Re: PSA: Clock drift and pkgin

On 2023-12-23 14:35, Mouse wrote:
>>>>         } else if (sec <= (LONG_MAX / 1000000))
>>>>                 ticks = (((sec * 1000000) + (unsigned long)usec + (tick - 1))
>>>>                     / tick) + 1;
>>> The delay is always rounded up to the resolution of the clock, so
>>> waiting for 1 microsecond waits at least 10ms.
> 
> But it is increased by 1 tick when it is an exact multiple of the clock
> resolution, too.  For sleeps, that makes some sense.  For timer
> reloads, it doesn't.

I would probably agree that for timer reloads it should not do that 
rounding up when the interval is evenly divisible. It is a very 
different case than a sleep. For the reload we know that this happens at 
a specific time.

> I could of course be wrong about that code being responsible, but
> reading realtimerexpire() makes me think not; it uses tshzto, which
> calls tstohz, which calls tvtohz, which is where the code quoted above
> comes from.  Maybe realtimerexpire should be using other code?

Agreed.

>> Two options are to increase HZ on the host as suggested, or halve HZ
>> on the guest.
> 
> I suppose actually fixing the bug isn't an option?
> 
> I don't know whether that would mean using different code for timer
> reloads and sleeps or what.  But 1.4T is demonstration-by-example that
> it is entirely possible to get this right, even in a tickful system.
> (I don't know whether 1.4T sleeps may be slightly too short; I haven't
> tested that.  But, even if so, fixing that should not involve breaking
> timer reloads.)

A tickless system do not fundamentally change anything either. You can't 
go below the resolution of a timer, and sleeps are supposed to be 
sleeping for *at least* the given time, but it could be more.

But in this case, we end up where the expected, and reasonable behaviour 
would be to get alarms/interrupts at the specified frequency, because it 
is the resolution of the clock, but we are in fact rounding it up to the 
double amount of time between alarms/interrupts. Not what I think anyone 
would have expected.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
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