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Subject[patch 05/12] syslets: core, documentation
From: Ingo Molnar <>

Add Documentation/syslet-design.txt with a high-level description
of the syslet concepts.

Signed-off-by: Ingo Molnar <>
Signed-off-by: Arjan van de Ven <>
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
+started by Ingo Molnar <>
+The goal of the syslet subsystem is to allow user-space to execute
+arbitrary system calls asynchronously. It does so by allowing user-space
+to execute "syslets" which are small scriptlets that the kernel can execute
+both securely and asynchronously without having to exit to user-space.
+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 first NULL one, atoms
+ 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.) ]
+The Syslet:
+A syslet is a program, represented by a graph of syslet atoms. The
+syslet atoms are chained to each other either via the atom->next pointer,
+or via the SYSLET_SKIP_TO_NEXT_ON_STOP flag.
+Running Syslets:
+Syslets can be run via the sys_async_exec() system call, which takes
+the first atom of the syslet as an argument. The kernel does not need
+to be told about the other atoms - it will fetch them on the fly as
+execution goes forward.
+A syslet might either be executed 'cached', or it might generate a
+'Cached' syslet execution means that the whole syslet was executed
+without blocking. The system-call returns the submitted atom's address
+in this case.
+If a syslet blocks while the kernel executes a system-call embedded in
+one of its atoms, the kernel will keep working on that syscall in
+parallel, but it immediately returns to user-space with a NULL pointer,
+so the submitting task can submit other syslets.
+Completion of asynchronous syslets:
+Completion of asynchronous syslets is done via the 'completion ring',
+which is a ringbuffer of syslet atom pointers in 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), and thus user-space has
+to make sure it completes all events it asked for.
+Waiting for completions:
+Syslet completions can be waited for via the sys_async_wait()
+system call - which takes the number of events it should wait for as
+a parameter. This system call will also return if the number of
+pending events goes down to zero.
+Sample Hello World syslet code:
+ * Set up a syslet atom:
+ */
+static void
+init_atom(struct syslet_uatom *atom, int nr,
+ void *arg_ptr0, void *arg_ptr1, void *arg_ptr2,
+ void *arg_ptr3, void *arg_ptr4, void *arg_ptr5,
+ void *ret_ptr, unsigned long flags, struct syslet_uatom *next)
+ atom->nr = nr;
+ atom->arg_ptr[0] = arg_ptr0;
+ atom->arg_ptr[1] = arg_ptr1;
+ atom->arg_ptr[2] = arg_ptr2;
+ atom->arg_ptr[3] = arg_ptr3;
+ atom->arg_ptr[4] = arg_ptr4;
+ atom->arg_ptr[5] = arg_ptr5;
+ atom->ret_ptr = ret_ptr;
+ atom->flags = flags;
+ atom->next = next;
+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,
+ 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;
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