This series introduces a simple coroutines framework and uses it to speed up the efi_init_obj_list() function. It is just an example to trigger discussions; hopefully other places in U-Boot can benefit from a similar treatment. Suggestions are welcome.
I came up with this idea after analyzing some profiling data (ftrace) taken during the execution of the "printenv -e" command on a Kria KV260 board. When the command is first invoked, efi_init_obj_list() is called which takes a significant amount of time (2.872 seconds). This time can be split into 0.570 s for efi_disks_register(), 0.811 s for efi_tcg2_register() and 1.418 s for efi_tcg2_do_initial_measurement(). All the other child functions are much quicker. Another interesting observation is that a large part of the time is actually spent waiting in __udelay(). More precisely: - For efi_disks_register(): 421 ms / 570 ms = 73.8 % spent in __udelay() - For efi_tcg2_register(): 805 ms / 811 ms = 99.1 % spent in __udelay() - For efi_tcg2_do_initial_measurement(): 1395.025 ms / 1418.372 ms = 98.3 % spent in __udelay()
Given the above data, it is reasonable to think that a nice speedup could be obtained if these functions could somehow be run in parallel. efi_tcg2_do_initial_measurement() unfortunately needs to be excluded because it depends on the first two. But efi_disks_register() and efi_tcg2_register() are clearly independant and are therefore good candidates for concurrent execution.
So, I considered two options:
- Spin up a secondary core to take care of one function while the other one runs on the main core - Introduce some kind of software scheduling
I quickly ruled out the first one for several reasons: initializing a secondary core is typically quite hardware-specific, it would not scale well if more functions than available cores would need to be run in parallel, it would make debugging harder etc. Software scheduling however can be accomplished quite easily, especially since we don't need to consider preemptive multitasking. Coroutines [1] for example can perfectly do the job. They provide a way to save and restore the execution context (registers and stack). Here is how it look like:
static void do_some_work(int v) { int i;
for (i = 0; i < 5; i++) { printf("%d", v); /* Save context and resume main thread */ co_yield(); } }
static void co1(void *arg) { do_some_work(1); /* Mark coroutine as "done" and resume main thread */ co_exit(); }
static void co2(void *arg) { do_some_work(2); co_exit(); }
void main_thread(void) { struct co *co1, *co2;
co1 = co_create(co1, ...); co2 = co_create(co2, ...);
do { printf("A"); if (!co1->done) { /* Invoke or resume first coroutine */ co_resume(co1); } printf("B"); if (!cor21->done) { /* Invoke or resume second coroutine */ co_resume(co2); } } while (!(co1->done && co2->done));
/* At this point, co1 and co2 have both called co_exit() */ }
The above example would print: A1B2A1B2A1B2A1B2A1B2.
- The first commit introduces the coroutine framework, loosely based on libaco [2]. The code was simplified and reformatted to better suit U-Boot. - The second commit modifies efi_init_obj_list() in order to turn efi_disks_register() and efi_tcg2_register() into coroutines when COROUTINES in enabled. On a KV260 board with a SD card inserted, this saves about .6 second (2.2 s instead of 2.8 s). - The third commit applies coroutines to usb_init(), which can significantly reduce the time it takes to scan multiple buses. Tested on arm64 QEMU with 4 XHCI buses: the USB scan takes 2.2 s instead of 5.8 s.
[1] https://en.wikipedia.org/wiki/Coroutine [2] https://github.com/hnes/libaco/
Changes in v2
- Remove x86 and x86_64 arch code since it is untested - Add missing SPDX license tag to arch/arm/cpu/armv8/co_switch.S - Change Apache-2.0 SPDX license tag to "Apache-2.0 OR GPL-2.0-or-later" - Apply coroutines to the USB bus scan in usb_init() Jerome Forissier (3): Introduce coroutines framework efi_loader: optimize efi_init_obj_list() with coroutines usb: scan multiple buses simultaneously with coroutines
arch/arm/cpu/armv8/Makefile | 1 + arch/arm/cpu/armv8/co_switch.S | 36 +++++++ drivers/usb/host/usb-uclass.c | 152 +++++++++++++++++++++++++++++- include/coroutines.h | 130 ++++++++++++++++++++++++++ lib/Kconfig | 10 ++ lib/Makefile | 2 + lib/coroutines.c | 165 +++++++++++++++++++++++++++++++++ lib/efi_loader/efi_setup.c | 113 +++++++++++++++++++++- lib/time.c | 14 ++- 9 files changed, 615 insertions(+), 8 deletions(-) create mode 100644 arch/arm/cpu/armv8/co_switch.S create mode 100644 include/coroutines.h create mode 100644 lib/coroutines.c