Re: [PATCH] sched/cputime: Make IRQ time accounting configurable at boot time

[Bart Van Assche]

On 6/16/23 00:45, Peter Zijlstra wrote:
> On Thu, Jun 15, 2023 at 01:37:26PM -0700, Bart Van Assche wrote:
> > Some producers of Android devices want IRQ time accounting enabled while
> > others want IRQ time accounting disabled. Hence, make IRQ time accounting
> > configurable at boot time.
>
> Why would they want this disabled? IRQ time accounting avoids a number
> of issues under high irq/softirq pressure.
>
> Disabling this makes no sense.

This is why disabling IRQ time accounting makes a ton of sense to me:
* If disabling IRQ time accounting would not be useful, there wouldn't
  be a kernel configuration option that controls whether it is enabled
  or disabled - it would be enabled all the time.
* If enabling IRQ time accounting would be essential, all Linux
  distributors would enable it. In the x86 kernels I checked, IRQ time
  accounting is disabled (Debian and openSUSE).
* For x86 there is already a kernel parameter for disabling IRQ time
  accounting (tsc=noirqtime).
* Modern hardware, e.g. UFSHCI 4.0 controllers, supports sending the
  completion interrupt to the CPU core that submitted the I/O. With such
  hardware IRQ overload (100% spent in IRQ handlers and 0% outside IRQ
  handlers) is impossible because the submitter is slowed down by the
  completion interrupts.
* The performance overhead of CONFIG_IRQ_TIME_ACCOUNTING is
  unacceptable. A quick test in an x86 VM shows that enabling
  CONFIG_IRQ_TIME_ACCOUNTING reduces IOPS by 10% (220K -> 200K). On an
  Android setup I measured an IOPS reduction of 40% (100K -> 60K) due
  to CONFIG_IRQ_TIME_ACCOUNTING. This is not acceptable.

Thanks,

Bart.