Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

Ming Lei
2019-09-05T09:06:36Z

On Wed, Sep 04, 2019 at 07:31:48PM +0200, Daniel Lezcano wrote:
> Hi,
>
> On 04/09/2019 19:07, Bart Van Assche wrote:
> > On 9/3/19 12:50 AM, Daniel Lezcano wrote:
> >> On 03/09/2019 09:28, Ming Lei wrote:
> >>> On Tue, Sep 03, 2019 at 08:40:35AM +0200, Daniel Lezcano wrote:
> >>>> It is a scheduler problem then ?
> >>>
> >>> Scheduler can do nothing if the CPU is taken completely by handling
> >>> interrupt & softirq, so seems not a scheduler problem, IMO.
> >>
> >> Why? If there is a irq pressure on one CPU reducing its capacity, the
> >> scheduler will balance the tasks on another CPU, no?
> >
> > Only if CONFIG_IRQ_TIME_ACCOUNTING has been enabled. However, I don't
> > know any Linux distro that enables that option. That's probably because
> > that option introduces two rdtsc() calls in each interrupt. Given the
> > overhead introduced by this option, I don't think this is the solution
> > Ming is looking for.
>
> Was this overhead reported somewhere ?

The syscall of gettimeofday() calls ktime_get_real_ts64() which finally
calls tk_clock_read() which calls rdtsc too.

But gettimeofday() is often used in fast path, and block IO_STAT needs to
read it too.

>
> > See also irqtime_account_irq() in kernel/sched/cputime.c.
>
> From my POV, this framework could be interesting to detect this situation.

Now we are talking about IRQ_TIME_ACCOUNTING instead of IRQ_TIMINGS, and the
former one could be used to implement the detection. And the only sharing
should be the read of timestamp.

Thanks,
Ming