[PATCH v2 0/2] sched/fair: Limit access to overutilized

Shrikanth Hegde
2024-02-28T07:16:48Z

When running a ISV workload on a large system (240 Cores, SMT8), it was
observed from perf profile that newidle_balance and enqueue_task_fair
were consuming more cycles. Perf annotate showed that most of the time
was spent on accessing overutilized field of root domain.

Aboorva was able to simulate similar perf profile by making some
changes to stress-ng --wait. Both newidle_balance and enqueue_task_fair
consume close to 5-7%. Perf annotate shows that most of the cycles are spent
in accessing rd,rd->overutilized field.

Overutilized was added for EAS(Energy aware scheduler) to choose either
EAS aware load balancing or regular load balance. As checked, on x86 and
powerpc both overload and overutilized share the same cacheline in rd.
Updating overutilized is not required for non-EAS platforms.

Patch 1/2 is the main patch. It helps in reducing the above said issue.
Both the functions don't show up in the profile. With patch comparison is in
changelog. With the patch stated problem in the ISV workload also got
solved and throughput has improved.
Patch 2/2 is only code refactoring to use the helper function instead of
direct access of the field, so one would come to know that it is accessed
only in EAS. This depends on 1/2 to be applied first

Thanks to Aboorva Devarajan and Nysal Jan K A for helping in recreating,
debugging this issue and verifying the patch.
Detailed perf annotate can be found in cover letter of v1.

v2 -> v1:
Chen Yu pointed out minor issue in code. corrected that code and updated
the changelog.

v1: https://lore.kernel.org/lkml/20240223150707.410417-1-sshegde@linux.ibm.com/

Shrikanth Hegde (2):
sched/fair: Add EAS checks before updating overutilized
sched/fair: Use helper function to access rd->overutilized

kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++------------
1 file changed, 36 insertions(+), 13 deletions(-)

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2.39.3