Re: [PATCH] sched, fair: Allow a per-cpu kthread waking a task to stack on the same CPU

[Mel Gorman]

On Tue, Jan 28, 2020 at 09:08:37PM +0800, Hillf Danton wrote:
> > Functionally it works, it's just slow. There is a cost to migration and
> > a cost to exiting from idle state and ramping up the CPU frequency.
> > 
> Yeah we need to pay some costs but are not they compensated by the ping
> and pong that waker and wakee are happy to play for ten minutes on
> different cpus sharing cache if you have no way to migrate waker?
> 

I could get into it depth but the changelog already mentions the cpufreq
implications and the consequences of round-robining around the machine
as a side-effect of how select_idle_sibling works. The data indicates
that we are not compensated by the migrations.

> Or back to the kworker case, a tradeoff needs to make between making kworker
> able to run on cache-sharing cpus and adding scheduling heuristics. IOW
> is cache affinity the key to the problem?
> 

The kworker is already running on CPUs sharing cache. That is not the
central issue.

-- 
Mel Gorman
SUSE Labs