Re: [PATCH 1/2] sched: Interrupt Aware Scheduler

[Rohit Jain]

On 05/12/2017 12:46 PM, Peter Zijlstra wrote:
> On Fri, May 12, 2017 at 11:04:26AM -0700, Rohit Jain wrote:
> > The patch avoids CPUs which might be considered interrupt-heavy when
> > trying to schedule threads (on the push side) in the system. Interrupt
> > Awareness has only been added into the fair scheduling class.
> >
> > It does so by, using the following algorithm:
> > --------------------------------------------------------------------------
> > 1) When the interrupt is getting processed, the start and the end times
> > are noted for the interrupt on a per-cpu basis.
> IRQ_TIME_ACCOUNTING you mean?

Yes. Exactly

> > 2) On a periodic basis the interrupt load is processed for each run
> > queue and this is mapped in terms of percentage in a global array. The
> > interrupt load for a given CPU is also decayed over time, so that the
> > most recent interrupt load has the biggest contribution in the interrupt
> > load calculations. This would mean the scheduler will try to avoid CPUs
> > (if it can) when scheduling threads which have been recently busy with
> > handling hardware interrupts.
> You mean like like how its already added to rt_avg? Which is then used
> to lower a CPU's capacity.

Right. The only difference I see is that it is not being used on the
enqueue side as of now.

> > 3) Any CPU which lies above the 80th percentile in terms of percentage
> > interrupt load is considered interrupt-heavy.
> >
> > 4) During idle CPU search from the scheduler perspective this
> > information is used to skip CPUs if better are available.
> >
> > 5) If none of the CPUs are better in terms of idleness and interrupt
> > load, then the interrupt-heavy CPU is considered to be the best
> > available CPU.
> I would much rather you work with the EAS people and extend the capacity
> awareness of those code paths. Then, per the existing logic, things
> should just work out.

Did you mean we should use the capacity as a metric on the enqueue side
and not introduce a new metric?

> It doesn't matter how the capacity is lowered, at some point you just
> don't want to put tasks on. It really doesn't matter if that's because
> IRQs, SoftIRQs, (higher priority) Real-Time tasks, thermal throttling or
> anything else.