[PATCH v2 06/14] uthread: add cooperative multi-tasking interface

[Ilias Apalodimas]

Hi Jerome,

On Tue, 25 Feb 2025 at 18:35, Jerome Forissier
<jerome.forissier at linaro.org> wrote:
>
> Add an new

'a'

>  internal API called uthread (Kconfig symbol: UTHREAD) which
> provides cooperative multi-tasking. The goal is to be able to improve
> the performance of some parts of U-Boot by overlapping lengthy
> operations, and also implement background jobs in the U-Boot shell.
> Each uthread has its own stack allocated on the heap. The default stack
> size is defined by the UTHREAD_STACK_SIZE symbol and is used when
> uthread_create() receives zero for the stack_sz argument.
>
> The implementation is based on context-switching via initjmp()/setjmp()/
> longjmp() and is inspired from barebox threads [1]. A notion of thread
> group helps with dependencies, such as when a thread needs to block
> until a number of other threads have returned.
>
> The name "uthread" comes from "user-space threads" because the
> scheduling happens with no help from a higher privileged mode, contrary
> to more complex models where kernel threads are defined. But the 'u'
> may as well stand for 'U-Boot' since the bootloader may actually be
> running at any privilege level and the notion of user vs. kernel may
> not make much sense in this context.
>
> [1] https://github.com/barebox/barebox/blob/master/common/bthread.c
>
> Signed-off-by: Jerome Forissier <jerome.forissier at linaro.org>
> ---
>  include/uthread.h |  44 ++++++++++++
>  lib/Kconfig       |  21 ++++++
>  lib/Makefile      |   2 +
>  lib/uthread.c     | 178 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 245 insertions(+)
>  create mode 100644 include/uthread.h
>  create mode 100644 lib/uthread.c
>
> diff --git a/include/uthread.h b/include/uthread.h
> new file mode 100644
> index 00000000000..f1f86d210d5
> --- /dev/null
> +++ b/include/uthread.h
> @@ -0,0 +1,44 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Copyright 2025 Linaro Limited
> + */
> +
> +#include <linux/types.h>
> +
> +#ifndef _UTHREAD_H_
> +#define _UTHREAD_H_
> +
> +#ifdef CONFIG_UTHREAD
> +
> +int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz,
> +                  unsigned int grp_id);
> +bool uthread_schedule(void);
> +unsigned int uthread_grp_new_id(void);
> +bool uthread_grp_done(unsigned int grp_id);
> +
> +#else
> +
> +static inline int uthread_create(void (*fn)(void *), void *arg, size_t stack_sz,
> +                                unsigned int grp_id)
> +{
> +       fn(arg);
> +       return 0;
> +}
> +
> +static inline bool uthread_schedule(void)
> +{
> +       return false;
> +}
> +
> +static inline unsigned int uthread_grp_new_id(void)
> +{
> +       return 0;
> +}
> +
> +static inline bool uthread_grp_done(unsigned int grp_id)
> +{
> +       return true;
> +}
> +
> +#endif /* CONFIG_UTHREAD */
> +#endif /* _UTHREAD_H_ */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 1a683dea670..b32740ecbcc 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -1255,6 +1255,27 @@ config PHANDLE_CHECK_SEQ
>           enable this config option to distinguish them using
>           phandles in fdtdec_get_alias_seq() function.
>
> +config UTHREAD
> +       bool "Enable thread support"
> +       depends on HAVE_INITJMP
> +       help
> +         Implement a simple form of cooperative multi-tasking based on
> +         context-switching via initjmp(), setjmp() and longjmp(). The
> +         uthread_ interface enables the main thread of execution to create
> +         one or more secondary threads and schedule them until they all have
> +         returned. At any point a thread may suspend its execution and
> +         schedule another thread, which allows for the efficient multiplexing
> +         of leghthy operations.
> +
> +config UTHREAD_STACK_SIZE
> +       int "Default uthread stack size"
> +       depends on UTHREAD
> +       default 32768
> +       help
> +         The default stak size for uthreads. Each uthread has its own stack.

stack size

> +         When the stack_sz argument to uthread_create() is zero then this
> +         value is used.
> +
>  endmenu
>
>  source "lib/fwu_updates/Kconfig"
> diff --git a/lib/Makefile b/lib/Makefile
> index a7bc2f3134a..3610694de7a 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -164,6 +164,8 @@ obj-$(CONFIG_LIB_ELF) += elf.o
>
>  obj-$(CONFIG_$(PHASE_)SEMIHOSTING) += semihosting.o
>
> +obj-$(CONFIG_UTHREAD) += uthread.o
> +
>  #
>  # Build a fast OID lookup registry from include/linux/oid_registry.h
>  #
> diff --git a/lib/uthread.c b/lib/uthread.c
> new file mode 100644
> index 00000000000..430d1c0de32
> --- /dev/null
> +++ b/lib/uthread.c
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2021 Ahmad Fatoum, Pengutronix
> + * Copyright (C) 2025 Linaro Limited
> + *
> + * An implementation of cooperative multi-tasking inspired from barebox threads
> + * https://github.com/barebox/barebox/blob/master/common/bthread.c
> + */
> +
> +#include <compiler.h>
> +#include <asm/setjmp.h>
> +#include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <malloc.h>
> +#include <stdint.h>
> +#include <uthread.h>
> +
> +static struct uthread {
> +       void (*fn)(void *);
> +       void *arg;
> +       jmp_buf ctx;
> +       void *stack;
> +       bool done;
> +       unsigned int grp_id;
> +       struct list_head list;
> +} main_thread = {
> +       .list = LIST_HEAD_INIT(main_thread.list),
> +};
> +
> +static struct uthread *current = &main_thread;
> +
> +/**
> + * uthread_trampoline() - Call the current thread's entry point then resume the
> + * main thread.
> + *
> + * This is a helper function which is used as the @func argument to the inijmp()

initjump

> + * function, and ultimately invoked via setjmp(). It does not return, but
> + * instead longjmp()'s back to the main thread.
> + */
> +static void __noreturn uthread_trampoline(void)
> +{
> +       struct uthread *curr = current;
> +
> +       curr->fn(curr->arg);
> +       curr->done = true;
> +       current = &main_thread;
> +       longjmp(current->ctx, 1);
> +       /* Not reached */
> +       while (true)
> +               ;
> +}
> +
> +/**
> + * uthread_free() - Free memory used by a uthread object.
> + */
> +static void uthread_free(struct uthread *uthread)
> +{
> +       if (!uthread)
> +               return;
> +       free(uthread->stack);
> +       free(uthread);
> +}
> +
> +/**
> + * uthread_create() - Create a uthread object and make it ready for execution
> + *
> + * Threads are automatically deleted when then return from their entry point.

when they

[...]
> +/**
> + * uthread_grp_new_id() - return a new ID for a thread group
> + *
> + * Return: the new thread group ID
> + */
> +unsigned int uthread_grp_new_id(void)
> +{
> +       static unsigned int id = 0;
> +
> +       return ++id;
> +}

This seems a bit weird. Why do we need this function?

> +
> +/**
> + * uthread_grp_done() - test if all threads in a group are done
> + *
> + * @grp: the ID of the thread group that should be considered
> + * Return: false if the group contains at least one runnable thread (i.e., one
> + * thread which entry point has not returned yet), true otherwise
> + */
> +bool uthread_grp_done(unsigned int grp_id)
> +{
> +       struct uthread *next;
> +
> +       list_for_each_entry(next, &main_thread.list, list) {
> +               if (next->grp_id == grp_id && !next->done)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> --
> 2.43.0
>


Apart from the minor typos and the function that I can't figure out
why we need this look pretty clean

Thnaks
/Ilias