[PATCH] Statclock aliasing by LAPIC

[John Baldwin]

On Friday 27 November 2009 6:42:50 pm Attilio Rao wrote:
> Handling all the three clocks (hardclock, softclock, profclock) within
> the LAPIC can lead to aliasing for the softclock and profclock because
> hz is sized to fit mainly hardclock. The fashion to handle all of them
> within the LAPIC was introduced in 2005 and before than the softclock
> and profclock were supposed to be handled in the rtc. Right now, too,
> there is the necessary support to handle profclock and statclock in
> atrtc which gets enabled if the LAPIC signals it can't take in charge
> the three clocks.
> The proposed patch reverts the situation preferring the atrtc to
> handle the statclock and profclock (then a different source from the
> LAPIC) and then avoid the aliasing problem:
> 
http://www.freebsd.org/~attilio/Sandvine/STABLE_8/statclock_aliasing/statclock_aliasing3.diff
> 
> In this patch, lapic_setup_clock() has been changed in order to return
> a three-states variable which identified if the LAPIC got in charge
> all the three clocks, just the hardclock or none of them (the current
> situation is just none/all) and the rtc handling runs subsequently.
> A tunable as been added to force LAPI to get in charge all the three
> clocks if needed.
> In ia32 atrtc compiling is linked to atpic compiling, so a compile
> time flag has been added to check if atpic is not present and in case
> force LAPIC to take in charge all the three clocks (which is still
> better than the 'safe belt values' still present in the rtc code).
> 
> Please note that statclock and profclock are widely used in our kernel
> (rusage is, for example, statclock driven) and fixing this would
> result in specific improvements (as a several-reported wrong CPU usage
> statistic in top).
> This bug has been found by Sandvine Incorporated.
> 
> Reviews, comments and testing are welcome.

Presumably in the RTC case lapic_timer_hz should always be hz and not some 
multiple of hz.  Also, did you check to make sure all the lock is present?  I 
think at one point I changed the locking for the RTC and/or ISA timer to just 
use critical_enter/exit so that UP machines running an SMP kernel wouldn't pay 
the locking overhead since the code was only used on UP machines.  It may very 
well be that I only changed that in a development branch though and not in 
HEAD.

-- 
John Baldwin