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

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

On 6/6/24 13:29, Rafael J. Wysocki wrote:
> On Thu, Jun 6, 2024 at 1:59 PM Christian Loehle
> <christian.loehle@arm.com> wrote:
>>
>> 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
> 
> Just to clarify, what does -1.0 mean here?

Good point, I should've mentioned.
This adds up the residencies just from the trace event cpu_idle.
-1 stands for PWR_EVENT_EXIT i.e. non-idle, it's quite useful
if we're talking absolute numbers but idle state ratios.
tldr: the time the CPU isn't in any idle state.

> [snip]