Subject: Re: [PATCH 3/3] sched: Aggressive balance in domains whose groups share package resources

From: Peter Zijlstra <[email-masked]>
Date: Mon, 28 Oct 2013 16:53:16 +0100

On Mon, Oct 21, 2013 at 05:15:02PM +0530, Vaidyanathan Srinivasan wrote:
> From: Preeti U Murthy <[email-masked]>
> 
> The current logic in load balance is such that after picking the
> busiest group, the load is attempted to be moved from the busiest cpu
> in that group to the dst_cpu. If the load cannot be moved from the
> busiest cpu to dst_cpu due to either tsk_cpus_allowed mask or cache
> hot tasks, then the dst_cpu is changed to be another idle cpu within
> the dst->grpmask. If even then, the load cannot be moved from the
> busiest cpu, then the source group is changed. The next busiest group
> is found and the above steps are repeated.
> 
> However if the cpus in the group share package resources, then when
> a load movement from the busiest cpu in this group fails as above,
> instead of finding the next busiest group to move load from, find the
> next busiest cpu *within the same group* from which to move load away.
> By doing so, a conscious effort is made during load balancing to keep
> just one cpu busy as much as possible within domains that have
> SHARED_PKG_RESOURCES flag set unless under scenarios of high load.
> Having multiple cpus busy within a domain which share package resource
> could lead to a performance hit.
> 
> A similar scenario arises in active load balancing as well. When the
> current task on the busiest cpu cannot be moved away due to task
> pinning, currently no more attempts at load balancing is made.

> This
> patch checks if the balancing is being done on a group whose cpus
> share package resources. If so, then check if the load balancing can
> be done for other cpus in the same group.

So I absolutely hate this patch... Also I'm not convinced I actually
understand the explanation above.

Furthermore; there is nothing special about spreading tasks for
SHARED_PKG_RESOURCES and special casing that one case is just wrong.

If anything it should be keyed off of SD_PREFER_SIBLING and or
cpu_power.