Re: [PATCH 0/6] cpuidle: teo: fixes and improvements

[Christian Loehle <christian.loehle@arm.com>]

On 6/6/24 10:00, Christian Loehle wrote:
> Hi all,
> so my investigation into teo lead to the following fixes and
> improvements. Logically they are mostly independent, that's why this
> cover letter is quite short, details are in the patches.
> 
> 1/6:
> As discussed, the utilization threshold is too high, while
> there are benefits in certain workloads, there are quite a few
> regressions, too.
> 2/6:
> Especially with the new util threshold, stopping tick makes little
> sense when utilized is detected, so don't.
> 3/6:
> Particularly with WFI, even if it's the only state, stopping the tick
> has benefits, so enable that in the early bail out.
> 4/6:
> Stopping the tick with 0 cost (if the idle state dictates it) is too
> aggressive IMO, so add 1ms constant cost.
> XXX: This has the issue of now being counted as idle_miss, so we could
> consider adding this to the states, too, but the simple implementation
> of this would have the downside that the cost is added to deeper states
> even if the tick is already off.
> 5/6:
> Remove the 'recent' intercept logic, see my findings in:
> https://lore.kernel.org/lkml/0ce2d536-1125-4df8-9a5b-0d5e389cd8af@arm.com/
> I haven't found a way to salvage this properly, so I removed it.
> The regular intercept seems to decay fast enough to not need this, but
> we could change it if that turns out to be the case.
> 6/6:
> The rest of the intercept logic had issues, too.
> See the commit.
> 
> TODO: add some measurements of common workloads and some simple sanity
> tests (like Vincent described in low utilization workloads if the
> state selection looks reasonable).
> I have some, but more (and more standardized) would be beneficial.
> 
> Happy for anyone to take a look and test as well.
> 
> Some numbers for context:
> Maybe some numbers for context, I'll probably add them to the cover letter.
> 
> Comparing:
> - IO workload (intercept heavy).
> - Timer workload very low utilization (check for deepest state)
> - hackbench (high utilization)
> all on RK3399 with CONFIG_HZ=100.
> target_residencies: 1, 900, 2000
> 
> 1. IO workload, 5 runs, results sorted, in read IOPS.
> fio --minimal --time_based --name=fiotest --filename=/dev/nvme0n1 --runtime=30 --rw=randread --bs=4k --ioengine=psync --iodepth=1 --direct=1 | cut -d \; -f 8;
> 
> teo fixed:
> /dev/nvme0n1
> [4597, 4673, 4727, 4741, 4756]
> /dev/mmcblk2
> [5753, 5832, 5837, 5911, 5949]
> /dev/mmcblk1
> [2059, 2062, 2070, 2071, 2080]
> 
> teo mainline:
> /dev/nvme0n1
> [3793, 3825, 3846, 3865, 3964]
> /dev/mmcblk2
> [3831, 4110, 4154, 4203, 4228]
> /dev/mmcblk1
> [1559, 1564, 1596, 1611, 1618]
> 
> menu:
> /dev/nvme0n1
> [2571, 2630, 2804, 2813, 2917]
> /dev/mmcblk2
> [4181, 4260, 5062, 5260, 5329]
> /dev/mmcblk1
> [1567, 1581, 1585, 1603, 1769]
> 
> 2. Timer workload (through IO for my convenience ;) )
> Results in read IOPS, fio same as above.
> echo "0 2097152 zero" | dmsetup create dm-zeros
> echo "0 2097152 delay /dev/mapper/dm-zeros 0 50" | dmsetup create dm-slow
> (Each IO is delayed by timer of 50ms, should be mostly in state2)
> 
> teo fixed:
> 3269 cpu_idle total
> 48 cpu_idle_miss
> 30 cpu_idle_miss above
> 18 cpu_idle_miss below
> 
> teo mainline:
> 3221 cpu_idle total
> 1269 cpu_idle_miss
> 22 cpu_idle_miss above
> 1247 cpu_idle_miss below
> 
> menu:
> 3433 cpu_idle total
> 114 cpu_idle_miss
> 61 cpu_idle_miss above
> 53 cpu_idle_miss below
> 
> Residencies:

Hmm, maybe actually including them would've been helpful too:
(Over 5s workload, only showing LITTLE cluster)
teo fixed:
idle_state 	
2.0 	4.813378
-1.0 	0.210820
1.0 	0.202778
0.0 	0.062426

teo mainline:
idle_state
1.0 	4.895766
-1.0 	0.098063
0.0 	0.253069

menu:
idle_state
2.0 	4.528356
-1.0 	0.241486
1.0 	0.345829
0.0 	0.202505

> 
> tldr: overall teo fixed spends more time in state2 while having
> fewer idle_miss than menu.
> teo mainline was just way too aggressive at selecting shallow states.
> 
> 3. Hackbench, 5 runs
> for i in $(seq 0 4); do hackbench -l 100 -g 100 ; sleep 1; done
> 
> teo fixed:
> Time: 4.807
> Time: 4.856
> Time: 5.072
> Time: 4.934
> Time: 4.962
> 
> teo mainline:
> Time: 4.945
> Time: 5.021
> Time: 4.927
> Time: 4.923
> Time: 5.137
> 
> menu:
> Time: 4.991
> Time: 4.884
> Time: 4.880
> Time: 4.946
> Time: 4.980
> 
> tldr: all comparable, teo mainline slightly worse
> 
> Kind Regards,
> Christian
> 
> Christian Loehle (6):
>   cpuidle: teo: Increase util-threshold
>   cpuidle: teo: Don't stop tick on utilized
>   cpuidle: teo: Don't always stop tick on one state
>   cpuidle: teo: Increase minimum time to stop tick
>   cpuidle: teo: Remove recent intercepts metric
>   cpuidle: teo: Don't count non-existent intercepts
> 
>  drivers/cpuidle/governors/teo.c | 121 +++++++++++++-------------------
>  1 file changed, 48 insertions(+), 73 deletions(-)
> 
> --
> 2.34.1
>