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Date: Wed, 7 Feb 2024 14:18:56 +0800
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From: Bitao Hu <yaoma@linux.alibaba.com>
Subject: Re: [PATCHv5 2/3] watchdog/softlockup: report the most frequent
 interrupts
To: Doug Anderson <dianders@chromium.org>
Cc: akpm@linux-foundation.org, pmladek@suse.com, kernelfans@gmail.com,
 liusong@linux.alibaba.com, linux-kernel@vger.kernel.org,
 yaoma@linux.alibaba.com
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 <20240206095902.56406-3-yaoma@linux.alibaba.com>
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Hi,

On 2024/2/7 05:42, Doug Anderson wrote:
> Hi,
> 
> On Tue, Feb 6, 2024 at 1:59 AM Bitao Hu <yaoma@linux.alibaba.com> wrote:
>>
>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>> index 71d5b6dfa358..26dc1ad86276 100644
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -18,6 +18,9 @@
>>   #include <linux/init.h>
>>   #include <linux/kernel_stat.h>
>>   #include <linux/math64.h>
>> +#include <linux/irq.h>
>> +#include <linux/irqdesc.h>
>> +#include <linux/bitops.h>
> 
> These are still not sorted alphabetically. "irq.h" and "irqdesc.h"
> should go between "init.h" and "kernel_stat.h". "bitops.h" is trickier
> because the existing headers are not quite sorted. Probably the best
> would be to fully sort them. They should end up like this:
> 
> #include <linux/bitops.h>
> #include <linux/cpu.h>
> #include <linux/init.h>
> #include <linux/irq.h>
> #include <linux/irqdesc.h>
> #include <linux/kernel_stat.h>
> #include <linux/kvm_para.h>
> #include <linux/math64.h>
> #include <linux/mm.h>
> #include <linux/module.h>
> #include <linux/nmi.h>
> #include <linux/stop_machine.h>
> #include <linux/sysctl.h>
> #include <linux/tick.h>
> 
> #include <linux/sched/clock.h>
> #include <linux/sched/debug.h>
> #include <linux/sched/isolation.h>
> 
> #include <asm/irq_regs.h>
>
Sorry, I misunderstood your point, thinking that they should only be
added between "init.h" and "module.h". I will arrange them in the
alphabetical order as you suggested.

> 
>> +static void start_counting_irqs(void)
>> +{
>> +       int i;
>> +       struct irq_desc *desc;
>> +       u32 *counts = __this_cpu_read(hardirq_counts);
>> +       int cpu = smp_processor_id();
>> +
>> +       if (!test_bit(cpu, softlockup_hardirq_cpus)) {
> 
> I don't think you need "softlockup_hardirq_cpus", do you? Just read
> "actual_nr_irqs" and see if it's non-zero? ...or read "hardirq_counts"
> and see if it's non-NULL?
Sure, the existing variables are sufficient for making a determination.
And may be I should swap it to make the decision logic here clearer,
like this (untested)?

bool is_counting_started(void)
{
     return !!__this_cpu_read(hardirq_counts);
}

if (!is_counting_started()) {
> 
> 
>> +               /*
>> +                * nr_irqs has the potential to grow at runtime. We should read
>> +                * it and store locally to avoid array out-of-bounds access.
>> +                */
>> +               __this_cpu_write(actual_nr_irqs, nr_irqs);
> 
> nit: IMO store nr_irqs in a local variable to avoid all of the
> "__this_cpu_read" calls everywhere. Then just write it once from your
> local variable.
OK.
> 
> 
>> +               counts = kmalloc_array(__this_cpu_read(actual_nr_irqs),
>> +                                      sizeof(u32),
>> +                                      GFP_ATOMIC);
> 
> should use "kcalloc()" so the array is zeroed. That way if the set of
> non-NULL "desc"s changes between calls you don't end up reading
> uninitialized memory.
OK, I will use "kcalloc()" here.
> 
> 
>> +static void stop_counting_irqs(void)
>> +{
>> +       u32 *counts = __this_cpu_read(hardirq_counts);
>> +       int cpu = smp_processor_id();
>> +
>> +       if (test_bit(cpu, softlockup_hardirq_cpus)) {
>> +               kfree(counts);
>> +               counts = NULL;
>> +               __this_cpu_write(hardirq_counts, counts);
> 
> nit: don't really need to set the local "counts" to NULL. Just:
> 
> __this_cpu_write(hardirq_counts, NULL);
> 
> ...and actually if you take my advice above and get rid of
> "softlockup_hardirq_cpus" then this function just becomes:
> 
>    kfree(__this_cpu_read(hardirq_counts));
>    __this_cpu_write(hardirq_counts, NULL);
> 
> Since kfree() handles when you pass it NULL...
OK.
> 
> 
>> +static void print_irq_counts(void)
>> +{
>> +       int i;
>> +       struct irq_desc *desc;
>> +       u32 counts_diff;
>> +       u32 *counts = __this_cpu_read(hardirq_counts);
>> +       int cpu = smp_processor_id();
>> +       struct irq_counts irq_counts_sorted[NUM_HARDIRQ_REPORT] = {
>> +               {-1, 0}, {-1, 0}, {-1, 0}, {-1, 0},
>> +       };
>> +
>> +       if (test_bit(cpu, softlockup_hardirq_cpus)) {
>> +               for_each_irq_desc(i, desc) {
>> +                       if (!desc)
>> +                               continue;
> 
> The "if" test above isn't needed. The "for_each_irq_desc()" macro
> already checks for NULL.
Thanks for your reminder.
> 
> 
> 
>> +                       /*
>> +                        * We need to bounds-check in case someone on a different CPU
>> +                        * expanded nr_irqs.
>> +                        */
>> +                       if (i < __this_cpu_read(actual_nr_irqs))
>> +                               counts_diff = desc->kstat_irqs ?
>> +                                       *this_cpu_ptr(desc->kstat_irqs) - counts[i] : 0;
>> +                       else
>> +                               counts_diff = desc->kstat_irqs ?
>> +                                       *this_cpu_ptr(desc->kstat_irqs) : 0;
> 
> Why do you need to test "kstat_irqs" for 0?
Although "alloc_desc" wil allocate both "desc" and "kstat_irqs" at the
same time, I refer to the usage of "kstat_irqs" in "show_interrupts"
from kernel/irq/proc.c, where it does perform a check.

kernel/irq/proc.c: show_interrupts
for_each_online_cpu(j)
	seq_printf(p, "%10u ", desc->kstat_irqs ?
				*per_cpu_ptr(desc->kstat_irqs, j) : 0);

I'm not sure why this is necessary, so I copied it as it is. Can we skip
the check in "print_irq_counts"?

> duplicate the math. In other words, I'd expect this (untested):
> 
> if (i < __this_cpu_read(actual_nr_irqs))
>    count = counts[i];
> else
>    count = 0;
> counts_diff = *this_cpu_ptr(desc->kstat_irqs) - count;
Agree.
> 
> I guess I'd also put "__this_cpu_read(actual_nr_irqs)" in a local
> variable like you do with counts...