Background:
 We've found that MCEs (specifically DRAM SBEs) tend to come in bunches,
 especially when we are trying really hard to stress the system out.  The
 current MCE poller uses a static interval which does not care whether it
 has or has not found MCEs recently.

Description:
 This patch makes the MCE poller adjust the polling interval dynamically.
 If we find an MCE, poll 2x faster (down to 10 ms).  When we stop finding
 MCEs, poll 2x slower (up to check_interval seconds).  The check_interval
 tunable becomes the max polling interval.  The "Machine check events
 logged" printk() is rate limited to the check_interval, which should be
 identical behavior to the old functionality.

Result:
 If you start to take a lot of correctable errors (not exceptions), you
 log them faster and more accurately (less chance of overflowing the MCA
 registers).  If you don't take a lot of errors, you will see no change.

Alternatives:
 I considered simply reducing the polling interval to 10 ms immediately
 and keeping it there as long as we continue to find errors.  This felt a
 bit heavy handed, but does perform significantly better for the default
 check_interval of 5 minutes (we're using a few seconds when testing for
 DRAM errors).  I could be convinced to go with this, if anyone felt it
 was not too aggressive.

Testing:
 I used an error-injecting DIMM to create lots of correctable DRAM errors
 and verified that the polling interval accelerates.  The printk() only
 happens once per check_interval seconds.

Patch:
 This patch is against 2.6.21-rc7.
Signed-Off-By: Tim Hockin <thockin@google.com>
Signed-off-by: Andi Kleen <ak@suse.de>