sched: question on group_capacity when SMT > 2
From: Michael Neuling
Date: Tue Mar 30 2010 - 22:42:12 EST
Peter & Ingo,
At the end of update_sg_lb_stats() we have this calculation:
sgs->group_capacity =
DIV_ROUND_CLOSEST(group->cpu_power, SCHED_LOAD_SCALE);
At the sibling level when we have SMT2, cpu_power will be 1178/2
(provided we are not scaling power with freq say) and SCHED_LOAD_SCALE
will be 1024, resulting in group_capacity being 1.
If we have SMT4, cpu_power will be 1178/4, hence group_capacity will end
up as 0. Shouldn't group capacity always be >= 1? Do we need to round
this up instead or make sure group_capacity is minimum of 1? Or do we
need to change the way we calculate cpu_power in the SMT > 2 case?
A similar problem could occur in SMT2 mode if cpu_power was scaled down
to less than 1024.
(FWIW the undocumented sprinkling of DIV_ROUND_blah in kernel/sched* is
somewhat disconcerting)
Regards,
Mikey
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