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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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12
kernel/dev/procfs.c
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@ -130,11 +130,12 @@ gen_meminfo(struct sbuf *sb)
static void
gen_uptime(struct sbuf *sb)
{
uint t = uptime(); // timer ticks, 100 Hz
sb_putu(sb, t / 100);
uint t = uptime(); // timer ticks, HZ of them per second
uint frac = (t % HZ) * 100 / HZ; // hundredths, whatever HZ happens to be
sb_putu(sb, t / HZ);
sb_putc(sb, '.');
sb_putu(sb, (t % 100) / 10);
sb_putu(sb, (t % 100) % 10);
sb_putu(sb, frac / 10);
sb_putu(sb, frac % 10);
sb_puts(sb, " 0.00\n"); // idle time: not tracked
}
@ -214,7 +215,8 @@ gen_version(struct sbuf *sb)
static void
gen_stat(struct sbuf *sb)
{
uint t = uptime();
// USER_HZ, like times() -- everything that parses /proc/stat divides by 100.
uint t = (uint)((uint64)uptime() * USER_HZ / HZ);
sb_puts(sb, "cpu ");
sb_putu(sb, t); sb_puts(sb, " 0 0 0 0 0 0 0 0 0\n");
// Online, for the same reason as cpuinfo: top and friends count the cpuN