Re: [patch 05/13] syslets: core, documentation

Randy Dunlap
2007-02-26T21:37:10Z

On Wed, 21 Feb 2007 22:15:21 +0100 Ingo Molnar wrote:

> From: Ingo Molnar <mingo@elte.hu>
>
> Add Documentation/syslet-design.txt with a high-level description
> of the syslet concepts.

Just a few comments & questions...

> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
> ---
> Documentation/syslet-design.txt | 137 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 137 insertions(+)
>
> Index: linux/Documentation/syslet-design.txt
> ===================================================================
> --- /dev/null
> +++ linux/Documentation/syslet-design.txt
> @@ -0,0 +1,137 @@
> +Syslets / asynchronous system calls
> +===================================
> +
> +the core syslet concepts are:
> +
> +The Syslet Atom:
> +----------------
> +
> +The syslet atom is a small, fixed-size (44 bytes on 32-bit) piece of
> +user-space memory, which is the basic unit of execution within the syslet
> +framework. A syslet represents a single system-call and its arguments.
> +In addition it also has condition flags attached to it that allows the
> +construction of larger programs (syslets) from these atoms.
> +
> +Arguments to the system call are implemented via pointers to arguments.
> +This not only increases the flexibility of syslet atoms (multiple syslets
> +can share the same variable for example), but is also an optimization:
> +copy_uatom() will only fetch syscall parameters up until the point it
> +meets the first NULL pointer. 50% of all syscalls have 2 or less
> +parameters (and 90% of all syscalls have 4 or less parameters).
> +
> + [ Note: since the argument array is at the end of the atom, and the
> + kernel will not touch any argument beyond the final NULL one, atoms

I would s/final/first/ since one could put many (unneeded) NULL pointers
in the argument array.

> + might be packed more tightly. (the only special case exception to
> + this rule would be SKIP_TO_NEXT_ON_STOP atoms, where the kernel will
> + jump a full syslet_uatom number of bytes.) ]
...

> +Completion of asynchronous syslets:
> +-----------------------------------
> +
> +Completion of asynchronous syslets is done via the 'completion ring',
> +which is a ringbuffer of syslet atom pointers user user-space memory,

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^^^^ ^^^^ ??

> +provided by user-space as an argument to the sys_async_exec() syscall.
> +The kernel fills in the ringbuffer starting at index 0, and user-space
> +must clear out these pointers. Once the kernel reaches the end of
> +the ring it wraps back to index 0. The kernel will not overwrite
> +non-NULL pointers (but will return an error), user-space has to
> +make sure it completes all events it asked for.

Last sentence is actually 2 sentences, so (e.g.) change the comma
to a semi-colon, xor begin the sentence with "Since".

How does the kernel return an error if the ring buffer is full?
Just a syscall negative error return code?

> +int main(int argc, char *argv[])
> +{
> + unsigned long int fd_out = 1; /* standard output */
> + char *buf = "Hello Syslet World!\n";
> + unsigned long size = strlen(buf);
> + struct syslet_uatom atom, *done;
> +
> + async_head_init();
> +
> + /*
> + * Simple syslet consisting of a single atom:
> + */
> + init_atom(&atom, __NR_sys_write, &fd_out, &buf, &size,
> + NULL, NULL, NULL, NULL, SYSLET_ASYNC, NULL);

init_atom() (was) above. sys_async_exec(), sys_async_wait() are
new syscalls. What are async_head_init() and async_head_exit()?

> + done = sys_async_exec(&atom);
> + if (!done) {
> + sys_async_wait(1);
> + if (completion_ring[curr_ring_idx] == &atom) {
> + completion_ring[curr_ring_idx] = NULL;
> + printf("completed an async syslet atom!\n");
> + }
> + } else {
> + printf("completed an cached syslet atom!\n");
> + }
> +
> + async_head_exit();
> +
> + return 0;
> +}

---
~Randy
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