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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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34
kernel/sys/sys_fs.c
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@ -15,6 +15,7 @@
#include "uaccess.h"
#include "errno.h"
#include "schfs.h" /* SCHFS_BLKSIZE, SCHFS_NAME_MAX — statfs reports them */
#include "gfx.h" /* gfx_cols/gfx_rows — TIOCGWINSZ reports them */
// -----------------------------------------------------------------------
// Linux-ABI-compatible struct stat (x86_64 "struct stat" from
@ -1042,10 +1043,41 @@ addr_t sys_ioctl(void)
if (request == TIOCGWINSZ) {
struct winsize *ws = (struct winsize*)user_ptr(argp, sizeof(*ws));
if (!ws) return -EFAULT;
ws->ws_row = 24; ws->ws_col = 80; ws->ws_xpixel = ws->ws_ypixel = 0;
// The console's real geometry, not a VT100's.
//
// This used to answer 24x80 unconditionally -- the size of a terminal
// SchOS has never drawn. Every program that lays out a screen asks
// this one question first: a line editor to decide where to wrap, `ls`
// to pick a column count, fastfetch to place its logo beside the text.
// At 1024x768 with the 8x16 font the console is 128x48, so all of them
// were formatting for a screen a third the area of the real one, and
// each looked like a separate bug in a separate program.
int cols = gfx_cols(), rows = gfx_rows();
// Serial-only boot, with no framebuffer attached: 80x24 is then a
// stated default rather than a wrong measurement.
ws->ws_col = cols > 0 ? (uint16)cols : 80;
ws->ws_row = rows > 0 ? (uint16)rows : 24;
ws->ws_xpixel = (uint16)gfx_width();
ws->ws_ypixel = (uint16)gfx_height();
return 0;
}
// The console's size is a property of the framebuffer, so a request to
// change it cannot be honoured -- but it is not an error either: programs
// set it to propagate a size they already learned. Accept and ignore.
if (request == TIOCSWINSZ) return 0;
// TCFLSH discards buffered input, output, or both. Unlike every other
// request here the argument is the queue selector *by value*, not a
// pointer -- passing it to user_ptr would fault on a legitimate call.
//
// Output needs no action: the console writes straight to the framebuffer
// and has no queue to drop. Input does, and answering 0 without draining
// it -- which is what the catch-all below did -- is how a keystroke typed
// during a command survives the flush and lands in the next prompt.
if (request == TCFLSH) {
if (argp == TCIFLUSH || argp == TCIOFLUSH)
cons_flush_input();
return 0;
}
// Controlling-terminal foreground process group.
//
// TIOCGPGRP used to answer with myproc()->pid, which happened to agree with