4.18-stable review patch. If anyone has any objections, please let me know.
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From: Steve Muckle smuckle@google.com
commit d0cdb3ce8834332d918fc9c8ff74f8a169ec9abe upstream.
When a task which previously ran on a given CPU is remotely queued to wake up on that same CPU, there is a period where the task's state is TASK_WAKING and its vruntime is not normalized. This is not accounted for in vruntime_normalized() which will cause an error in the task's vruntime if it is switched from the fair class during this time.
For example if it is boosted to RT priority via rt_mutex_setprio(), rq->min_vruntime will not be subtracted from the task's vruntime but it will be added again when the task returns to the fair class. The task's vruntime will have been erroneously doubled and the effective priority of the task will be reduced.
Note this will also lead to inflation of all vruntimes since the doubled vruntime value will become the rq's min_vruntime when other tasks leave the rq. This leads to repeated doubling of the vruntime and priority penalty.
Fix this by recognizing a WAKING task's vruntime as normalized only if sched_remote_wakeup is true. This indicates a migration, in which case the vruntime would have been normalized in migrate_task_rq_fair().
Based on a similar patch from John Dias joaodias@google.com.
Suggested-by: Peter Zijlstra peterz@infradead.org Tested-by: Dietmar Eggemann dietmar.eggemann@arm.com Signed-off-by: Steve Muckle smuckle@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Chris Redpath Chris.Redpath@arm.com Cc: John Dias joaodias@google.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Miguel de Dios migueldedios@google.com Cc: Morten Rasmussen Morten.Rasmussen@arm.com Cc: Patrick Bellasi Patrick.Bellasi@arm.com Cc: Paul Turner pjt@google.com Cc: Quentin Perret quentin.perret@arm.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Todd Kjos tkjos@google.com Cc: kernel-team@android.com Fixes: b5179ac70de8 ("sched/fair: Prepare to fix fairness problems on migration") Link: http://lkml.kernel.org/r/20180831224217.169476-1-smuckle@google.com Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/sched/fair.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
--- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -10019,7 +10019,8 @@ static inline bool vruntime_normalized(s * - A task which has been woken up by try_to_wake_up() and * waiting for actually being woken up by sched_ttwu_pending(). */ - if (!se->sum_exec_runtime || p->state == TASK_WAKING) + if (!se->sum_exec_runtime || + (p->state == TASK_WAKING && p->sched_remote_wakeup)) return true;
return false;