Re: [PATCH] tick/broadcast: Do not set oneshot_mask except was_periodic was true

[Victor Hassan <victor@allwinnertech.com>]

On 4/3/2023 6:26 PM, Frederic Weisbecker wrote:
> On Tue, Mar 28, 2023 at 02:36:29PM +0800, Victor Hassan wrote:
>> If a broadcast timer is registered after the system switched to oneshot
>> mode, a hang_task err could occur like that:
>>
>> INFO: task kworker/u15:0:7 blocked for more than 120 seconds.
>>        Tainted: G            E     5.15.41-android13-8-00002-xxx #1
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:kworker/u16:0   state:D stack: 9808 pid:  7 ppid: 2 flags:0x00000008
>> Workqueue: events_unbound deferred_probe_work_func.cfi_jt
>> Call trace:
>>   __switch_to+0y240/0x490
>>   __schedule+0x620/0xafc
>>   schedule+0x110/0x204
>>   schedule_hrtimeout_range_clock+0x9c/0x118
>>   usleep_range_state+0x150/0x1ac
>>   _regulator_do_enable+0x528/0x878
>>   set_machine_constraints+0x6a0/0xf2c
>>   regulator_register+0x3ac/0x7ac
>>   devm_regulator_register+0xbc/0x120
>>   pmu_ext_regulator_probe+0xb0/0x1b4 [pmu_ext_regulator]
>>   platform_probe+0x70/0x194
>>   really_proe+0x320/0x68c
>>   __driver_probe_device+0x204/0x260
>>   driver_probe_device+0x48/0x1e0
>>
>> When the new broadcast timer was registered after the system switched
>> to oneshot mode, the broadcast timer was not used as periodic. If the
>> oneshot mask was set incorrectly, all cores which did not enter cpu_idle
>> state can't enter cpu_idle normally, causing the hrtimer mechanism to
>> break.
>>
>> This patch fixes the issue by moving the update action about oneshot
>> mask to a more strict conditions. The tick_broadcast_setup_oneshot would
>> be called in two typical condition, and they all will work.
>>
>> 1. tick_handle_periodic -> tick_broadcast_setup_oneshot
>>
>> The origin broadcast was periodic, so it can set the oneshot_mask bits
>> for those waiting for periodic broadcast and program the broadcast timer
>> to fire.
>>
>> 2. tick_install_broadcast_device -> tick_broadcast_setup_oneshot
>>
>> The origin broadcast was oneshot, so the cores which enter the cpu_idle
>> already used the oneshot_mask bits. It is unnecessary to update the
>> oneshot_mask.
>>
>> Fixes: 9c336c9935cf ("tick/broadcast: Allow late registered device to enter oneshot mode")
>>
>> Signed-off-by: Victor Hassan <victor@allwinnertech.com>
>> ---
>>   kernel/time/tick-broadcast.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
>> index 93bf2b4e47e5..fdbbba487978 100644
>> --- a/kernel/time/tick-broadcast.c
>> +++ b/kernel/time/tick-broadcast.c
>> @@ -1041,12 +1041,13 @@ static void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
>>   		 */
>>   		cpumask_copy(tmpmask, tick_broadcast_mask);
>>   		cpumask_clear_cpu(cpu, tmpmask);
>> -		cpumask_or(tick_broadcast_oneshot_mask,
>> -			   tick_broadcast_oneshot_mask, tmpmask);
>>   
>>   		if (was_periodic && !cpumask_empty(tmpmask)) {
>>   			ktime_t nextevt = tick_get_next_period();
>>   
>> +			cpumask_or(tick_broadcast_oneshot_mask,
>> +				   tick_broadcast_oneshot_mask, tmpmask);
>> +
> 
> Good catch, it looks like one issue that can trigger is due to the resulting
> ignored calls to tick_broadcast_exit(). Indeed if the cpu is already in
> tick_broadcast_oneshot_mask then cpuidle won't call the exit

Hi Frederic,
Thanks for the review. Still I have a few questions and would like to 
discuss them further.

> 
> Leading to such race:
> 
> * CPU 1 stop its tick, next event is in one hour
> * CPU 0 registers new broadcast and sets CPU 1 in tick_broadcast_oneshot_mask
> * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter() is ignored because
>    the CPU is already in tick_broadcast_oneshot_mask

Yes.

> * CPU 1 goes to sleep
> * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1
>    is in one hour, program the broadcast to that deadline
> * CPU 1 gets an interrupt that enqueues a new timer expiring in the next jiffy
> * CPU 1 don't call tick_broadcast_exit and thus don't remove itself from
>    tick_broadcast_oneshot_mask

I'm not sure about this... Actually, I believe CPU 1 *will* call 
tick_broadcast_exit in this condition because I cannot find a limitation 
on this execution path.

> * CPU 1 re-enters in cpuidle_enter_state(), tick_broadcast_enter() is again
>    ignored so the new timer isn't propagated to the broadcast.
> * CPU 1 goes to sleep and won't be woken before one hour.
> 
> 
>    Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> 
> 
> Thanks.
> 
> 
> 
> 
>>   			clockevents_switch_state(bc, CLOCK_EVT_STATE_ONESHOT);
>>   			tick_broadcast_init_next_event(tmpmask, nextevt);
>>   			tick_broadcast_set_event(bc, cpu, nextevt);
>> -- 
>> 2.29.0
>>