Read: swtch.S and proc.c (focus on the code that switches
between processes, specifically scheduler
and sched
).
Also process creation: sys_fork() and copyproc().
Hand-In Procedure
You are to turn in this homework during lecture. Please write up your answers to the exercises below and hand them in to a 6.828 staff member at the beginning of lecture.
Introduction
In this homework you will investigate how the kernel switches between two processes.
Assignment:
Suppose a process that is running in the kernel
calls sched()
, which ends up jumping
into scheduler()
.
Turn in:
Where is the stack that sched()
executes on?
Turn in:
Where is the stack that scheduler()
executes on?
Turn in:
When sched()
calls swtch()
,
does that call to swtch()
ever return? If so, when?
Now think back to lecture 2 and the invariants
that gcc expects any function, including swtch
, to
maintain. Compare these invariants with what swtch
actually implements, and the state that our kernel maintains in
a struct context
.
Turn in: Could swtch
do less work and still be
correct? Could we reduce the size of a struct context
?
Provide concrete examples if yes, or argue for why not.
Surround the call to swtch()
in scheduler()
with calls
to cprintf()
like this:
cprintf("a"); swtch(&cpu->scheduler, &proc->context); cprintf("b");
Similarly,
surround the call to swtch()
in sched()
with calls
to cprintf()
like this:
cprintf("c"); swtch(&proc->context, cpu->scheduler); cprintf("d");
Rebuild your kernel and boot it on QEMU. With a few exceptions you should see a regular four-character pattern repeated over and over.
Turn in: What is the four-character pattern?
Turn in: The very first characters are ac
. Why does
this happen?
This completes the homework.