1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
|
<html>
<head>
<title>Lab: Alarm and uthread</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: Alarm and uthread</h1>
This lab will familiarize you with the implementation of system calls
and switching between threads of execution. In particular, you will
implement new system calls (<tt>sigalarm</tt> and <tt>sigreturn</tt>)
and switching between threads in a user-level thread package.
<h2>Warmup: RISC-V assembly</h2>
<p>For this lab it will be important to understand a bit of RISC-V assembly.
<p>Add a file user/call.c with the following content, modify the
Makefile to add the program to the user programs, and compile (make
fs.img). The Makefile also produces a binary and a readable
assembly a version of the program in the file user/call.asm.
<pre>
#include "kernel/param.h"
#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"
int g(int x) {
return x+3;
}
int f(int x) {
return g(x);
}
void main(void) {
printf(1, "%d %d\n", f(8)+1, 13);
exit();
}
</pre>
<p>Read through user/call.asm and understand it. The instruction manual
for RISC-V is in the doc directory (doc/riscv-spec-v2.2.pdf). Here
are some questions that you should answer for yourself:
<ul>
<li>Which registers contain arguments to functions? Which
register holds 13 in the call to <tt>printf</tt>? Which register
holds the second argument? Which register holds the third one? Etc.
<li>Where is the function call to <tt>f</tt> from main? Where
is the call to <tt>g</tt>?
(Hint: the compiler may inline functions.)
<li>At what address is the function <tt>printf</tt> located?
<li>What value is in the register <tt>ra</tt> just after the <tt>jalr</tt>
to <tt>printf</tt> in <tt>main</tt>?
</ul>
<h2>Warmup: system call tracing</h2>
<p>In this exercise you will modify the xv6 kernel to print out a line
for each system call invocation. It is enough to print the name of the
system call and the return value; you don't need to print the system
call arguments.
<p>
When you're done, you should see output like this when booting
xv6:
<pre>
...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
write -> 1
</pre>
<p>
That's init forking and execing sh, sh making sure only two file descriptors are
open, and sh writing the $ prompt. (Note: the output of the shell and the
system call trace are intermixed, because the shell uses the write syscall to
print its output.)
<p> Hint: modify the syscall() function in kernel/syscall.c.
<p>Run the xv6 programs you wrote in earlier labs and inspect the system call
trace. Are there many system calls? Which system calls correspond
to code in the applications you wrote?
<p>Optional: print the system call arguments.
<h2>Alarm</h2>
<p>
In this exercise you'll add a feature to xv6 that periodically alerts
a process as it uses CPU time. This might be useful for compute-bound
processes that want to limit how much CPU time they chew up, or for
processes that want to compute but also want to take some periodic
action. More generally, you'll be implementing a primitive form of
user-level interrupt/fault handlers; you could use something similar
to handle page faults in the application, for example.
<p>
You should add a new <tt>sigalarm(interval, handler)</tt> system call.
If an application calls <tt>sigalarm(n, fn)</tt>, then after every
<tt>n</tt> "ticks" of CPU time that the program consumes, the kernel
should cause application function
<tt>fn</tt> to be called. When <tt>fn</tt> returns, the application
should resume where it left off. A tick is a fairly arbitrary unit of
time in xv6, determined by how often a hardware timer generates
interrupts.
<p>
You should put the following test program in <tt>user/alarmtest.c</tt>:
<pre>
#include "kernel/param.h"
#include "kernel/types.h"
#include "kernel/stat.h"
#include "kernel/riscv.h"
#include "user/user.h"
void test0();
void test1();
void periodic();
int
main(int argc, char *argv[])
{
test0();
test1();
exit();
}
void test0()
{
int i;
printf(1, "test0 start\n");
sigalarm(2, periodic);
for(i = 0; i < 1000*500000; i++){
if((i % 250000) == 0)
write(2, ".", 1);
}
sigalarm(0, 0);
printf(1, "test0 done\n");
}
void
periodic()
{
printf(1, "alarm!\n");
}
void __attribute__ ((noinline)) foo(int i, int *j) {
if((i % 2500000) == 0) {
write(2, ".", 1);
}
*j += 1;
}
void test1() {
int i;
int j;
printf(1, "test1 start\n");
j = 0;
sigalarm(2, periodic);
for(i = 0; i < 1000*500000; i++){
foo(i, &j);
}
if(i != j) {
printf(2, "i %d should = j %d\n", i, j);
exit();
}
printf(1, "test1 done\n");
}
</pre>
The program calls <tt>sigalarm(2, periodic1)</tt> in <tt>test0</tt> to
ask the kernel to force a call to <tt>periodic()</tt> every 2 ticks,
and then spins for a while.
You can see the assembly
code for alarmtest in user/alarmtest.asm, which may be handy
for debugging.
When you've finished the lab,
<tt>alarmtest</tt> should produce output like this:
<pre>
$ alarmtest
test0 start
...................................................alarm!
.............................................................alarm!
(repeated many times)
test0 done
test1 start
..alarm!
..alarm!
..alarm!
(repeated many times)
test1 done
$
</pre>
<p>
At first, however, you'll see that alarmtest only prints periods,
and doesn't print "alarm!".
<p>The main challenge will be to arrange that the handler is invoked
when the process's alarm interval expires. You'll need to modify
usertrap() in kernel/trap.c so that when a
process's alarm interval expires, the process executes
the handler. How can you do that? You will need to understand
how system calls work (i.e., the code in kernel/trampoline.S
and kernel/trap.c). Which register contains the address to which
system calls return?
<p>Your solution will be only a few lines of code, but it may be tricky to
get it right.
<h3>test0: invoke handler</h3>
<p>Get started by modifying the kernel to jump to the alarm handler in
user space, which will cause test0 to print "alarm!". Don't worry yet
what happens after the "alarm!" output; it's OK for now if your
program crashes after printing "alarm!". Here are some hints:
<ul>
<li>You'll need to modify the Makefile to cause <tt>alarmtest.c</tt>
to be compiled as an xv6 user program.
<li>The right declaration to put in <tt>user/user.h</tt> is:
<pre>
int sigalarm(int ticks, void (*handler)());
</pre>
<li>Update user/sys.pl (which generates user/usys.S),
kernel/syscall.h, and kernel/syscall.c
to allow <tt>alarmtest</tt> to invoke the sigalarm system
call.
<li>Your <tt>sys_sigalarm()</tt> should store the alarm interval and
the pointer to the handler function in new fields in the <tt>proc</tt>
structure, defined in <tt>kernel/proc.h</tt>.
<li>You'll need to keep track of how many ticks have passed since the
last call (or are left until the next call) to a process's alarm
handler; you'll need a new field in <tt>struct proc</tt> for this
too. You can initialize <tt>proc</tt> fields in <tt>allocproc()</tt>
in <tt>proc.c</tt>.
<li>Every tick, the hardware clock forces an interrupt, which is handled
in <tt>usertrap()</tt>; you should add some code here.
<li>You only want to manipulate a process's alarm ticks if there's a a
timer interrupt; you want something like
<pre>
if(which_dev == 2) ...
</pre>
<li>Only invoke the alarm function if the process has a
timer outstanding. Note that the address of the user's alarm
function might be 0 (e.g., in alarmtest.asm, <tt>periodic</tt> is at
address 0).
<li>It will be easier to look at traps with gdb if you tell qemu to
use only one CPU, which you can do by running
<pre>
make CPUS=1 qemu
</pre>
<li>You've succeeded if alarmtest prints "alarm!".
</ul>
<h3>test1(): resume interrupted code</h3>
Chances are that alarmtest crashes at some point after it prints
"alarm!". Depending on how your solution works, that point may be in
test0, or it may be in test1. Crashes are likely caused
by the alarm handler (<tt>periodic</tt> in alarmtest.c) returning
to the wrong point in the user program.
<p>
Your job now is to ensure that, when the alarm handler is done,
control returns to
the instruction at which the user program was originally
interrupted by the timer interrupt. You must also ensure that
the register contents are restored to values they held
at the time of the interrupt, so that the user program
can continue undisturbed after the alarm.
<p>Your solution is likely to require you to save and restore
registers---what registers do you need to save and restore to resume
the interrupted code correctly? (Hint: it will be many).
Several approaches are possible; one convenient plan is to add another
system call <tt>sigreturn</tt> that the user-space alarm handler calls when it is
done, and which restores registers and returns to the original
interrupted user instruction.
Some hints:
<ul>
<li>Add a new <tt>sigreturn</tt> system call.
<li>Have <tt>usertrap</tt> save enough state in
<tt>struct proc</tt> when the timer goes off
that <tt>sigreturn</tt> can correctly return to the
interrupted user code.
<li>Prevent re-entrant calls to the handler----if a handler hasn't
returned yet, the kernel shouldn't call it again.
</ul>
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
make sure you didn't break any other parts of the kernel.
<h2>Uthread: switching between threads</h2>
<p>Download <a href="uthread.c">uthread.c</a> and <a
href="uthread_switch.S">uthread_switch.S</a> into your xv6 directory.
Make sure <tt>uthread_switch.S</tt> ends with <tt>.S</tt>, not
<tt>.s</tt>. Add the
following rule to the xv6 Makefile after the _forktest rule:
<pre>
$U/_uthread: $U/uthread.o $U/uthread_switch.o
$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_uthread $U/uthread.o $U/uthread_switch.o $(ULIB)
$(OBJDUMP) -S $U/_uthread > $U/uthread.asm
</pre>
Make sure that the blank space at the start of each line is a tab,
not spaces.
<p>
Add <tt>_uthread</tt> in the Makefile to the list of user programs defined by UPROGS.
<p>Run xv6, then run <tt>uthread</tt> from the xv6 shell. The xv6 kernel will print an error message about <tt>uthread</tt> encountering a page fault.
<p>Your job is to complete <tt>uthread_switch.S</tt>, so that you see output similar to
this (make sure to run with CPUS=1):
<pre>
~/classes/6828/xv6$ make CPUS=1 qemu
...
$ uthread
my thread running
my thread 0x0000000000002A30
my thread running
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
my thread 0x0000000000004A40
my thread 0x0000000000002A30
...
my thread 0x0000000000002A88
my thread 0x0000000000004A98
my thread: exit
my thread: exit
thread_schedule: no runnable threads
$
</pre>
<p><tt>uthread</tt> creates two threads and switches back and forth between
them. Each thread prints "my thread ..." and then yields to give the other
thread a chance to run.
<p>To observe the above output, you need to complete <tt>uthread_switch.S</tt>, but before
jumping into <tt>uthread_switch.S</tt>, first understand how <tt>uthread.c</tt>
uses <tt>uthread_switch</tt>. <tt>uthread.c</tt> has two global variables
<tt>current_thread</tt> and <tt>next_thread</tt>. Each is a pointer to a
<tt>thread</tt> structure. The thread structure has a stack for a thread and a
saved stack pointer (<tt>sp</tt>, which points into the thread's stack). The
job of <tt>uthread_switch</tt> is to save the current thread state into the
structure pointed to by <tt>current_thread</tt>, restore <tt>next_thread</tt>'s
state, and make <tt>current_thread</tt> point to where <tt>next_thread</tt> was
pointing to, so that when <tt>uthread_switch</tt> returns <tt>next_thread</tt>
is running and is the <tt>current_thread</tt>.
<p>You should study <tt>thread_create</tt>, which sets up the initial stack for
a new thread. It provides hints about what <tt>uthread_switch</tt> should do.
Note that <tt>thread_create</tt> simulates saving all callee-save registers
on a new thread's stack.
<p>To write the assembly in <tt>thread_switch</tt>, you need to know how the C
compiler lays out <tt>struct thread</tt> in memory, which is as
follows:
<pre>
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- current_thread
......
......
--------------------
| 4 bytes for state|
--------------------
| stack size bytes |
| for stack |
--------------------
| 8 bytes for sp |
-------------------- <--- next_thread
</pre>
The variables <tt>&next_thread</tt> and <tt>¤t_thread</tt> each
contain the address of a pointer to <tt>struct thread</tt>, and are
passed to <tt>thread_switch</tt>. The following fragment of assembly
will be useful:
<pre>
ld t0, 0(a0)
sd sp, 0(t0)
</pre>
This saves <tt>sp</tt> in <tt>current_thread->sp</tt>. This works because
<tt>sp</tt> is at
offset 0 in the struct.
You can study the assembly the compiler generates for
<tt>uthread.c</tt> by looking at <tt>uthread.asm</tt>.
<p>To test your code it might be helpful to single step through your
<tt>uthread_switch</tt> using <tt>riscv64-linux-gnu-gdb</tt>. You can get started in this way:
<pre>
(gdb) file user/_uthread
Reading symbols from user/_uthread...
(gdb) b *0x230
</pre>
0x230 is the address of uthread_switch (see uthread.asm). When you
compile it may be at a different address, so check uthread_asm.
You may also be able to type "b uthread_switch". <b>XXX This doesn't work
for me; why?</b>
<p>The breakpoint may (or may not) be triggered before you even run
<tt>uthread</tt>. How could that happen?
<p>Once your xv6 shell runs, type "uthread", and gdb will break at
<tt>thread_switch</tt>. Now you can type commands like the following to inspect
the state of <tt>uthread</tt>:
<pre>
(gdb) p/x *next_thread
$1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}
</pre>
What address is <tt>0x168</tt>, which sits on the bottom of the stack
of <tt>next_thread</tt>?
With "x", you can examine the content of a memory location
<pre>
(gdb) x/x next_thread->sp
0x4a28 <all_thread+16304>: 0x00000168
</pre>
Why does that print <tt>0x168</tt>?
<h3>Optional challenges</h3>
<p>The user-level thread package interacts badly with the operating system in
several ways. For example, if one user-level thread blocks in a system call,
another user-level thread won't run, because the user-level threads scheduler
doesn't know that one of its threads has been descheduled by the xv6 scheduler. As
another example, two user-level threads will not run concurrently on different
cores, because the xv6 scheduler isn't aware that there are multiple
threads that could run in parallel. Note that if two user-level threads were to
run truly in parallel, this implementation won't work because of several races
(e.g., two threads on different processors could call <tt>thread_schedule</tt>
concurrently, select the same runnable thread, and both run it on different
processors.)
<p>There are several ways of addressing these problems. One is
using <a href="http://en.wikipedia.org/wiki/Scheduler_activations">scheduler
activations</a> and another is to use one kernel thread per
user-level thread (as Linux kernels do). Implement one of these ways
in xv6. This is not easy to get right; for example, you will need to
implement TLB shootdown when updating a page table for a
multithreaded user process.
<p>Add locks, condition variables, barriers,
etc. to your thread package.
</body>
</html>
|
