`sleep` is now implemented in terms of `nanosleep`. `nanosleep` is now
more precise and handles overflow when calculating wakeup time. I don't
think anything was depending on this, but I could see a program sleeping
for max time to block until signal.
We now appreciate sa_mask and SA_NODEFER and change the signal mask for
the duration of signal handler. This is done by making a sigprocmask
syscall at the end of the signal handler. Back-to-back signals will
still grow stack as original registers are popped AFTER the block mask
is updated. I guess this is why linux has sigreturn(?).
Current cpu index is stored at either segment. If userspace sets that
segment, kernel will not overwrite it on every reschedule. This is fine
as long as user program does not use anything that relies on it :)
We need to have interrupts enabled when signal kills the process as
process does mutex locking. Also signals are now only checked when
returning to userspace in the same place where userspace segments are
loaded.
Process's memory regions are now behind an rwlock instead of using the
full process lock. This allows most pointer validations to not block as
write operations to memory regions are rare.
Thread's userspace stack is now part of process's memory regions. This
simplifies code that explicitly looped over threads to see if the
accessed address was inside a thread's stack.
Only drawback of this is that MemoryRegions don't support guard pages,
so userspace stackoverflow will be handeled as cleanly as it was prior
to this.
This patch also fixes some unnecessary locking of the process lock and
moves locking to the internal helper functions instead of asserting that
the lock is held. Also we now make sure loaded ELF regions are in sorted
order as we previously expected.
There is no need to save and load sse state on every interrupt. Instead
we can use CR0.TS to make threads trigger an interrupt when they use sse
instructions. This can be used to only save and load sse state when
needed.
Processor now keeps track of its current "sse thread" and the scheduler
either enabled or disabled sse based on which thread it is starting up.
When a thread dies, it checks if it was the current sse thread to avoid
use after free bugs. When load balancing, processor has to save the
thread's sse state before sending it to a new processor (if it was the
current sse thread). This ensures thread's sse state will be correct
when the new processor ends up loading it.
Doing a yield no longer raises a software interrupt. Instead it just
saves all the callee saved registers, ip, sp and return value. Because
yield is only called in the kernel, it can just restore registers and
jump to the target address. There is never a need to use iret :)
If the processor has invariant TSC it can be used to measure time. We
keep track of the last nanosecond and TSC values and offset them based
on the current TSC. This allows getting current time in userspace.
The implementation maps a single RO page to every processes' address
space. The page contains the TSC info which gets updated every 100 ms.
If the processor does not have invariant TSC, this page will not
indicate the capability for TSC based timing.
There was the problem about how does a processor know which cpu it is
running without doing syscall. TSC counters may or may not be
synchronized between cores, so we need a separate TSC info for each
processor. I ended up adding sequence of bytes 0..255 at the start of
the shared page. When a scheduler gets a new thread, it updates the
threads gs/fs segment to point to the byte corresponding to the current
cpu.
This TSC based timing is also used in kernel. With 64 bit HPET this
probably does not bring much of a benefit, but on PIT or 32 bit HPET
this removes the need to aquire a spinlock to get the current time.
This change does force the userspace to not use gs/fs themselves and
they are both now reserved. Other one is used for TLS (this can be
technically used if user does not call libc code) and the other for
the current processor index (cannot be used as kernel unconditionally
resets it after each load balance).
I was looking at how many times timer's current time was polled
(userspace and kernel combined). When idling in window manager, it was
around 8k times/s. When running doom it peaked at over 1 million times
per second when loading and settled at ~30k times/s.
SIGWINCH and SIGCANCEL ended up interrupting functions even when they
were marked as SIG_DFL. Now resizing the userspace terminal emulator
does not get interrupted!
Kernel can just use raw threads, pretty muchs the only thing that
process provides is syscalls which kernel threads of course don't
need.
Also this makes init process have pid 1 :D
All block functions now take an optional mutex parameter that is
atomically unlocked instead of having the user unlock it before hand.
This prevents a ton of race conditions everywhere in the code!
Bananymous
DcraftBgandBananymous