+linux-renesas-soc In hope that the issue reported in [1] was seen by other members of Renesas community. [1] https://lkml.org/lkml/2019/9/18/711 On Wed, Sep 18, 2019 at 04:41:37PM +0200, Balasubramani Vivekanandan wrote: > I was investigating a rcu stall warning on ARM64 Renesas Rcar3 > platform. On analysis I found that rcu stall warning was because the > rcu_preempt kthread was starved of cpu time. rcu_preempt was blocked in > the function schedule_timeout() and never woken up. On further > investigation I found that local timer interrupts were not happening on > the cpu where the rcu_preempt kthread was blocked. So the rcu_preempt > was not woken up after timeout. > I continued my analysis to debug why the timer failed on the cpu. I > found that when cpu goes through idle state cycle, the timer failure > happens. When the cpu enters the idle state it subscribes to the tick > broadcast clock and shutsdown the local timer. Then on exit from idle > state the local timer is programmed to fire interrupts. But I found that > the during the error scenario, cpu fails to program the local timer on > exit from idle state. The below code in > __tick_broadcast_oneshot_control() is where the idle code exit path goes > through and fails to program the timer hardware > > now = ktime_get(); > if (dev->next_event <= now) { > cpumask_set_cpu(cpu, tick_broadcast_force_mask); > goto out; > } > > The value in next_event will be earlier than current time because the > tick broadcast clock did not wake up the cpu on its subcribed > timeout. Later when the cpu is woken up due to some other event this > condition will arise. After the cpu woken up, any further timeout > requests by any task on the cpu might fail to program the timer > hardware because the value in next_event will be earlier than the > current time. > Then I focussed on why the tick broadcast clock failed to wake up the > cpu. I noticed a race condition in the hrtimer based tick broadcast > clock. The race condition results in a condition where the tick > broadcast hrtimer is never restarted. I have created a patch to fix the > race condition. Please review > > Balasubramani Vivekanandan (1): > tick: broadcast-hrtimer: Fix a race in bc_set_next > > kernel/time/tick-broadcast-hrtimer.c | 58 ++++++++++++++++++++++------ > kernel/time/tick-broadcast.c | 2 + > 2 files changed, 48 insertions(+), 12 deletions(-) > > -- > 2.17.1 > -- Best Regards, Eugeniu
