-*- text -*-
-From: "Waqar Mohsin" <wmohsin@gmail.com>
-Subject: 3 questions about switch_threads() in switch.S
-To: blp@cs.stanford.edu, joshwise@stanford.edu
-Date: Fri, 3 Mar 2006 17:09:21 -0800
+* Bochs is not fully reproducible.
-QUESTION 1
-
-In the section
-
- # Save current stack pointer to old thread's stack, if any.
- movl SWITCH_CUR(%esp), %eax
- test %eax, %eax
- jz 1f
- movl %esp, (%eax,%edx,1)
-1:
+Godmar says:
- # Restore stack pointer from new thread's stack.
- movl SWITCH_NEXT(%esp), %ecx
- movl (%ecx,%edx,1), %esp
+- In Project 2, we're missing tests that pass arguments to system calls
+that span multiple pages, where some are mapped and some are not.
+An implementation that only checks the first page, rather than all pages
+that can be touched during a call to read()/write() passes all tests.
-why are we saving the current stack pointer only if the "cur" thread pointer
-is non-NULL ? Isn't it gauranteed to be non-NULL because switch_threads() is
-only called form schedule(), where we have
+- In Project 2, we're missing a test that would fail if they assumed
+that contiguous user-virtual addresses are laid out contiguously
+in memory. The loading code should ensure that non-contiguous
+physical pages are allocated for the data segment (at least.)
- struct thread *cur = running_thread ();
+- Need some tests that test that illegal accesses lead to process
+termination. I have written some, will add them. In P2, obviously,
+this would require that the students break this functionality since
+the page directory is initialized for them, still it would be good
+to have.
-which should always be non-NULL (given the way kernel pool is laid out).
+- There does not appear to be a test that checks that they close all
+fd's on exit. Idea: add statistics & self-diagnostics code to palloc.c
+and malloc.c. Self-diagnostics code could be used for debugging.
+The statistics code would report how much kernel memory is free.
+Add a system call "get_kernel_memory_information". User programs
+could engage in a variety of activities and notice leaks by checking
+the kernel memory statistics.
-QUESTION 2
+---
- # This stack frame must match the one set up by thread_create().
- pushl %ebx
- pushl %ebp
- pushl %esi
- pushl %edi
+From: "Godmar Back" <godmar@gmail.com>
+Subject: priority donation tests
+To: "Ben Pfaff" <blp@cs.stanford.edu>
+Date: Fri, 3 Mar 2006 11:02:08 -0500
+
+Ben,
-I find the comment confusing. thread_create() is a special case: the set of
-registers popped from switch_threads stack frame for a newly created thread
-are all zero, so their order shouldn't dictate the order above.
+it seems the priority donation tests are somewhat incomplete and allow
+incorrect implementations to pass with a perfect score.
-I think all that matters is that the order of pops at the end of
-switch_threads() is the opposite of the pushes at the beginning (as shown
-above).
+We are seeing the following wrong implementations pass all tests:
-QUESTION 3
+- Implementations that assume locks are released in the opposite order
+in which they're acquired. The students implement this by
+popping/pushing on the donation list.
-Is it true that struct switch_threads_frame does NOT strictly require
+- Implementations that assume that the priority of a thread waiting on
+a semaphore or condition variable cannot change between when the
+thread was blocked and when it is unblocked. The students implement
+this by doing an insert into an ordered list on block, rather than
+picking the maximum thread on unblock.
- struct thread *cur; /* 20: switch_threads()'s CUR argument. */
- struct thread *next; /* 24: switch_threads()'s NEXT argument. */
-at the end ?
+Neither of these two cases is detected; do you currently check for
+these mistakes manually?
-When a newly created thread's stack pointer is installed in switch_threads(),
-all we do is pop the saved registers and return to switch_entry() which pops
-off and discards the above two simulated (and not used) arguments to
-switch_threads().
+I wrote a test that checks for the first case; it is here:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple-2.patch
-If we remove these two from struct switch_threads_frame and don't do a
+[...]
- # Discard switch_threads() arguments.
- addl $8, %esp
-in switch_entry(), things should still work. Am I right ?
+I also wrote a test case for the second scenario:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-sema.c
+http://people.cs.vt.edu/~gback/pintos/priority-donate-sema.ck
-Thanks
-Waqar
+I put the other tests up here:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple2.c
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple2.ck
From: "Godmar Back" <godmar@gmail.com>
-Subject: thread_yield in irq handler
+Subject: multiple threads waking up at same clock tick
To: "Ben Pfaff" <blp@cs.stanford.edu>
-Date: Wed, 22 Feb 2006 22:18:50 -0500
+Date: Wed, 1 Mar 2006 08:14:47 -0500
-Ben,
+Greg Benson points out another potential TODO item for P1.
-you write in your Tour of Pintos:
+----
+One thing I recall:
-"Second, an interrupt handler must not call any function that can
-sleep, which rules out thread_yield(), lock_acquire(), and many
-others. This is because external interrupts use space on the stack of
-the kernel thread that was running at the time the interrupt occurred.
-If the interrupt handler tried to sleep and that thread resumed, then
-the two uses of the single stack would interfere, which cannot be
-allowed."
+The alarm tests do not test to see if multiple threads are woken up if
+their timers have expired. That is, students can write a solution
+that just wakes up the first thread on the sleep queue rather than
+check for additional threads. Of course, the next thread will be
+woken up on the next tick. Also, this might be hard to test.
-Is the last sentence really true?
+---
+Way to test this: (from Godmar Back)
-I thought the reason that you couldn't sleep is that you would put
-effectively a random thread/process to sleep, but I don't think it
-would cause problems with the kernel stack. After all, it doesn't
-cause this problem if you call thread_yield at the end of
-intr_handler(), so why would it cause this problem earlier.
+Thread A with high priority spins until 'ticks' changes, then calls to
+timer_sleep(X), Thread B with lower priority is then resumed, calls
+set_priority to make its priority equal to that of thread A, then
+calls timer_sleep(X), all of that before the next clock interrupt
+arrives.
-As for thread_yield(), my understanding is that the reason it's called
-at the end is to ensure it's done after the interrupt is acknowledged,
-which you can't do until the end because Pintos doesn't handle nested
-interrupts.
+On wakeup, each thread records wake-up time and calls yield
+immediately, forcing the scheduler to switch to the other
+equal-priority thread. Both wake-up times must be the same (and match
+the planned wake-up time.)
- - Godmar
+PS:
+I actually tested it and it's hard to pass with the current ips setting.
+The bounds on how quickly a thread would need to be able to return after
+sleep appear too tight. Need another idea.
From: "Godmar Back" <godmar@gmail.com>
* Add FS persistence test(s).
-* lock_acquire(), lock_release() don't need additional intr_dis/enable
- calls, because the semaphore protects lock->holder.
-
-
-
* process_death test needs improvement
* Internal tests.
. opendir/readdir/closedir
. everything needed for getcwd()
+
+To add partition support:
+
+- Find four partition types that are more or less unused and choose to
+ use them for Pintos. (This is implemented.)
+
+- Bootloader reads partition tables of all BIOS devices to find the
+ first that has the "Pintos kernel" partition type. (This is
+ implemented.) Ideally the bootloader would make sure there is
+ exactly one such partition, but I didn't implement that yet.
+
+- Bootloader reads kernel into memory at 1 MB using BIOS calls. (This
+ is implemented.)
+
+- Kernel arguments have to go into a separate sector because the
+ bootloader is otherwise too big to fit now? (I don't recall if I
+ did anything about this.)
+
+- Kernel at boot also scans partition tables of all the disks it can
+ find to find the ones with the four Pintos partition types (perhaps
+ not all exist). After that, it makes them available to the rest of
+ the kernel (and doesn't allow access to other devices, for safety).
+
+- "pintos" and "pintos-mkdisk" need to write a partition table to the
+ disks that they create. "pintos-mkdisk" will need to take a new
+ parameter specifying the type. (I might have partially implemented
+ this, don't remember.)
+
+- "pintos" should insist on finding a partition header on disks handed
+ to it, for safety.
+
+- Need some way for "pintos" to assemble multiple disks or partitions
+ into a single image that can be copied directly to a USB block
+ device. (I don't know whether I came up with a good solution yet or
+ not, or whether I implemented any of it.)
+
+To add USB support:
+
+- Needs to be able to scan PCI bus for UHCI controller. (I
+ implemented this partially.)
+
+- May want to be able to initialize USB controllers over CardBus
+ bridges. I don't know whether this requires additional work or if
+ it's useful enough to warrant extra work. (It's of special interest
+ for me because I have a laptop that only has USB via CardBus.)
+
+- There are many protocol layers involved: SCSI over USB-Mass Storage
+ over USB over UHCI over PCI. (I may be forgetting one.) I don't
+ know yet whether it's best to separate the layers or to merge (some
+ of) them. I think that a simple and clean organization should be a
+ priority.
+
+- VMware can likely be used for testing because it can expose host USB
+ devices as guest USB devices. This is safer and more convenient
+ than using real hardware for testing.
+
+- Should test with a variety of USB keychain devices because there
+ seems to be wide variation among them, especially in the SCSI
+ protocols they support. Should try to use a "lowest-common
+ denominator" SCSI protocol if any such thing really exists.
+
+- Might want to add a feature whereby kernel arguments can be given
+ interactively, rather than passed on-disk. Needs some though.
projects / pintos-anon / blobdiff