Re: [PATCH V2 06/10] cpufreq: governor: Keep single copy of information common to policy->cpus

[Viresh Kumar <viresh.kumar@linaro.org>]

On 02-12-15, 13:43, Saravana Kannan wrote:
> There's a separate thread where we proposed a fix to deferrable
> timers that are stored globally if they are not CPU bound. That way,
> even if one CPU is up, they get handled. But TGLX had some valid
> concerns with cache thrashing and impact on some network code. So,
> last I heard, he was going to rewrite and fixed the deferrable timer
> problem by having the "orphaned" (because CPU has gone idle)
> deferrable timers being adopted by other CPUs while the original CPU
> is idle.
> 
> Once that's fixed, we just need one timer per policy. Long story
> short, CPU freq is working around a poor API semantic of deferrable
> timers.

There is another idea that I have.

Lets sacrifice idleness of CPU0 (which is already considered as
housekeeping CPU in scheduler) to save us from all the complexity we
have today.

Suppose we have 16 CPUs, with 4 CPUs per policy and hence 4 policies.
- Keep a single delayed-work (non-deferrable) per policy and queue them as:
  queue_work_on(CPU0).
- This will work because any CPU can calculate the load of other CPUs,
  and there is no dependency on the local CPU.
- CPU0 will hence get interrupted, check if the policy->cpus are idle
  or not, and if not, update their frequency (perhaps with an IPI).

Not sure if this will be better performance wise though.

-- 
viresh