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Message-ID: <bd69d9dc-0494-4975-bebd-881f3560e659@arm.com>
Date: Mon, 10 Feb 2025 14:15:07 +0000
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>,
Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Daniel Lezcano <daniel.lezcano@...aro.org>,
Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
Aboorva Devarajan <aboorvad@...ux.ibm.com>
Subject: Re: [RFT][PATCH v1 0/5] cpuidle: menu: Avoid discarding useful
information when processing recent idle intervals
On 2/6/25 14:21, Rafael J. Wysocki wrote:
> Hi Everyone,
>
> This work had been triggered by a report that commit 0611a640e60a ("eventpoll:
> prefer kfree_rcu() in __ep_remove()") had caused the critical-jOPS metric of
> the SPECjbb 2015 benchmark [1] to drop by around 50% even though it generally
> reduced kernel overhead. Indeed, it was found during further investigation
> that the total interrupt rate while running the SPECjbb workload had fallen as
> a result of that commit by 55% and the local timer interrupt rate had fallen by
> almost 80%.
>
> That turned out to cause the menu cpuidle governor to select the deepest idle
> state supplied by the cpuidle driver (intel_idle) much more often which added
> significantly more idle state latency to the workload and that led to the
> decrease of the critical-jOPS score.
>
> Interestingly enough, this problem was not visible when the teo cpuidle
> governor was used instead of menu, so it appeared to be specific to the
> latter. CPU wakeup event statistics collected while running the workload
> indicated that the menu governor was effectively ignoring non-timer wakeup
> information and all of its idle state selection decisions appeared to be
> based on timer wakeups only. Thus, it appeared that the reduction of the
> local timer interrupt rate caused the governor to predict a idle duration
> much more often while running the workload and the deepest idle state was
> selected significantly more often as a result of that.
>
> A subsequent inspection of the get_typical_interval() function in the menu
> governor indicated that it might return UINT_MAX too often which then caused
> the governor's decisions to be based entirely on information related to timers.
>
> Generally speaking, UINT_MAX is returned by get_typical_interval() if it
> cannot make a prediction based on the most recent idle intervals data with
> sufficiently high confidence, but at least in some cases this means that
> useful information is not taken into account at all which may lead to
> significant idle state selection mistakes. Moreover, this is not really
> unlikely to happen.
>
> One issue with get_typical_interval() is that, when it eliminates outliers from
> the sample set in an attempt to reduce the standard deviation (and so improve
> the prediction confidence), it does that by dropping high-end samples only,
> while samples at the low end of the set are retained. However, the samples
> at the low end very well may be the outliers and they should be eliminated
> from the sample set instead of the high-end samples. Accordingly, the
> likelihood of making a meaningful idle duration prediction can be improved
> by making it also eliminate low-end samples if they are farther from the
> average than high-end samples. This is done in patch [4/5].
>
> Another issue is that get_typical_interval() gives up after eliminating 1/4
> of the samples if the standard deviation is still not as low as desired (within
> 1/6 of the average or within 20 us if the average is close to 0), but the
> remaining samples in the set still represent useful information at that point
> and discarding them altogether may lead to suboptimal idle state selection.
>
> For instance, the largest idle duration value in the get_typical_interval()
> data set is the maximum idle duration observed recently and it is likely that
> the upcoming idle duration will not exceed it. Therefore, in the absence of
> a better choice, this value can be used as an upper bound on the target
> residency of the idle state to select. Patch [5/5] works along these lines,
> but it takes the maximum data point remaining after the elimination of
> outliers.
>
> The first two patches in the series are straightforward cleanups (in fact,
> the first patch is kind of reversed by patch [4/5], but it is there because
> it can be applied without the latter) and patch [3/5] is a cosmetic change
> made in preparation for the subsequent ones.
>
> This series turns out to restore the SPECjbb critical-jOPS metric on affected
> systems to the level from before commit 0611a640e60a and it also happens to
> increase its max-jOPS metric by around 3%.
>
> For easier reference/testing it is present in the git branch at
>
> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/log/?h=experimental/menu
>
> based on the cpuidle material that went into 6.14-rc1.
>
> If possible, please let me know if it works for you.
>
> Thanks!
>
>
> [1] Link: https://www.spec.org/jbb2015/
5/5 shows significant IO workload improvements (the shorter wakeup scenario is
much more likely to be picked up now).
I don't see a significant regression in idle misses so far, I'll try Android
backports soon and some other system.
Here's a full dump, sorry it's from a different system (rk3588, only two idle
states), apparently eth networking is broken on 6.14-rc1 now on rk3399 :(
For dm-delay 51ms (dm-slow) the command is (8 CPUs)
fio --minimal --time_based --group_reporting --name=fiotest --filename=/dev/mapper/dm-slow --runtime=30s --numjobs=16 --rw=randread --bs=4k --ioengine=psync --iodepth=1 --direct=1
For the rest:
fio --minimal --time_based --name=fiotest --filename=/dev/mmcblk1 --runtime=30s --rw=randread --bs=4k --ioengine=psync --iodepth=1 --direct=1
device gov iter iops idles idle_misses idle_miss_ratio belows aboves
mapper/dm-slow menu 0 307 99648 318 0.003 318 0
mapper/dm-slow menu 1 307 100948 389 0.004 389 0
mapper/dm-slow menu 2 307 99512 380 0.004 380 0
mapper/dm-slow menu 3 307 99212 307 0.003 307 0
mapper/dm-slow menu 4 307 100156 343 0.003 343 0
mapper/dm-slow menu-1 0 307 97434 260 0.003 260 0
mapper/dm-slow menu-1 1 307 94628 324 0.003 324 0
mapper/dm-slow menu-1 2 307 98004 248 0.003 248 0
mapper/dm-slow menu-1 3 307 97524 263 0.003 263 0
mapper/dm-slow menu-1 4 307 97048 304 0.003 304 0
mapper/dm-slow menu-2 0 307 98340 376 0.004 376 0
mapper/dm-slow menu-2 1 307 96246 275 0.003 275 0
mapper/dm-slow menu-2 2 307 96456 317 0.003 317 0
mapper/dm-slow menu-2 3 307 100054 268 0.003 268 0
mapper/dm-slow menu-2 4 307 93378 288 0.003 288 0
mapper/dm-slow menu-3 0 307 95140 303 0.003 303 0
mapper/dm-slow menu-3 1 307 95858 318 0.003 318 0
mapper/dm-slow menu-3 2 307 100528 302 0.003 302 0
mapper/dm-slow menu-3 3 307 98274 311 0.003 311 0
mapper/dm-slow menu-3 4 307 98428 327 0.003 327 0
mapper/dm-slow menu-4 0 307 100340 304 0.003 304 0
mapper/dm-slow menu-4 1 307 101628 359 0.004 359 0
mapper/dm-slow menu-4 2 307 100624 281 0.003 281 0
mapper/dm-slow menu-4 3 307 99824 340 0.003 340 0
mapper/dm-slow menu-4 4 307 98318 290 0.003 290 0
mapper/dm-slow menu-5 0 307 96842 310 0.003 310 0
mapper/dm-slow menu-5 1 307 98884 271 0.003 271 0
mapper/dm-slow menu-5 2 307 99706 259 0.003 259 0
mapper/dm-slow menu-5 3 307 93096 270 0.003 270 0
mapper/dm-slow menu-5 4 307 101590 333 0.003 333 0
mapper/dm-slow menu-m 0 307 94270 297 0.003 297 0
mapper/dm-slow menu-m 1 307 99820 355 0.004 355 0
mapper/dm-slow menu-m 2 307 99284 313 0.003 313 0
mapper/dm-slow menu-m 3 307 99320 288 0.003 288 0
mapper/dm-slow menu-m 4 307 99666 269 0.003 269 0
mmcblk1 menu 0 818 227246 32716 0.144 32716 0
mmcblk1 menu 1 818 252552 33582 0.133 33582 0
mmcblk1 menu 2 825 255822 31958 0.125 31958 0
mmcblk1 menu 3 822 255814 33374 0.130 33374 0
mmcblk1 menu 4 822 253200 33310 0.132 33310 0
mmcblk1 menu-1 0 822 254768 33545 0.132 33545 0
mmcblk1 menu-1 1 819 249476 33289 0.133 33289 0
mmcblk1 menu-1 2 823 256152 32838 0.128 32838 0
mmcblk1 menu-1 3 824 231098 31120 0.135 31120 0
mmcblk1 menu-1 4 820 254590 33189 0.130 33189 0
mmcblk1 menu-2 0 824 256084 32927 0.129 32927 0
mmcblk1 menu-2 1 806 240166 33672 0.140 33672 0
mmcblk1 menu-2 2 808 253178 33963 0.134 33963 0
mmcblk1 menu-2 3 822 240628 32860 0.137 32860 0
mmcblk1 menu-2 4 811 251522 33478 0.133 33478 0
mmcblk1 menu-3 0 810 251914 32477 0.129 32477 0
mmcblk1 menu-3 1 811 253324 32344 0.128 32344 0
mmcblk1 menu-3 2 826 239634 31478 0.131 31478 0
mmcblk1 menu-3 3 811 252462 33810 0.134 33810 0
mmcblk1 menu-3 4 806 231730 33646 0.145 33646 0
mmcblk1 menu-4 0 826 231986 32301 0.139 32301 0
mmcblk1 menu-4 1 821 256988 34290 0.133 34290 0
mmcblk1 menu-4 2 805 247456 35092 0.142 35092 0
mmcblk1 menu-4 3 807 255072 35291 0.138 35291 0
mmcblk1 menu-4 4 808 255076 35222 0.138 35222 0
mmcblk1 menu-5 0 861 308822 26267 0.085 26267 0
mmcblk1 menu-5 1 835 288153 26496 0.092 26496 0
mmcblk1 menu-5 2 841 304148 26916 0.088 26916 0
mmcblk1 menu-5 3 858 304838 26347 0.086 26347 0
mmcblk1 menu-5 4 859 303370 26090 0.086 26090 0
mmcblk1 menu-m 0 811 243486 33215 0.136 33215 0
mmcblk1 menu-m 1 827 256902 32863 0.128 32863 0
mmcblk1 menu-m 2 807 249032 34080 0.137 34080 0
mmcblk1 menu-m 3 809 253537 33718 0.133 33718 0
mmcblk1 menu-m 4 824 241996 32842 0.136 32842 0
mmcblk1 teo 0 874 346720 18326 0.053 18326 0
mmcblk1 teo 1 889 350712 19364 0.055 19364 0
mmcblk1 teo 2 874 341195 19004 0.056 19004 0
mmcblk1 teo 3 870 343718 18770 0.055 18770 0
mmcblk1 teo 4 871 321152 18415 0.057 18415 0
nvme0n1 menu 0 11546 819014 110717 0.135 110717 0
nvme0n1 menu 1 10507 745534 86297 0.116 86297 0
nvme0n1 menu 2 11758 829030 110667 0.133 110667 0
nvme0n1 menu 3 10762 768898 93655 0.122 93655 0
nvme0n1 menu 4 11719 820536 110456 0.135 110456 0
nvme0n1 menu-1 0 11409 811374 111285 0.137 111285 0
nvme0n1 menu-1 1 11432 805208 108621 0.135 108621 0
nvme0n1 menu-1 2 11154 781534 100566 0.129 100566 0
nvme0n1 menu-1 3 10180 724944 73523 0.101 73523 0
nvme0n1 menu-1 4 11667 827804 110505 0.133 110505 0
nvme0n1 menu-2 0 11091 791998 105824 0.134 105824 0
nvme0n1 menu-2 1 10664 748122 90282 0.121 90282 0
nvme0n1 menu-2 2 10921 773806 95668 0.124 95668 0
nvme0n1 menu-2 3 11445 807918 112475 0.139 112475 0
nvme0n1 menu-2 4 10629 761546 90181 0.118 90181 0
nvme0n1 menu-3 0 10330 723824 74813 0.103 74813 0
nvme0n1 menu-3 1 10242 717762 74187 0.103 74187 0
nvme0n1 menu-3 2 10579 754108 86841 0.115 86841 0
nvme0n1 menu-3 3 10161 730416 76722 0.105 76722 0
nvme0n1 menu-3 4 11665 820052 112621 0.137 112621 0
nvme0n1 menu-4 0 11279 789456 106411 0.135 106411 0
nvme0n1 menu-4 1 11095 766714 98036 0.128 98036 0
nvme0n1 menu-4 2 11003 786088 98979 0.126 98979 0
nvme0n1 menu-4 3 10371 746978 77039 0.103 77039 0
nvme0n1 menu-4 4 10761 770218 89958 0.117 89958 0
nvme0n1 menu-5 0 13243 926672 514 0.001 514 0
nvme0n1 menu-5 1 14235 985852 1054 0.001 1054 0
nvme0n1 menu-5 2 13032 911560 506 0.001 506 0
nvme0n1 menu-5 3 13074 917252 691 0.001 691 0
nvme0n1 menu-5 4 13361 933126 466 0.000 466 0
nvme0n1 menu-m 0 10290 739468 73692 0.100 73692 0
nvme0n1 menu-m 1 10647 763144 80430 0.105 80430 0
nvme0n1 menu-m 2 11067 790362 98525 0.125 98525 0
nvme0n1 menu-m 3 11337 806888 102446 0.127 102446 0
nvme0n1 menu-m 4 11519 818128 110233 0.135 110233 0
nvme0n1 teo 0 14267 994532 273 0.000 273 0
nvme0n1 teo 1 13857 965726 395 0.000 395 0
nvme0n1 teo 2 12762 892900 311 0.000 311 0
nvme0n1 teo 3 13056 900172 269 0.000 269 0
nvme0n1 teo 4 13687 956048 240 0.000 240 0
sda menu 0 1943 1044428 162298 0.155 162298 0
sda menu 1 1601 860152 232733 0.271 232733 0
sda menu 2 1947 1089550 154879 0.142 154879 0
sda menu 3 1917 992278 146316 0.147 146316 0
sda menu 4 1706 947224 257686 0.272 257686 0
sda menu-1 0 1981 1109204 174590 0.157 174590 0
sda menu-1 1 1778 989142 271685 0.275 271685 0
sda menu-1 2 1759 955310 252735 0.265 252735 0
sda menu-1 3 1818 985389 180365 0.183 180365 0
sda menu-1 4 1782 915060 247016 0.270 247016 0
sda menu-2 0 1877 959734 181691 0.189 181691 0
sda menu-2 1 1718 961724 262950 0.273 262950 0
sda menu-2 2 1751 949092 259223 0.273 259223 0
sda menu-2 3 1808 1011822 211016 0.209 211016 0
sda menu-2 4 1734 959348 261769 0.273 261769 0
sda menu-3 0 1723 952826 260493 0.273 260493 0
sda menu-3 1 1718 931974 254462 0.273 254462 0
sda menu-3 2 1773 984232 239335 0.243 239335 0
sda menu-3 3 1741 969477 265131 0.273 265131 0
sda menu-3 4 1735 970372 263907 0.272 263907 0
sda menu-4 0 1911 1030290 170538 0.166 170538 0
sda menu-4 1 1769 972168 233029 0.240 233029 0
sda menu-4 2 1737 969896 260880 0.269 260880 0
sda menu-4 3 1738 941298 253874 0.270 253874 0
sda menu-4 4 1701 953710 258250 0.271 258250 0
sda menu-5 0 2463 1349556 26158 0.019 26158 0
sda menu-5 1 2359 1344306 80343 0.060 80343 0
sda menu-5 2 2280 1306554 115670 0.089 115670 0
sda menu-5 3 2573 1420702 4765 0.003 4765 0
sda menu-5 4 2348 1355996 70428 0.052 70428 0
sda menu-m 0 1738 962150 261205 0.271 261205 0
sda menu-m 1 1667 922214 238208 0.258 238208 0
sda menu-m 2 1696 911352 255364 0.280 255364 0
sda menu-m 3 1840 1006556 193333 0.192 193333 0
sda menu-m 4 1681 919693 251029 0.273 251029 0
sda teo 0 2503 1427634 25997 0.018 25997 0
sda teo 1 2424 1401434 35228 0.025 35228 0
sda teo 2 2527 1454382 27546 0.019 27546 0
sda teo 3 2481 1430128 16678 0.012 16678 0
sda teo 4 2589 1481254 13389 0.009 13389 0
nullb0 menu 0 337827 88502 200 0.002 200 0
nullb0 menu 1 337833 87476 188 0.002 188 0
nullb0 menu 2 336378 88862 92 0.001 92 0
nullb0 menu 3 336022 86174 188 0.002 188 0
nullb0 menu 4 335158 87880 156 0.002 156 0
nullb0 menu-1 0 334663 89150 199 0.002 199 0
nullb0 menu-1 1 338526 88184 111 0.001 111 0
nullb0 menu-1 2 336671 89210 211 0.002 211 0
nullb0 menu-1 3 337454 82408 198 0.002 198 0
nullb0 menu-1 4 338256 86994 118 0.001 118 0
nullb0 menu-2 0 336636 82202 165 0.002 165 0
nullb0 menu-2 1 337580 77918 171 0.002 171 0
nullb0 menu-2 2 336260 89198 226 0.003 226 0
nullb0 menu-2 3 338440 85444 215 0.003 215 0
nullb0 menu-2 4 333633 87244 119 0.001 119 0
nullb0 menu-3 0 336890 88096 122 0.001 122 0
nullb0 menu-3 1 335804 68502 79 0.001 79 0
nullb0 menu-3 2 336863 87258 195 0.002 195 0
nullb0 menu-3 3 337091 76452 127 0.002 127 0
nullb0 menu-3 4 336142 80664 83 0.001 83 0
nullb0 menu-4 0 336840 86936 128 0.001 128 0
nullb0 menu-4 1 334498 88792 113 0.001 113 0
nullb0 menu-4 2 336736 88542 104 0.001 104 0
nullb0 menu-4 3 336476 64548 70 0.001 70 0
nullb0 menu-4 4 337513 84776 107 0.001 107 0
nullb0 menu-5 0 338498 89216 135 0.002 135 0
nullb0 menu-5 1 335087 87424 85 0.001 85 0
nullb0 menu-5 2 336965 75456 179 0.002 179 0
nullb0 menu-5 3 337415 88112 114 0.001 114 0
nullb0 menu-5 4 332365 76456 82 0.001 82 0
nullb0 menu-m 0 337718 88018 125 0.001 125 0
nullb0 menu-m 1 337801 86584 164 0.002 164 0
nullb0 menu-m 2 336760 84262 102 0.001 102 0
nullb0 menu-m 3 337524 87902 147 0.002 147 0
nullb0 menu-m 4 333724 87916 117 0.001 117 0
nullb0 teo 0 336215 88312 231 0.003 231 0
nullb0 teo 1 337653 88802 266 0.003 266 0
nullb0 teo 2 337198 87960 234 0.003 234 0
nullb0 teo 3 338716 88516 227 0.003 227 0
nullb0 teo 4 336334 88978 261 0.003 261 0
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