On Tue, Jan 12, 2021 at 04:09:53PM -0800, Paul E. McKenney wrote:
> On Tue, Jan 12, 2021 at 01:59:48PM +0000, Mark Rutland wrote:
> > Hi,
> >
> > While fuzzing arm64 with Syzkaller (under QEMU+KVM) over a number of
> > releases,
> > I've occasionally seen some ridiculously long stalls (20+ seconds), where it
> > appears that a CPU is stuck in a hard IRQ context. As this gets detected
> > after
> > the CPU returns to the interrupted context, it's difficult to identify where
> > exactly the stall is coming from.
> >
> > These patches are intended to help tracking this down, with a WARN() if an
> > IRQ
> > handler takes longer than a given timout (1 second by default), logging the
> > specific IRQ and handler function. While it's possible to achieve similar
> > with
> > tracing, it's harder to integrate that into an automated fuzzing setup.
> >
> > I've been running this for a short while, and haven't yet seen any of the
> > stalls with this applied, but I've tested with smaller timeout periods in
> > the 1
> > millisecond range by overloading the host, so I'm confident that the check
> > works.
> >
> > Thanks,
> > Mark.
>
> Nice!
>
> Acked-by: Paul E. McKenney <paul...@kernel.org>
>
> I added the patch below to add a three-second delay to the scheduling
> clock interrupt handler. This executed, but did not cause your warning
> to be emitted, probably because rcutorture runs under qemu/KVM. So no
> Tested-by, not yet, anyway.
I think this is because on x86, APIC timer interrupts are handled in
arch code without going through the usual IRQ management infrastructure.
A dump_stack() in rcu_sched_clock_irq() shows:
[ 75.131594] rcu: rcu_sched_clock_irq: 3-second delay.
[ 75.132557] CPU: 2 PID: 135 Comm: sh Not tainted 5.11.0-rc3+ #12
[ 75.133610] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[ 75.135639] Call Trace:
[ 75.136100] dump_stack+0x57/0x6a
[ 75.136713] rcu_sched_clock_irq+0x76d/0x880
[ 75.137493] update_process_times+0x77/0xb0
[ 75.138254] tick_sched_handle.isra.17+0x2b/0x40
[ 75.139105] tick_sched_timer+0x36/0x70
[ 75.139803] ? tick_sched_handle.isra.17+0x40/0x40
[ 75.140665] __hrtimer_run_queues+0xf8/0x230
[ 75.141441] hrtimer_interrupt+0xfc/0x240
[ 75.142169] ? asm_sysvec_apic_timer_interrupt+0xa/0x20
[ 75.143117] __sysvec_apic_timer_interrupt+0x58/0xf0
[ 75.144017] sysvec_apic_timer_interrupt+0x27/0x80
[ 75.144892] asm_sysvec_apic_timer_interrupt+0x12/0x20
Here __sysvec_apic_timer_interrupt() calls local_apic_timer_interrupt()
which calls the clock_event_device::event_handler() directly. Since that
never goes via an irqaction handler, the code I add is never invoked in
this path. I believe this is true for a number of IRQs on x86 (e.g.
IPIs). A slow handler for a peripheral interrupt should still be caught,
though.
On arm64, timer interrupts (and IIUC IPIs too) go though the usual IRQ
management code, and so delays there get caught:
[ 311.703932] rcu: rcu_sched_clock_irq: 3-second delay.
[ 311.705012] CPU: 3 PID: 199 Comm: bash Not tainted
5.11.0-rc3-00003-gbe60490b2295-dirty #13
[ 311.706694] Hardware name: linux,dummy-virt (DT)
[ 311.707688] Call trace:
[ 311.708233] dump_backtrace+0x0/0x1a0
[ 311.709053] show_stack+0x18/0x70
[ 311.709774] dump_stack+0xd0/0x12c
[ 311.710468] rcu_sched_clock_irq+0x7d4/0xcf0
[ 311.711356] update_process_times+0x9c/0xec
[ 311.712288] tick_sched_handle+0x34/0x60
[ 311.713191] tick_sched_timer+0x4c/0xa4
[ 311.714043] __hrtimer_run_queues+0x140/0x1e0
[ 311.715012] hrtimer_interrupt+0xe8/0x290
[ 311.715943] arch_timer_handler_virt+0x38/0x4c
[ 311.716951] handle_percpu_devid_irq+0x94/0x190
[ 311.717953] __handle_domain_irq+0x7c/0xe0
[ 311.718890] gic_handle_irq+0xc0/0x140
[ 311.719729] el0_irq_naked+0x4c/0x54
[ 314.720833] ------------[ cut here ]------------
[ 314.721950] IRQ 11 handler arch_timer_handler_virt took 3016901740 ns
[ 314.723421] WARNING: CPU: 3 PID: 199 at kernel/irq/internals.h:140
handle_percpu_devid_irq+0x158/0x190
I think our options are:
1) Live with it, and don't check these special cases.
2) Rework the special cases to go though the regular irqaction
processing.
3) Open-code checks in each special case.
4) Add a helper/wrapper function that can be called in each special
case, and update each one accordingly.
... and I reckon some mixture of #3 and #4 is plausible. We could add a
__handle_check_irq_function() or similar and use that to wrap the call
to local_apic_timer_interrupt() from sysvec_apic_timer_interrupt(), but
I'm not sure exactly what that needs to look like to cover any other
special cases.
Thanks,
Mark.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e04e336..dac8c7a 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2606,6 +2606,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> */
> void rcu_sched_clock_irq(int user)
> {
> + static atomic_t invctr;
> +
> trace_rcu_utilization(TPS("Start scheduler-tick"));
> lockdep_assert_irqs_disabled();
> raw_cpu_inc(rcu_data.ticks_this_gp);
> @@ -2623,6 +2625,14 @@ void rcu_sched_clock_irq(int user)
> invoke_rcu_core();
> lockdep_assert_irqs_disabled();
>
> + if (atomic_inc_return(&invctr) % 0x3ffff == 0) {
> + int i;
> +
> + pr_alert("%s: 3-second delay.\n", __func__);
> + for (i = 0; i < 3000; i++)
> + udelay(1000);
> + }
> +
> trace_rcu_utilization(TPS("End scheduler-tick"));
> }
>
