Hi folks,
Problem
=======
Abhijeet pointed out that the following sequence of trace events can be
observed:
cat-1676 [001] d..3 103.010411: sched_waking: comm=grep pid=1677 prio=120 target_cpu=004
grep-1677 [004] d..2 103.010440: sched_switch: prev_comm=grep prev_pid=1677 prev_prio=120 prev_state=R 0x0 ==> next_comm=swapper/4 next_pid=0 next_prio=120
<idle>-0 [004] dNh3 103.0100459: sched_wakeup: comm=grep pid=1677 prio=120 target_cpu=004
IOW, not-yet-woken task gets presented as runnable and switched out in
favor of the idle task... Dietmar and I had a look, turns out this sequence
can happen:
p->__state = TASK_INTERRUPTIBLE;
__schedule()
deactivate_task(p);
ttwu()
READ !p->on_rq
p->__state=TASK_WAKING
trace_sched_switch()
__trace_sched_switch_state()
task_state_index()
return 0;
TASK_WAKING isn't in TASK_REPORT, hence why task_state_index(p) returns 0.
This can happen as of commit c6e7bd7afaeb ("sched/core: Optimize ttwu()
spinning on p->on_cpu") which punted the TASK_WAKING write to the other
side of
smp_cond_load_acquire(&p->on_cpu, !VAL);
in ttwu().
Uwe reported on #linux-rt what I think is a similar issue with
TASK_RTLOCK_WAIT on PREEMPT_RT; again that state isn't in TASK_REPORT so
you get prev_state=0 despite the task blocking on a lock.
Both of those are very confusing for tooling or even human observers.
Patches
=======
For the TASK_WAKING case, pushing the prev_state read in __schedule() down
to __trace_sched_switch_state() seems sufficient. That's patch 1.
For TASK_RTLOCK_WAIT, it's a bit trickier. One could use ->saved_state as
prev_state, but
a) that is also susceptible to racing (see ttwu() / ttwu_state_match())
b) some lock substitutions (e.g. rt_spin_lock()) leave ->saved_state as
TASK_RUNNING.
Patch 2 adds TASK_RTLOCK_WAIT to TASK_REPORT instead.
Testing
=======
Briefly tested on an Arm Juno via ftrace+hackbench against
o tip/sched/core@82762d2af31a
o v5.16-rt15-rebase w/ CONFIG_PREEMPT_RT
I also had a look at the __schedule() disassembly as suggested by Peter; on x86
prev_state gets pushed to the stack and popped prior to the trace event static
call, the rest seems mostly unchanged (GCC 9.3).
Revisions
=========
v2 -> v3
++++++++
o Dropped TASK_RTLOCK_WAIT from TASK_REPORT, made it appear as
TASK_UNINTERRUPTIBLE instead (Eric)
RFC v1 -> v2
++++++++++++
o Collected tags (Steven, Sebastian)
o Patched missed trace_switch event clients (Steven)
Cheers,
Valentin
Valentin Schneider (2):
sched/tracing: Don't re-read p->state when emitting sched_switch event
sched/tracing: Report TASK_RTLOCK_WAIT tasks as TASK_UNINTERRUPTIBLE
include/linux/sched.h | 19 ++++++++++++++++---
include/trace/events/sched.h | 11 +++++++----
kernel/sched/core.c | 4 ++--
kernel/trace/fgraph.c | 4 +++-
kernel/trace/ftrace.c | 4 +++-
kernel/trace/trace_events.c | 8 ++++++--
kernel/trace/trace_osnoise.c | 4 +++-
kernel/trace/trace_sched_switch.c | 1 +
kernel/trace/trace_sched_wakeup.c | 1 +
9 files changed, 42 insertions(+), 14 deletions(-)
--
2.25.1