Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding issue in timekeeping core

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 2012-09-17 at 16:49 -0700, Andy Lutomirski wrote:

> I haven't looked in any great detail, but the approach looks sensible
> and should slow down the vsyscall code.
> 
> That being said, as long as you're playing with this, here are a
> couple thoughts:
> 
> 1. The TSC-reading code does this:
> 
> 	ret = (cycle_t)vget_cycles();
> 
> 	last = VVAR(vsyscall_gtod_data).clock.cycle_last;
> 
> 	if (likely(ret >= last))
> 		return ret;
> 
> I haven't specifically benchmarked the cost of that branch, but I
> suspect it's a fairly large fraction of the total cost of
> vclock_gettime.  IIUC, the point is that there might be a few cycles
> worth of clock skew even on systems with otherwise usable TSCs, and we
> don't want a different CPU to return complete garbage if the cycle
> count is just below cycle_last.
> 
> A different formulation would avoid the problem: set cycle_last to,
> say, 100ms *before* the time of the last update_vsyscall, and adjust
> the wall_time, etc variables accordingly.  That way a few cycles (or
> anything up to 100ms) or skew won't cause an overflow.  Then you could
> kill that branch.
> 

I'm curious... If the task gets preempted after reading ret, and doesn't
get to run again for another 200ms, would that break it?

-- Steve