When we check if a task has been switched out since the last scan, we might
have a race condition on the following scenario:
- the task is freshly created and scheduled
- it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out
- check_hung_task() scans this task and will report a false positive because
t->nvcsw + t->nivcsw == t->last_switch_count == 0
Add a check for such cases.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Impact: saves sizeof(long) bytes per task_struct
By guaranteeing that sysctl_hung_task_timeout_secs have elapsed between
tasklist scans we can avoid using timestamps.
Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Since the tasklist is protected by rcu list operations, it is safe
to convert the read_lock()s to rcu_read_lock().
Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Impact: extend the scope of hung-task checks
Changed the default value of hung_task_check_count to PID_MAX_LIMIT.
hung_task_batch_count added to put an upper bound on the critical
section. Every hung_task_batch_count checks, the rcu lock is never
held for a too long time.
Keeping the critical section small minimizes time preemption is disabled
and keeps rcu grace periods small.
To prevent following a stale pointer, get_task_struct is called on g and t.
To verify that g and t have not been unhashed while outside the critical
section, the task states are checked.
The design was proposed by Frédéric Weisbecker.
Signed-off-by: Mandeep Singh Baines <msb@google.com>
Suggested-by: Frédéric Weisbecker <fweisbec@gmail.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Impact: fix potential false panic
A potential race exists if sysctl_hung_task_timeout_secs is reset to 0
while inside check_hung_uniterruptible_tasks(). If check_task() is
entered, a comparison with 0 will result in a false hung_task being
detected.
If sysctl_hung_task_panic is set, the system will panic.
Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Decoupling allows:
* hung tasks check to happen at very low priority
* hung tasks check and softlockup to be enabled/disabled independently
at compile and/or run-time
* individual panic settings to be enabled disabled independently
at compile and/or run-time
* softlockup threshold to be reduced without increasing hung tasks
poll frequency (hung task check is expensive relative to softlock watchdog)
* hung task check to be zero over-head when disabled at run-time
Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
