linux-kernel - Re: Discussion: quick_pit

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Message-ID: <20150610161157.18875.qmail@ns.horizon.com>
Date:	10 Jun 2015 12:11:57 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	adrian.hunter@...el.com, mingo@...nel.org
Cc:	ak@...ux.intel.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	linux@...izon.com, luto@...capital.net, tglx@...utronix.de,
	torvalds@...ux-foundation.org
Subject: Re: Discussion: quick_pit_calibrate is slow

> Especially any 'measure the minimum time' approach measuring more than
> a single PIT tick would be senstive to false positives.

Just to be clear, I'm not measuring the minimum of any timer ticks;
it's the timer *access* that's being measured.  It's literally the
duration of a single inb() or outb().  Even the *value* is unimportant.

It's just a way to figure out the skew between the TSC and the peripheral.

Think of the peripheral access like a network ping.  I'm asking
a remote computer for its time, and measuring *my* time at the start and
end of the ping.

There's a minimum possible ping time, achieved where queueing delays
are zero.  If I could achieve that minimum time reliably, synchronizing
clocks rates would be trivial.

To measure offset, I have to make some arbitrary assumption about
the duration of the two legs of the round trip.  Typically I assume that
they're equal, so that the remote time corresponds to the point
halfway between my two time measurements.

(For timer rate measurements, it turns out that this assumption
doesn't affect the result at all, so it can be made arbitrarily.)

If a ping takes more than the minimum possible time, then what's
actually most likely is that *one* of the two legs was delayed, and
the correct time is not in the middle of the ping time range, but
hard up against one edge.

I.e. 

	t1 = get_cycles();
	byte = inb(peripheral);
	t2 = get_cycles();
	interval = t2 - t1;
	tmin = min(interval, tmin);

The instant of the inb() is most likely t1 + tmin/2, or t2 - tmin/2,
might be elsewhere in that interval, but is definitely not (if tmin has
been measured accurately) outside that interval.
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