[Bug 223914] [PATCH] Fix bug in which the long term ULE load balancer is executed only once

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=223914

            Bug ID: 223914
           Summary: [PATCH] Fix bug in which the long term ULE load
                    balancer is executed only once
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Keywords: patch
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: justinien.bouron at epfl.ch
          Keywords: patch

Created attachment 188323
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=188323&action=edit
Patch for the ULE load balancer solving the issue of this bug report.

The load balancer of ULE has a bug which cause it to be executed only once at
startup, and it will *never* be executed again.

The reason is simple, when the sched_balance function is called for the first
time, the smp is not yet setup, and the function returns.
However the balance_tick variable is not resetted before the return and thus
will remains 0 for ever.

Because the condition in sched_clock to trigger the load balancer is :
" balance_ticks && --balance_ticks == 0 ", once balance_ticks is 0, this
condition will never be met, and thus the load balancer will never be executed
again.

To convince yourself you can even put a panic right before the call to
sched_balance_group in sched_balance, it will never panic ...

The fix is rather simple, just move the line resetting the balance_ticks value
*before* the if branch "if (smp_started == 0 || rebalance == 0)". That way we
are sure that the load balancer is re-scheduled.

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