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SubjectRe: [PATCH] kernel/irq/proc: /proc/interrupts irq-off latency reduction
On Thu, 5 Apr 2018, Nicholas Piggin wrote:
> Reading /proc/interrupts shows up as a latency source of several
> hundred microseconds on a 2 socket 176 thread system, because it
> iterates twice over all CPUs under an irq lock pulling in remote
> cachelines, doing lots of seq_printf, etc.
>
> On a 16 socket system with higher latencies to remote cachelines,
> latencies around 10ms would be seen here. irqbalanced periodically
> reads this file, and the latency spikes are noticed on smaller
> systems. Also the file is unprivileged, so it's important to reduce
> this latency.
>
> Avoid holding the lock while iterating over CPUs to get their stats.
> The desc should be protected with irq_lock_sparse().

s/should/_IS_/

>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
> kernel/irq/proc.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
> index e8f374971e37..bbc4004b74bc 100644
> --- a/kernel/irq/proc.c
> +++ b/kernel/irq/proc.c
> @@ -544,17 +544,22 @@ int show_interrupts(struct seq_file *p, void *v)
> if (!desc)
> goto outsparse;
>
> - raw_spin_lock_irqsave(&desc->lock, flags);
> for_each_online_cpu(j)
> any_count |= kstat_irqs_cpu(i, j);
> +
> action = desc->action;

This is a bad idea. The action can vanish between here and the code further
below which derefences it.

You need to take desc->request_mutex for protection. That serializes
against concurrent request/free_irq().

> - if ((!action || irq_desc_is_chained(desc)) && !any_count)
> - goto out;
> + if (!any_count) {
> + raw_spin_lock_irqsave(&desc->lock, flags);
> + if (!action || irq_desc_is_chained(desc))
> + goto out;
> + raw_spin_unlock_irqrestore(&desc->lock, flags);
> + }
>
> seq_printf(p, "%*d: ", prec, i);
> for_each_online_cpu(j)
> seq_printf(p, "%10u ", kstat_irqs_cpu(i, j));
>
> + raw_spin_lock_irqsave(&desc->lock, flags);
> if (desc->irq_data.chip) {
> if (desc->irq_data.chip->irq_print_chip)
> desc->irq_data.chip->irq_print_chip(&desc->irq_data, p);

And if you hold request_mutex you can drop the spinlock right before
printing desc->name.

Thanks,

tglx


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