Re: [RFC PATCH v1] sched/fair: limit load balance redo times at the same sched_domain level

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Jan 25, 2021 at 02:02:58PM +0800, Aubrey Li wrote:
> A long-tail load balance cost is observed on the newly idle path,
> this is caused by a race window between the first nr_running check
> of the busiest runqueue and its nr_running recheck in detach_tasks.
> 
> Before the busiest runqueue is locked, the tasks on the busiest
> runqueue could be pulled by other CPUs and nr_running of the busiest
> runqueu becomes 1, this causes detach_tasks breaks with LBF_ALL_PINNED
> flag set, and triggers load_balance redo at the same sched_domain level.
> 
> In order to find the new busiest sched_group and CPU, load balance will
> recompute and update the various load statistics, which eventually leads
> to the long-tail load balance cost.
> 
> This patch introduces a variable(sched_nr_lb_redo) to limit load balance
> redo times, combined with sysctl_sched_nr_migrate, the max load balance
> cost is reduced from 100+ us to 70+ us, measured on a 4s x86 system with
> 192 logical CPUs.
> 
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>

If redo_max is a constant, why is it not a #define instead of increasing
the size of lb_env?

-- 
Mel Gorman
SUSE Labs