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

[Andy Lutomirski]

On Thu, Sep 20, 2012 at 7:31 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 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?

Only if cycle_last changes while preempted (or from a different CPU).
That case is covered by the seqlock in do_realtime and do_monotonic.

--Andy
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