Re: [PATCH] sched/fair: update_pick_idlest() Select group with lowest group_util when idle_cpus are equal

[Mel Gorman <mgorman@suse.de>]

On Thu, Jul 02, 2020 at 11:27:52AM +0200, Dietmar Eggemann wrote:
> On 17/06/2020 16:52, Peter Puhov wrote:
> > On Wed, 17 Jun 2020 at 06:50, Valentin Schneider
> > <valentin.schneider@arm.com> wrote:
> >>
> >>
> >> On 16/06/20 17:48, peter.puhov@linaro.org wrote:
> >>> From: Peter Puhov <peter.puhov@linaro.org>
> >>> We tested this patch with following benchmarks:
> >>>   perf bench -f simple sched pipe -l 4000000
> >>>   perf bench -f simple sched messaging -l 30000
> >>>   perf bench -f simple  mem memset -s 3GB -l 15 -f default
> >>>   perf bench -f simple futex wake -s -t 640 -w 1
> >>>   sysbench cpu --threads=8 --cpu-max-prime=10000 run
> >>>   sysbench memory --memory-access-mode=rnd --threads=8 run
> >>>   sysbench threads --threads=8 run
> >>>   sysbench mutex --mutex-num=1 --threads=8 run
> >>>   hackbench --loops 20000
> >>>   hackbench --pipe --threads --loops 20000
> >>>   hackbench --pipe --threads --loops 20000 --datasize 4096
> >>>
> >>> and found some performance improvements in:
> >>>   sysbench threads
> >>>   sysbench mutex
> >>>   perf bench futex wake
> >>> and no regressions in others.
> >>>
> >>
> >> One nitpick for the results of those: condensing them in a table form would
> >> make them more reader-friendly. Perhaps something like:
> >>
> >> | Benchmark        | Metric   | Lower is better? | BASELINE | SERIES | DELTA |
> >> |------------------+----------+------------------+----------+--------+-------|
> >> | Sysbench threads | # events | No               |    45526 |  56567 |  +24% |
> >> | Sysbench mutex   | ...      |                  |          |        |       |
> >>
> >> If you want to include more stats for each benchmark, you could have one table
> >> per (e.g. see [1]) - it'd still be a more readable form (or so I believe).
> 
> Wouldn't Unix Bench's 'execl' and 'spawn' be the ultimate test cases
> for those kind of changes?
> 
> I only see minor improvements with tip/sched/core as base on hikey620
> (Arm64 octa-core).
> 
> 				base		w/ patch
> ./Run spawn -c 8 -i 10		 633.6		 635.1
> 
> ./Run execl -c 8 -i 10		1187.5		1190.7	
> 
> 
> At the end of find_idlest_group(), when comparing local and idlest, it
> is explicitly mentioned that number of idle_cpus is used instead of
> utilization.
> The comparision between potential idle groups and local & idlest group
> should probably follow the same rules.
> 

There is the secondary hazard that what update_sd_pick_busiest returns
is checked later by find_busiest_group when considering the imbalance
between NUMA nodes. This particular patch splits basic communicating tasks
cross-node again at fork time so cross node communication is reintroduced
(same applies if sum_nr_running is used instead of group_util).

-- 
Mel Gorman
SUSE Labs