On Mon, Feb 27, 2023 at 06:26:00PM -0600, Scott Cheloha wrote:
> On Tue, Feb 28, 2023 at 10:18:16AM +1100, Jonathan Gray wrote:
> > On Mon, Feb 27, 2023 at 04:57:04PM -0600, Scott Cheloha wrote:
> > > ticks and jiffies start at zero. During boot in initclocks(), we
> > > reset them:
> > >
> > > /* sys/kern/kern_clock.c */
> > >
> > > 89 int ticks;
> > > 90 static int psdiv, pscnt; /* prof => stat divider
> > > */
> > > 91 int psratio; /* ratio: prof / stat */
> > > 92
> > > 93 volatile unsigned long jiffies; /* XXX Linux API for
> > > drm(4) */
> > > 94
> > > 95 /*
> > > 96 * Initialize clock frequencies and start both clocks running.
> > > 97 */
> > > 98 void
> > > 99 initclocks(void)
> > > 100 {
> > > 101 ticks = INT_MAX - (15 * 60 * hz);
> > > 102 jiffies = ULONG_MAX - (10 * 60 * hz);
> > > 103
> > > 104 /* [... ] */
> > >
> > > The idea here (committed by dlg@) is sound. We reset ticks and
> > > jiffies to near-rollover values to catch buggy code misusing them.
> > >
> > > But! That jump from zero to whatever violates valid assumptions made
> > > by correct code, too.
> >
> > Assumptions made by what code? Does it exist in the tree?
>
> First, even if the code did not exist, wouldn't it be simpler to not
> do the jump? No?
There are enough problems to fix without chasing ones that
don't exist.
>
> Second, with rare exception, all kernel code using ticks/jiffies
> assumes ticks/jiffies does not advance more than once every 1/hz
> seconds on average.
>
> In timeout_add(9), we assign an absolute expiration time relative
> to the current value of ticks. Code calling timeout_add(9) before
> initclocks() cannot account for the jump in initclocks().
What code calling timeout_add() before initclocks()?
>
> There is probably equivalent code in drm(4) making the same
> assumption.
The vast majority of drm runs after root is mounted.
>
> Relatedly, in cpu_relax() we increment jiffies if the kernel is
> cold:
>
> sys/dev/pci/drm/include/linux/processor.h
>
> 12 static inline void
> 13 cpu_relax(void)
> 14 {
> 15 CPU_BUSY_CYCLE();
> 16 if (cold) {
> 17 delay(tick);
> 18 jiffies++;
> 19 }
> 20 }
>
