clocksource: Retry clock read if long delays detected

clocksource: Retry clock read if long delays detected

When the clocksource watchdog marks a clock as unstable, this might
be due to that clock being unstable or it might be due to delays that
happen to occur between the reads of the two clocks.  Yes, interrupts are
disabled across those two reads, but there are no shortage of things that
can delay interrupts-disabled regions of code ranging from SMI handlers
to vCPU preemption.  It would be good to have some indication as to why
the clock was marked unstable.

This commit therefore re-reads the watchdog clock on either side of
the read from the clock under test.  If the watchdog clock shows an
excessive time delta between its pair of reads, the reads are retried.
The maximum number of retries is specified by a new kernel boot
parameter clocksource.max_read_retries, which defaults to three, that
is, up to four reads, one initial and up to three retries.  If retries
were required, a message is printed on the console.  If the number of
retries is exceeded, the clock under test will be marked unstable.
However, the probability of this happening due to various sorts of
delays is quite small.  In addition, the reason (clock-read delays)
for the unstable marking will be apparent.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Reported-by: Chris Mason <clm@fb.com>
[ paulmck: Per-clocksource retries per Neeraj Upadhyay feedback. ]
[ paulmck: Don't reset injectfail per Neeraj Upadhyay feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>