Re: [PATCH] sched: Resolve sd_idle and first_idle_cpu Catch-22 - v1

[Peter Zijlstra]

On Fri, 2011-02-04 at 13:25 -0800, Venkatesh Pallipadi wrote:
> Consider a system with { [ (A B) (C D) ] [ (E F) (G H) ] },
> () denoting SMT siblings, [] cores on same socket and {} system wide
> Further, A, C and D are idle, B is busy and one of EFGH has excess load.
> 
> With sd_idle logic, a check in rebalance_domains() converts tick
> based load balance requests from CPU A to busy load balance for core
> and above domains (lower rate of balance and higher load_idx).

the if (load_balance())
	idle = CPU_NOT_IDLE;
bit, right?

> With first_idle_cpu logic, when CPU C or D tries to balance across domains
> the logic finds CPU A as first idle CPU in the group and nominates CPU A to
> idle balance across sockets.

Right..

> But, sd_idle above would not allow CPU A to do cross socket idle balance
> as CPU A switches its higher level balancing to busy balance.

Because it fails the sd->flags & SD_SHARE_CPUPOWER test at the beginning
of load_balance() and hence sd_idle will remain 0, right?

I'm just not quite sure how we then end up returning !0 for
load_balance(), both branches returning -1 seem conditional on
SD_SHARE_CPUPOWER but the [ (A B) (C D) ], domain doesn't have that set.

> So, this can result is no cross socket balancing for extended periods.

Which is bad

> The fix here adds additional check to detect sd_idle logic in
> first_idle_cpu code path. We will now nominate (in order or preference):
> * First fully idle CPU
> * First semi-idle CPU
> * First CPU
> 
> Note that this solution works fine for 2 SMT siblings case and won't be
> perfect in picking proper semi-idle in case of more than 2 SMT threads.

All these SMT exceptions make my head hurt, can't we clean that up
instead of making them worse?

Why is SMT treaded differently from say a shared cache? In both cases we
want to spread the load as wide as possible to provide as much of the
resources to the few runnable tasks.

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