) && (PARAMS(cond)), \
+ PARAMS(cond), \
PARAMS(void *__data, proto), \
PARAMS(__data, args))

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