Re: [PATCH] nohz: Fix missing tick reprog while interrupting inline timer softirq

From: Grygorii Strashko
Date: Fri Aug 24 2018 - 12:14:47 EST

On 08/24/2018 01:17 AM, Greg KH wrote:
> On Thu, Aug 23, 2018 at 05:57:06PM -0500, Grygorii Strashko wrote:
>> Hi
>> On 07/31/2018 05:52 PM, Frederic Weisbecker wrote:
>>> Before updating the full nohz tick or the idle time on IRQ exit, we
>>> check first if we are not in a nesting interrupt, whether the inner
>>> interrupt is a hard or a soft IRQ.
>>> There is a historical reason for that: the dyntick idle mode used to
>>> reprogram the tick on IRQ exit, after softirq processing, and there was
>>> no point in doing that job in the outer nesting interrupt because the
>>> tick update will be performed through the end of the inner interrupt
>>> eventually, with even potential new timer updates.
>>> One corner case could show up though: if an idle tick interrupts a softirq
>>> executing inline in the idle loop (through a call to local_bh_enable())
>>> after we entered in dynticks mode, the IRQ won't reprogram the tick
>>> because it assumes the softirq executes on an inner IRQ-tail. As a
>>> result we might put the CPU in sleep mode with the tick completely
>>> stopped whereas a timer can still be enqueued. Indeed there is no tick
>>> reprogramming in local_bh_enable(). We probably asssumed there was no bh
>>> disabled section in idle, although there didn't seem to be debug code
>>> ensuring that.
>>> Nowadays the nesting interrupt optimization still stands but only concern
>>> full dynticks. The tick is stopped on IRQ exit in full dynticks mode
>>> and we want to wait for the end of the inner IRQ to reprogramm the tick.
>>> But in_interrupt() doesn't make a difference between softirqs executing
>>> on IRQ tail and those executing inline. What was to be considered a
>>> corner case in dynticks-idle mode now becomes a serious opportunity for
>>> a bug in full dynticks mode: if a tick interrupts a task executing
>>> softirq inline, the tick reprogramming will be ignored and we may exit
>>> to userspace after local_bh_enable() with an enqueued timer that will
>>> never fire.
>>> To fix this, simply keep reprogramming the tick if we are in a hardirq
>>> interrupting softirq. We can still figure out a way later to restore
>>> this optimization while excluding inline softirq processing.
>>> Reported-by: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>
>>> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>> Tested-by: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>
>>> ---
>>> kernel/softirq.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>> diff --git a/kernel/softirq.c b/kernel/softirq.c
>>> index 900dcfe..0980a81 100644
>>> --- a/kernel/softirq.c
>>> +++ b/kernel/softirq.c
>>> @@ -386,7 +386,7 @@ static inline void tick_irq_exit(void)
>>> /* Make sure that timer wheel updates are propagated */
>>> if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
>>> - if (!in_interrupt())
>>> + if (!in_irq())
>>> tick_nohz_irq_exit();
>>> }
>>> #endif
>> This patch was back ported to the Stable linux-4.14.y and It causes regression -
>> flood of "NOHZ: local_softirq_pending" messages on all TI boards during boot (NFS boot):
>> [ 4.179796] NOHZ: local_softirq_pending 2c2 in sirq 256
>> [ 4.185051] NOHZ: local_softirq_pending 2c2 in sirq 256
>> the same is not reproducible with LKML - seems due to changes in tick-sched.c
>> __tick_nohz_idle_enter()/tick_nohz_irq_exit().
> What changes do you think fixed this?

not sure. But it seems set of changes from Rafael J. Wysocki:

ff7de62 nohz: Avoid duplication of code related to got_idle_tick
296bb1e cpuidle: menu: Refine idle state selection for running tick
554c8aa sched: idle: Select idle state before stopping the tick
23a8d88 time: tick-sched: Split tick_nohz_stop_sched_tick()
45f1ff5 cpuidle: Return nohz hint from cpuidle_select()
2aaf709 sched: idle: Do not stop the tick upfront in the idle loop
0e77676 time: tick-sched: Reorganize idle tick management code
b7eaf1a cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely

>> I've generated backtrace from can_stop_idle_tick() (see below) and seems this
>> patch makes tick_nohz_irq_exit() call unconditional in case of nested interrupt:
>> gic_handle_irq
>> |- irq_exit
>> |- preempt_count_sub(HARDIRQ_OFFSET); <-- [1]
>> |-__do_softirq
>> <irqs enabled>
>> |- gic_handle_irq()
>> |- irq_exit()
>> |- tick_irq_exit()
>> if (!in_irq()) <-- My understanding is that this condition will be always true due to [1]
>> tick_nohz_irq_exit();
>> |-__tick_nohz_idle_enter()
>> |- can_stop_idle_tick()
>> Sry, not sure if my conclusion is right and how can it be fixed.
> Any pointers to a patch that might need to be backported would be
> appreciated.

Author: Frederic Weisbecker <frederic@xxxxxxxxxx>
Date: Fri Aug 3 15:31:34 2018 +0200

nohz: Fix missing tick reprogram when interrupting an inline softirq

commit 0a0e0829f990120cef165bbb804237f400953ec2 upstream.