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47252cfbac
While testing the tracer preemptoff, I hit this strange trace:
<...>-259 0...1 0us : schedule <-worker_thread
<...>-259 0d..1 0us : rcu_note_context_switch <-__schedule
<...>-259 0d..1 0us : rcu_sched_qs <-rcu_note_context_switch
<...>-259 0d..1 0us : rcu_preempt_qs <-rcu_note_context_switch
<...>-259 0d..1 0us : _raw_spin_lock <-__schedule
<...>-259 0d..1 0us : preempt_count_add <-_raw_spin_lock
<...>-259 0d..2 0us : do_raw_spin_lock <-_raw_spin_lock
<...>-259 0d..2 1us : deactivate_task <-__schedule
<...>-259 0d..2 1us : update_rq_clock.part.84 <-deactivate_task
<...>-259 0d..2 1us : dequeue_task_fair <-deactivate_task
<...>-259 0d..2 1us : dequeue_entity <-dequeue_task_fair
<...>-259 0d..2 1us : update_curr <-dequeue_entity
<...>-259 0d..2 1us : update_min_vruntime <-update_curr
<...>-259 0d..2 1us : cpuacct_charge <-update_curr
<...>-259 0d..2 1us : __rcu_read_lock <-cpuacct_charge
<...>-259 0d..2 1us : __rcu_read_unlock <-cpuacct_charge
<...>-259 0d..2 1us : clear_buddies <-dequeue_entity
<...>-259 0d..2 1us : account_entity_dequeue <-dequeue_entity
<...>-259 0d..2 2us : update_min_vruntime <-dequeue_entity
<...>-259 0d..2 2us : update_cfs_shares <-dequeue_entity
<...>-259 0d..2 2us : hrtick_update <-dequeue_task_fair
<...>-259 0d..2 2us : wq_worker_sleeping <-__schedule
<...>-259 0d..2 2us : kthread_data <-wq_worker_sleeping
<...>-259 0d..2 2us : pick_next_task_fair <-__schedule
<...>-259 0d..2 2us : check_cfs_rq_runtime <-pick_next_task_fair
<...>-259 0d..2 2us : pick_next_entity <-pick_next_task_fair
<...>-259 0d..2 2us : clear_buddies <-pick_next_entity
<...>-259 0d..2 2us : pick_next_entity <-pick_next_task_fair
<...>-259 0d..2 2us : clear_buddies <-pick_next_entity
<...>-259 0d..2 2us : set_next_entity <-pick_next_task_fair
<...>-259 0d..2 3us : put_prev_entity <-pick_next_task_fair
<...>-259 0d..2 3us : check_cfs_rq_runtime <-put_prev_entity
<...>-259 0d..2 3us : set_next_entity <-pick_next_task_fair
gnome-sh-1031 0d..2 3us : finish_task_switch <-__schedule
gnome-sh-1031 0d..2 3us : _raw_spin_unlock_irq <-finish_task_switch
gnome-sh-1031 0d..2 3us : do_raw_spin_unlock <-_raw_spin_unlock_irq
gnome-sh-1031 0...2 3us!: preempt_count_sub <-_raw_spin_unlock_irq
gnome-sh-1031 0...1 582us : do_raw_spin_lock <-_raw_spin_lock
gnome-sh-1031 0...1 583us : _raw_spin_unlock <-drm_gem_object_lookup
gnome-sh-1031 0...1 583us : do_raw_spin_unlock <-_raw_spin_unlock
gnome-sh-1031 0...1 583us : preempt_count_sub <-_raw_spin_unlock
gnome-sh-1031 0...1 584us : _raw_spin_unlock <-drm_gem_object_lookup
gnome-sh-1031 0...1 584us+: trace_preempt_on <-drm_gem_object_lookup
gnome-sh-1031 0...1 603us : <stack trace>
=> preempt_count_sub
=> _raw_spin_unlock
=> drm_gem_object_lookup
=> i915_gem_madvise_ioctl
=> drm_ioctl
=> do_vfs_ioctl
=> SyS_ioctl
=> entry_SYSCALL_64_fastpath
As I'm tracing preemption disabled, it seemed incorrect that the trace
would go across a schedule and report not being in the scheduler.
Looking into this I discovered the problem.
schedule() calls preempt_disable() but the preempt_schedule() calls
preempt_enable_notrace(). What happened above was that the gnome-shell
task was preempted on another CPU, migrated over to the idle cpu. The
tracer stared with idle calling schedule(), which called
preempt_disable(), but then gnome-shell finished, and it enabled
preemption with preempt_enable_notrace() that does stop the trace, even
though preemption was enabled.
The purpose of the preempt_disable_notrace() in the preempt_schedule()
is to prevent function tracing from going into an infinite loop.
Because function tracing can trace the preempt_enable/disable() calls
that are traced. The problem with function tracing is:
NEED_RESCHED set
preempt_schedule()
preempt_disable()
preempt_count_inc()
function trace (before incrementing preempt count)
preempt_disable_notrace()
preempt_enable_notrace()
sees NEED_RESCHED set
preempt_schedule() (repeat)
Now by breaking out the preempt off/on tracing into their own code:
preempt_disable_check() and preempt_enable_check(), we can add these to
the preempt_schedule() code. As preemption would then be disabled, even
if they were to be traced by the function tracer, the disabled
preemption would prevent the recursion.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160321112339.6dc78ad6@gandalf.local.home
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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| .. | ||
| bpf | ||
| configs | ||
| debug | ||
| events | ||
| gcov | ||
| irq | ||
| livepatch | ||
| locking | ||
| power | ||
| printk | ||
| rcu | ||
| sched | ||
| time | ||
| trace | ||
| .gitignore | ||
| acct.c | ||
| async.c | ||
| audit.c | ||
| audit.h | ||
| audit_fsnotify.c | ||
| audit_tree.c | ||
| audit_watch.c | ||
| auditfilter.c | ||
| auditsc.c | ||
| backtracetest.c | ||
| bounds.c | ||
| capability.c | ||
| cgroup.c | ||
| cgroup_freezer.c | ||
| cgroup_pids.c | ||
| compat.c | ||
| configs.c | ||
| context_tracking.c | ||
| cpu.c | ||
| cpu_pm.c | ||
| cpuset.c | ||
| crash_dump.c | ||
| cred.c | ||
| delayacct.c | ||
| dma.c | ||
| elfcore.c | ||
| exec_domain.c | ||
| exit.c | ||
| extable.c | ||
| fork.c | ||
| freezer.c | ||
| futex.c | ||
| futex_compat.c | ||
| groups.c | ||
| hung_task.c | ||
| irq_work.c | ||
| jump_label.c | ||
| kallsyms.c | ||
| kcmp.c | ||
| Kconfig.freezer | ||
| Kconfig.hz | ||
| Kconfig.locks | ||
| Kconfig.preempt | ||
| kcov.c | ||
| kexec.c | ||
| kexec_core.c | ||
| kexec_file.c | ||
| kexec_internal.h | ||
| kmod.c | ||
| kprobes.c | ||
| ksysfs.c | ||
| kthread.c | ||
| latencytop.c | ||
| Makefile | ||
| membarrier.c | ||
| memremap.c | ||
| module-internal.h | ||
| module.c | ||
| module_signing.c | ||
| notifier.c | ||
| nsproxy.c | ||
| padata.c | ||
| panic.c | ||
| params.c | ||
| pid.c | ||
| pid_namespace.c | ||
| profile.c | ||
| ptrace.c | ||
| range.c | ||
| reboot.c | ||
| relay.c | ||
| resource.c | ||
| seccomp.c | ||
| signal.c | ||
| smp.c | ||
| smpboot.c | ||
| smpboot.h | ||
| softirq.c | ||
| stacktrace.c | ||
| stop_machine.c | ||
| sys.c | ||
| sys_ni.c | ||
| sysctl.c | ||
| sysctl_binary.c | ||
| task_work.c | ||
| taskstats.c | ||
| test_kprobes.c | ||
| torture.c | ||
| tracepoint.c | ||
| tsacct.c | ||
| uid16.c | ||
| up.c | ||
| user-return-notifier.c | ||
| user.c | ||
| user_namespace.c | ||
| utsname.c | ||
| utsname_sysctl.c | ||
| watchdog.c | ||
| workqueue.c | ||
| workqueue_internal.h | ||