Tr: xtime_lock

[Benjamin Herrenschmidt]

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Sujet: xtime_lock
Envoyé: vendredi 19 juillet 2002 15:31
De: Jeroen T. Vermeulen <jtv at xs4all.nl>
À: Benjamin Herrenschmidt <benh at kernel.crashing.org>

Hi Ben,

I was just trying to learn a bit more about the kernel's locking
primitives and execution model, when I came across xtime_lock and your
comment about wanting to get rid of it (arch/ppc/kernel/time.c).

So here's my question: would it be feasible (and/or pointful) to replace
the xtime_lock in do_gettimeofday() by a simple optimistic approach?  If
the machine is fast enough compared to timer resolution, I imagine one
could just read the clock twice and compare the results to verify that
you got consistent data.  Of course whether that saves you any time is
another matter; perhaps it would make sense with SMP but not for
uniprocessor machines, or something.

A detail that could make it more feasible AFAICS is that you don't
need to re-read the least significant--and most volatile!--part of the
time value for the "second opinion" check, because you only get an
actual inconsistent result if there's been a rollover while you were
reading.  Or am I forgetting to take nonlinear clock changes into
account?  OTOH they mean you get weird results anyway...

Oh, and on a side note, why the loop to convert "superfluous" usecs to
secs?  Is this because the number of iterations is so low that the
conditional is faster than a division plus a remainder operation?

I mean

	while (usec >= 1000000) {
	  	sec++;
		usec -= 1000000;
	}

vs.

	sec += (usec / 1000000);
	usec %= 1000000;


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