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@linutronix.de Signed-off-by: Frederic Weisbecker frederic@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@kernel.org Tested-by: Anna-Maria Gleixner anna-maria@linutronix.de
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();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.
commit Author: Frederic Weisbecker frederic@kernel.org Date: Fri Aug 3 15:31:34 2018 +0200
nohz: Fix missing tick reprogram when interrupting an inline softirq
commit 0a0e0829f990120cef165bbb804237f400953ec2 upstream.