[PATCH 28/40] ARM: sched_clock: provide common infrastructure for sched_clock()

On Fri, 17 Dec 2010, Russell King - ARM Linux wrote:

Provide common sched_clock() infrastructure for platforms to use to
create a 64-bit ns based sched_clock() implementation from a counter
running at a non-variable clock rate.

This implementation is based upon maintaining an epoch for the counter
and an epoch for the nanosecond time.  When we desire a sched_clock()
time, we calculate the number of counter ticks since the last epoch
update, convert this to nanoseconds and add to the epoch nanoseconds.

We regularly refresh these epochs within the counter wrap interval.
We perform a similar calculation as above, and store the new epochs.

We read and write the epochs in such a way that sched_clock() can easily
(and locklessly) detect when an update is in progress, and repeat the
loading of these constants when they're known not to be stable.  The
one caveat is that sched_clock() is not called in the middle of an
update.

How is this ensured?  I think this is currently a problem.

Right now there is nothing that prevents  update_sched_clock() from 
being interrupted in the middle of cd->epoch_cyc and cd->epoch_cyc_copy.  
Let's say an interrupt happens when those two are different.

Within an interrupt handler, you may find calls to printk(), which may 
also end up calling sched_clock() for time stamping. The sched_clock() 
function will call cyc_to_fixed_sched_clock() which will deadlock.

Maybe you should at least mask interrupts in update_sched_clock() so 
that the epoch update is not interruptible.


Nicolas