#10945 NORM 1.75-so: Minutes-long freezes with 2.6.39 tree on XO-1.75 A2/A3

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#10945: Minutes-long freezes with 2.6.39 tree on XO-1.75 A2/A3
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           Reporter:  martin.langhoff  |       Owner:  buytenh      
               Type:  defect           |      Status:  closed       
           Priority:  normal           |   Milestone:  1.75-software
          Component:  kernel           |     Version:  1.75-A3      
         Resolution:  fixed            |    Keywords:               
        Next_action:  never set        |    Verified:  0            
Deployment_affected:                   |   Blockedby:               
           Blocking:                   |  
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Changes (by buytenh):

  * status:  new => closed
  * resolution:  => fixed


Comment:

 I forward-ported the timer handling fix to the 2.6.39 branch in the xo1.75
 -kernel-bringup repository and committed it there.  (It cleanly cherry-
 picks onto the arm-3.0 branch in the olpc-3.0 repository, but I don't seem
 to have the right permission bits to commit to that repository.)

 The problem in the current upstream mmp timer handling code that appears
 to be triggered here is that it handles clockevents by setting up a
 comparator on the free-running clocksource timer to match and trigger an
 interrupt at 'current_time + delta', which if delta is small enough can
 lead to 'current_time + delta' already being in the past when comparator
 setup has finished, and the requested event will not trigger.

 The fix for this is using two timers, one as a free-running clocksource,
 and one interrupt-generating timer to handle clockevents, but this fix is
 somewhat invasive.  A safer fix for 3.0 may be to disallow
 CONFIG_NO_HZ/CONFIG_HIGH_RES_TIMERS on mmp.

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Ticket URL: <http://dev.laptop.org/ticket/10945#comment:16>
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