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kernel: the clock ran 7x slow because the GPU never acknowledged its interrupt

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fastfetch reported 3 seconds of uptime after 15 real ones, `time` claimed
3.12s for a 20-second command, and ping's one-second interval was closer to
six. One cause, not three: `ticks` was advancing at ~14.5 Hz instead of 100.

It was not the timer. virtio-gpu-pci shares IRQ 11 with the e1000 and
registered no interrupt handler at all. PCI INTx is level-triggered, so the
device held the line asserted, the kernel signalled EOI, and it fired again
immediately -- 72,000 interrupts a second, 83% of CPU0, starving the tick.
The e1000's handler read ICR as zero half a million times, correctly
answering "not mine", which is what made it look like a network-free problem.

Reading virtio's ISR byte clears it and drops the line. That is the fix.

irq_dispatch now measures each line's rate against the TSC -- not ticks,
which is the first casualty of a storm -- and names the attached handlers
once above 20000/s. It reproduces the original fault as a single line.

Found along the way, and fixed because they were real:

  - lapicinit loaded TICR with xv6's uncalibrated 10000000, whose own comment
    admitted it was never measured. calibrate_timers() now measures the LAPIC
    and the TSC against the 8254 PIT. (It came back at 999.9 MHz, so this was
    not the bug -- but the next machine is not obliged to be 1 GHz.)
  - microdelay() was an empty function body. Every microdelay(1000) in the
    xHCI reset path and the USB hub power-on sequence returned instantly.
  - HZ is a constant in param.h now, not the literal 100 written out at a
    dozen ticks-to-seconds conversions. USER_HZ is separate and deliberately
    does not follow it: musl answers sysconf(_SC_CLK_TCK) with 100 always, so
    times() and /proc/stat scale at the syscall boundary the way Linux does.
  - TIOCGWINSZ answered a hardcoded 24x80. gfx_cols()/gfx_rows() have known
    the real geometry all along; at 1024x768 the console is 128x48, so every
    program that lays out a screen was formatting for the wrong one.
  - TCFLSH fell through to the catch-all `return 0`, claiming to discard
    input without discarding any.

uptime now advances 5.01 seconds per 5 real seconds. schfs-check passes.

Full investigation, including the two wrong hypotheses and the measurement
flaw that protected them, in docs/history/kernel.md section 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-12 21:34:36 +02:00
commit 9d4cdb44fc
17 changed files with 479 additions and 37 deletions
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11
kernel/sys/sys_poll.c
View file

@ -91,7 +91,10 @@ sys_poll(void)
if (nfds > 0 && copy_from_user(fds, ufds, nfds * sizeof(struct pollfd)) < 0)
return -EFAULT;
int timeout_ticks = timeout_ms < 0 ? -1 : (timeout_ms + 9) / 10;
// Rounded up: a caller asking for any wait at all must not get zero ticks,
// which poll() below treats as "return immediately".
int timeout_ticks = timeout_ms < 0 ? -1
: (timeout_ms + MSEC_PER_TICK - 1) / MSEC_PER_TICK;
int r = do_poll(fds, nfds, timeout_ticks);
if (r >= 0 && nfds > 0 &&
@ -123,7 +126,7 @@ sys_ppoll(void)
int64 ts[2]; // {tv_sec, tv_nsec}
if (copy_from_user(ts, utsp, sizeof(ts)) < 0)
return -EFAULT;
timeout_ticks = (int)(ts[0] * 100 + ts[1] / 10000000);
timeout_ticks = (int)(ts[0] * HZ + ts[1] / NSEC_PER_TICK);
}
int r = do_poll(fds, nfds, timeout_ticks);
@ -178,7 +181,7 @@ sys_select(void)
int64 tv[2]; // {tv_sec, tv_usec}
if (copy_from_user(tv, utv, sizeof(tv)) < 0)
return -EFAULT;
timeout_ticks = (int)(tv[0] * 100 + tv[1] / 10000);
timeout_ticks = (int)(tv[0] * HZ + tv[1] / USEC_PER_TICK);
}
for (;;) {
@ -252,7 +255,7 @@ sys_pselect6(void)
int64 ts[2];
if (copy_from_user(ts, utsp, sizeof(ts)) < 0)
return -EFAULT;
timeout_ticks = (int)(ts[0] * 100 + ts[1] / 10000000);
timeout_ticks = (int)(ts[0] * HZ + ts[1] / NSEC_PER_TICK);
}
for (;;) {