gpiolib: Avoid the hotplug performance reduction #6862
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The 6.9 kernel introduced a large patchset [1] designed to make gpiochip usage safe in the presence of potential hotplugging events. The changes included protecting every gpiochip access with a claim of an interlock. Running on a Pi 5 these changes reduce GPIO performance from userspace by around 10%. The penalty would be proportionally higher from kernel, as seen by SPI speed reductions. Patch the gpiolib implementation to remove the protection of gpiochip accesses. By providing alternative implementations of the relevant macros, the changes are localised and therefore easier to verify. See: raspberrypi#6854 [1] https://lwn.net/Articles/960024/ Signed-off-by: Phil Elwell <[email protected]>
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Here are the results of running a performance test on a Pi 5. The test drives a GPIO high and low 1 million times in a tight loop, repeating the process 10 times and calculating the mean and standard deviation of the per-pass execution time (in microseconds): You can see that there is still a small performance regression with the patched kernel, but it has been reduced from ~11% to less than 1%. |
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Many thanks for the patch, downloaded and running with no obvious issues. Testing with 1E6 dsequential samples running a time-stamp, GPIO -> out, 3uSec delay, SPI Read Back on an AD7606 using Python code, priority set as chrt 99 gives the following sample rates :- 6.6.31 = 37.0 kHz 6.12.25 (unpatched) = 34.9k Hz (~6% performance drop from 6.6.31) 6.12.28 (GPIO Patch applied) = 35.5 kHz (~4% performance drop from 6.6.31) Patched performance is clearly an improvement over unpached 6.12.25 thanks, not quite the 1% hit for pure GPIO high-low toggles in a loop, though we are alternating GPIO / SPI read so a slightly more complex situation. Histograms to follow. |
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This seems like a step in the right direction for performance. |
kernel: usb: xhci: set Input Slot Context bit in xhci_fixup_endpoint See: raspberrypi/linux#6850 kernel: configs: Enable more MediaTek drivers See: raspberrypi/linux#6857 kernel: firmware/raspberrypi: raise timeout to 3s See: raspberrypi/linux#6859 kernel: overlays: sc16is75x: Disable spidev0 first See: raspberrypi/linux#6858 kernel: gpiolib: Avoid the hotplug performance reduction See: raspberrypi/linux#6862
kernel: usb: xhci: set Input Slot Context bit in xhci_fixup_endpoint See: raspberrypi/linux#6850 kernel: configs: Enable more MediaTek drivers See: raspberrypi/linux#6857 kernel: firmware/raspberrypi: raise timeout to 3s See: raspberrypi/linux#6859 kernel: overlays: sc16is75x: Disable spidev0 first See: raspberrypi/linux#6858 kernel: gpiolib: Avoid the hotplug performance reduction See: raspberrypi/linux#6862
The 6.9 kernel introduced a large patchset [1] designed to make gpiochip usage safe in the presence of potential hotplugging events. The changes included protecting every gpiochip access with a claim of an interlock.
Running on a Pi 5 these changes reduce GPIO performance from userspace by around 10%. The penalty would be proportionally higher from kernel, as seen by SPI speed reductions.
Patch the gpiolib implementation to remove the protection of gpiochip accesses. By providing alternative implementations of the relevant macros, the changes are localised and therefore easier to verify.
See: #6854
[1] https://lwn.net/Articles/960024/