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SubjectRe: [PATCH v3 4/4] /proc/stat: Call kstat_irqs_usr() only for active IRQs
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On 01/11/2019 04:02 PM, Thomas Gleixner wrote:
> On Fri, 11 Jan 2019, Matthew Wilcox wrote:
>> On Fri, Jan 11, 2019 at 08:19:33PM +0100, Thomas Gleixner wrote:
>>> On Fri, 11 Jan 2019, Thomas Gleixner wrote:
>>>> --- a/kernel/irq/internals.h
>>>> +++ b/kernel/irq/internals.h
>>>> @@ -246,6 +246,7 @@ static inline void kstat_incr_irqs_this_
>>>> {
>>>> __this_cpu_inc(*desc->kstat_irqs);
>>>> __this_cpu_inc(kstat.irqs_sum);
>>>> + desc->tot_count++;
>>> There is one issue here. True percpu interrupts, like the timer interrupts
>>> on ARM(64), will access that in parallel. But that's not rocket science to
>>> fix.
>> I was wondering about that from an efficiency point of view. Since
>> interrupts are generally targetted to a single CPU, there's no cacheline
>> bouncing to speak of, except for interrupts like TLB shootdown on x86.
>> It might make sense for the percpu interrupts to still sum them at read
>> time, and not sum them at interrupt time.
> Yes, for regular interrupts the stats are properly serialized and the patch
> works just fine. For true percpu interrupts we just can avoid the update of
> tot_count and sum them up as before. The special vectors on x86 are
> accounted separately anyway and not part of the problem here. That
> show_all_irqs() code only deals with device interrupts which are backed by
> irq descriptors. TLB/IPI/.... are different beasts and handled low level in
> the architecture code. Though ARM and some others have these interrupts as
> regular Linux interrupt numbers. That's why we need to care.
>
> Updated untested patch below.
>
> Thanks,
>
> tglx
>
> 8<-------------
>
> fs/proc/stat.c | 28 +++++++++++++++++++++++++---
> include/linux/irqdesc.h | 3 ++-
> kernel/irq/chip.c | 12 ++++++++++--
> kernel/irq/internals.h | 8 +++++++-
> kernel/irq/irqdesc.c | 7 ++++++-
> 5 files changed, 50 insertions(+), 8 deletions(-)
>
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -79,6 +79,30 @@ static u64 get_iowait_time(int cpu)
>
> #endif
>
> +static void show_irq_gap(struct seq_file *p, int gap)
> +{
> + static const char zeros[] = " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0";
> +
> + while (gap > 0) {
> + int inc = min_t(int, gap, ARRAY_SIZE(zeros) / 2);
> +
> + seq_write(p, zeros, 2 * inc);
> + gap -= inc;
> + }
> +}
> +
> +static void show_all_irqs(struct seq_file *p)
> +{
> + int i, next = 0;
> +
> + for_each_active_irq(i) {
> + show_irq_gap(p, i - next);
> + seq_put_decimal_ull(p, " ", kstat_irqs_usr(i));
> + next = i + 1;
> + }
> + show_irq_gap(p, nr_irqs - next);
> +}
> +
> static int show_stat(struct seq_file *p, void *v)
> {
> int i, j;
> @@ -156,9 +180,7 @@ static int show_stat(struct seq_file *p,
> }
> seq_put_decimal_ull(p, "intr ", (unsigned long long)sum);
>
> - /* sum again ? it could be updated? */
> - for_each_irq_nr(j)
> - seq_put_decimal_ull(p, " ", kstat_irqs_usr(j));
> + show_all_irqs(p);
>
> seq_printf(p,
> "\nctxt %llu\n"
> --- a/include/linux/irqdesc.h
> +++ b/include/linux/irqdesc.h
> @@ -65,9 +65,10 @@ struct irq_desc {
> unsigned int core_internal_state__do_not_mess_with_it;
> unsigned int depth; /* nested irq disables */
> unsigned int wake_depth; /* nested wake enables */
> + unsigned int tot_count;
> unsigned int irq_count; /* For detecting broken IRQs */
> - unsigned long last_unhandled; /* Aging timer for unhandled count */
> unsigned int irqs_unhandled;
> + unsigned long last_unhandled; /* Aging timer for unhandled count */
> atomic_t threads_handled;
> int threads_handled_last;
> raw_spinlock_t lock;
> --- a/kernel/irq/chip.c
> +++ b/kernel/irq/chip.c
> @@ -855,7 +855,11 @@ void handle_percpu_irq(struct irq_desc *
> {
> struct irq_chip *chip = irq_desc_get_chip(desc);
>
> - kstat_incr_irqs_this_cpu(desc);
> + /*
> + * PER CPU interrupts are not serialized. Do not touch
> + * desc->tot_count.
> + */
> + __kstat_incr_irqs_this_cpu(desc);
>
> if (chip->irq_ack)
> chip->irq_ack(&desc->irq_data);
> @@ -884,7 +888,11 @@ void handle_percpu_devid_irq(struct irq_
> unsigned int irq = irq_desc_get_irq(desc);
> irqreturn_t res;
>
> - kstat_incr_irqs_this_cpu(desc);
> + /*
> + * PER CPU interrupts are not serialized. Do not touch
> + * desc->tot_count.
> + */
> + __kstat_incr_irqs_this_cpu(desc);
>
> if (chip->irq_ack)
> chip->irq_ack(&desc->irq_data);
> --- a/kernel/irq/internals.h
> +++ b/kernel/irq/internals.h
> @@ -242,12 +242,18 @@ static inline void irq_state_set_masked(
>
> #undef __irqd_to_state
>
> -static inline void kstat_incr_irqs_this_cpu(struct irq_desc *desc)
> +static inline void __kstat_incr_irqs_this_cpu(struct irq_desc *desc)
> {
> __this_cpu_inc(*desc->kstat_irqs);
> __this_cpu_inc(kstat.irqs_sum);
> }
>
> +static inline void kstat_incr_irqs_this_cpu(struct irq_desc *desc)
> +{
> + __kstat_incr_irqs_this_cpu(desc);
> + desc->tot_count++;
> +}
> +
> static inline int irq_desc_get_node(struct irq_desc *desc)
> {
> return irq_common_data_get_node(&desc->irq_common_data);
> --- a/kernel/irq/irqdesc.c
> +++ b/kernel/irq/irqdesc.c
> @@ -119,6 +119,7 @@ static void desc_set_defaults(unsigned i
> desc->depth = 1;
> desc->irq_count = 0;
> desc->irqs_unhandled = 0;
> + desc->tot_count = 0;
> desc->name = NULL;
> desc->owner = owner;
> for_each_possible_cpu(cpu)
> @@ -919,11 +920,15 @@ unsigned int kstat_irqs_cpu(unsigned int
> unsigned int kstat_irqs(unsigned int irq)
> {
> struct irq_desc *desc = irq_to_desc(irq);
> - int cpu;
> unsigned int sum = 0;
> + int cpu;
>
> if (!desc || !desc->kstat_irqs)
> return 0;
> + if (!irq_settings_is_per_cpu_devid(desc) &&
> + !irq_settings_is_per_cpu(desc))
> + return desc->tot_count;
> +
> for_each_possible_cpu(cpu)
> sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
> return sum;
>
>
That looks good to me. Thanks for providing a more simple solution.

BTW, if the percpu IRQ is known at allocation time, maybe we should just
not allocate a percpu count for the corresponding descriptor.

Cheers,
Longman


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