Re: [PATCH] sched/fair: use dst group while checking imbalance for NUMA balancer

[Mel Gorman <mgorman@suse.de>]

On Mon, Sep 07, 2020 at 07:27:08PM +1200, Barry Song wrote:
> Something is wrong. In find_busiest_group(), we are checking if src has
> higher load, however, in task_numa_find_cpu(), we are checking if dst
> will have higher load after balancing. It seems it is not sensible to
> check src.
> It maybe cause wrong imbalance value, for example, if
> dst_running = env->dst_stats.nr_running + 1 results in 3 or above, and
> src_running = env->src_stats.nr_running - 1 results in 1;
> The current code is thinking imbalance as 0 since src_running is smaller
> than 2.
> This is inconsistent with load balancer.
> 

It checks the conditions if the move was to happen. Have you evaluated
this for a NUMA balancing load and confirmed it a) balances properly and
b) does not increase the scan rate trying to "fix" the problem?

-- 
Mel Gorman
SUSE Labs