Re: [PATCHv5 1/3] watchdog/softlockup: low-overhead detection of interrupt

[Bitao Hu]

Hi,

On 2024/2/7 05:41, Doug Anderson wrote:
> Hi,
>
> On Tue, Feb 6, 2024 at 1:59 AM Bitao Hu <yaoma@xxxxxxxxxxxxxxxxx> wrote:
> > The following softlockup is caused by interrupt storm, but it cannot be
> > identified from the call tree. Because the call tree is just a snapshot
> > and doesn't fully capture the behavior of the CPU during the soft lockup.
> >    watchdog: BUG: soft lockup - CPU#28 stuck for 23s! [fio:83921]
> >    ...
> >    Call trace:
> >      __do_softirq+0xa0/0x37c
> >      __irq_exit_rcu+0x108/0x140
> >      irq_exit+0x14/0x20
> >      __handle_domain_irq+0x84/0xe0
> >      gic_handle_irq+0x80/0x108
> >      el0_irq_naked+0x50/0x58
> >
> > Therefore，I think it is necessary to report CPU utilization during the
> > softlockup_thresh period (report once every sample_period, for a total
> > of 5 reportings), like this:
> >    watchdog: BUG: soft lockup - CPU#28 stuck for 23s! [fio:83921]
> >    CPU#28 Utilization every 4s during lockup:
> >      #1: 0% system, 0% softirq, 100% hardirq, 0% idle
> >      #2: 0% system, 0% softirq, 100% hardirq, 0% idle
> >      #3: 0% system, 0% softirq, 100% hardirq, 0% idle
> >      #4: 0% system, 0% softirq, 100% hardirq, 0% idle
> >      #5: 0% system, 0% softirq, 100% hardirq, 0% idle
> >    ...
> >
> > This would be helpful in determining whether an interrupt storm has
> > occurred or in identifying the cause of the softlockup. The criteria for
> > determination are as follows:
> >    a. If the hardirq utilization is high, then interrupt storm should be
> >    considered and the root cause cannot be determined from the call tree.
> >    b. If the softirq utilization is high, then we could analyze the call
> >    tree but it may cannot reflect the root cause.
> >    c. If the system utilization is high, then we could analyze the root
> >    cause from the call tree.
> >
> > Signed-off-by: Bitao Hu <yaoma@xxxxxxxxxxxxxxxxx>
> > ---
> >   kernel/watchdog.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 89 insertions(+)
>
> On v4 you got Liu Song's Reviewed-by and I don't think this is
> massively different than v4. I would have expected you to carry the
> tag forward. In any case ,I guess Liu Song can give it again.. >
>
> > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > index 81a8862295d6..71d5b6dfa358 100644
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -16,6 +16,8 @@
> >   #include <linux/cpu.h>
> >   #include <linux/nmi.h>
> >   #include <linux/init.h>
> > +#include <linux/kernel_stat.h>
> > +#include <linux/math64.h>
> >   #include <linux/module.h>
> >   #include <linux/sysctl.h>
> >   #include <linux/tick.h>
> > @@ -333,6 +335,90 @@ __setup("watchdog_thresh=", watchdog_thresh_setup);
> >
> >   static void __lockup_detector_cleanup(void);
> >
> > +#ifdef CONFIG_IRQ_TIME_ACCOUNTING
> > +#define NUM_STATS_GROUPS       5
> > +#define NUM_STATS_PER_GROUP    4
> > +enum stats_per_group {
> > +       STATS_SYSTEM,
> > +       STATS_SOFTIRQ,
> > +       STATS_HARDIRQ,
> > +       STATS_IDLE,
>
> nit: I still would have left "NUM_STATS_PER_GROUP" here instead of as
> a separate #define.
OK.
> > +static void print_cpustat(void)
> > +{
> > +       int i, group;
> > +       u8 tail = __this_cpu_read(cpustat_tail);
>
> Sorry for not noticing before, but why are you using
> "__this_cpu_read()" instead of "this_cpu_read()"? In other words, why
> do you need the double-underscore version everywhere? I don't think
> you do, do you?
I also struggled with which version of the operation to use. The one
without double-underscores provides preemption/interrupt protection,
but in watchdog.c, the version with double-underscores is used. I
analyzed that it is also safe to use the version without
preemption/interrupt protection in my code, so to maintain consistency
with watchdog.c, I ues the version with double-underscores.

Is my approach reasonable? If not, I will switch to using the
non-underscored version.
> > +       u64 sample_period_second = sample_period;
> > +
> > +       do_div(sample_period_second, NSEC_PER_SEC);
> > +       /*
> > +        * We do not want the "watchdog: " prefix on every line,
> > +        * hence we use "printk" instead of "pr_crit".
> > +        */
> > +       printk(KERN_CRIT "CPU#%d Utilization every %llus during lockup:\n",
> > +               smp_processor_id(), sample_period_second);
> > +       for (i = 0; i < NUM_STATS_GROUPS; i++) {
> > +               group = (tail + i) % NUM_STATS_GROUPS;
> > +               printk(KERN_CRIT "\t#%d: %3u%% system,\t%3u%% softirq,\t"
> > +                       "%3u%% hardirq,\t%3u%% idle\n", i+1,
>
> nit: though I don't care too much in this case, I think kernel folks
> slightly prefer "i + 1" instead of "i+1". Running
> "./scripts/checkpatch.pl --strict" will give a warning about this, for
> instance. Actually, "./scripts/checkpatch.pl --strict" has a few extra
> style nits that you could consider fixing.
Thanks for your reminder. I will use "./scripts/checkpatch.pl --strict"
to check and correct these patches.
> > +static void report_cpu_status(void)
> > +{
> > +       print_cpustat();
> > +}
>
> I don't understand why you need the extra wrapper. You didn't have it
> on v3 and I don't see any reason why you introduced it. Ah, I see, in
> the next patch you add something to it. OK, I guess it's fine to
> introduce it here.
Yes, I add this wrapper to prepare for the next patch, to avoid
predeclaring of "print_irq_counts".


> -Doug