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0346625cd7
When the genpd governor for CPUs, tries to select the most optimal idle state for a group of CPUs managed in a PM domain, it fails far too often. On a Dragonboard 410c, which is an arm64 based platform with 4 CPUs in one cluster that is using PSCI OS-initiated mode, we can observe that we often fail when trying to enter the selected idle state. This is certainly a suboptimal behaviour that leads to many unnecessary requests being sent to the PSCI FW. A simple dd operation that reads from the eMMC, to generate some IRQs and I/O handling helps us to understand the problem, while also monitoring the rejected counters in debugfs for the corresponding idle states of the genpd in question. Menu governor: cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 1451 437 91 149 0 S1 65194 558 149 172 0 dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500 524288000 bytes (500.0MB) copied, 3.562698 seconds, 140.3MB/s cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 2694 1073 265 892 1 S1 74567 829 561 790 0 The dd completed in ~3.6 seconds and rejects increased with 586. Teo governor: cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 4976 2096 392 1721 2 S1 160661 1893 1309 1904 0 dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500 524288000 bytes (500.0MB) copied, 3.543225 seconds, 141.1MB/s cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 5192 2194 433 1830 2 S1 167677 2891 3184 4729 0 The dd completed in ~3.6 seconds and rejects increased with 1916. The main reason to the above problem is pending IPIs for one of the CPUs that is affected by the idle state that the genpd governor selected. This leads to that the PSCI FW refuses to enter it. To improve the behaviour, let's start to take into account pending IPIs for CPUs in the genpd governor, hence we fallback to use the shallower per CPU idle state. Re-testing with this change shows a significant improved behaviour. - Menu governor: cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 2556 878 19 368 1 S1 69974 596 10 152 0 dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500 524288000 bytes (500.0MB) copied, 3.522010 seconds, 142.0MB/s cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 3360 1320 28 819 1 S1 70168 710 11 267 0 The dd completed in ~3.5 seconds and rejects increased with 10. - Teo governor cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 5145 1861 39 938 1 S1 188887 3117 51 1975 0 dd if=/dev/mmcblk0 of=/dev/null bs=1M count=500 524288000 bytes (500.0MB) copied, 3.653100 seconds, 136.9MB/s cat /sys/kernel/debug/pm_genpd/power-domain-cluster/idle_states State Time Spent(ms) Usage Rejected Above Below S0 5260 1923 42 1002 1 S1 190849 4033 52 2892 0 The dd completed in ~3.7 seconds and rejects increased with 4. Note that, the rejected counters in genpd are also being accumulated in the rejected counters that are managed by cpuidle, yet on a per CPU idle states basis. Comparing these counters before/after this change, through cpuidle's sysfs interface shows the similar improvements. Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.