6.6-stable review patch. If anyone has any objections, please let me know.
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From: Aboorva Devarajan aboorvad@linux.ibm.com
[ Upstream commit 07d815701274d156ad8c7c088a52e01642156fb8 ]
On virtualized PowerPC (pseries) systems, where only one polling state (Snooze) and one deep state (CEDE) are available, selecting CEDE when the predicted idle duration is less than the target residency of CEDE state can hurt performance. In such cases, the entry/exit overhead of CEDE outweighs the power savings, leading to unnecessary state transitions and higher latency.
Menu governor currently contains a special-case rule that prioritizes the first non-polling state over polling, even when its target residency is much longer than the predicted idle duration. On PowerPC/pseries, where the gap between the polling state (Snooze) and the first non-polling state (CEDE) is large, this behavior causes performance regressions.
Refine that special case by adding an extra requirement: the first non-polling state can only be chosen if its target residency is below the defined RESIDENCY_THRESHOLD_NS. If this condition is not satisfied, polling is allowed instead, avoiding suboptimal non-polling state entries.
This change is limited to the single special-case rule for the first non-polling state. The general non-polling state selection logic in the menu governor remains unchanged.
Performance improvement observed with pgbench on PowerPC (pseries) system: +---------------------------+------------+------------+------------+ | Metric | Baseline | Patched | Change (%) | +---------------------------+------------+------------+------------+ | Transactions/sec (TPS) | 495,210 | 536,982 | +8.45% | | Avg latency (ms) | 0.163 | 0.150 | -7.98% | +---------------------------+------------+------------+------------+
CPUIdle state usage: +--------------+--------------+-------------+ | Metric | Baseline | Patched | +--------------+--------------+-------------+ | Total usage | 12,735,820 | 13,918,442 | | Above usage | 11,401,520 | 1,598,210 | | Below usage | 20,145 | 702,395 | +--------------+--------------+-------------+
Above/Total and Below/Total usage percentages: +------------------------+-----------+---------+ | Metric | Baseline | Patched | +------------------------+-----------+---------+ | Above % (Above/Total) | 89.56% | 11.49% | | Below % (Below/Total) | 0.16% | 5.05% | | Total cpuidle miss (%) | 89.72% | 16.54% | +------------------------+-----------+---------+
The results indicate that restricting CEDE selection to cases where its residency matches the predicted idle time reduces mispredictions, lowers unnecessary state transitions, and improves overall throughput.
Reviewed-by: Christian Loehle christian.loehle@arm.com Signed-off-by: Aboorva Devarajan aboorvad@linux.ibm.com [ rjw: Changelog edits, rebase ] Link: https://patch.msgid.link/20251006013954.17972-1-aboorvad@linux.ibm.com Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/cpuidle/governors/menu.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c index 8c591dde61023..27010eee6d1bc 100644 --- a/drivers/cpuidle/governors/menu.c +++ b/drivers/cpuidle/governors/menu.c @@ -354,12 +354,13 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev, }
/* - * Use a physical idle state, not busy polling, unless a timer - * is going to trigger soon enough or the exit latency of the - * idle state in question is greater than the predicted idle - * duration. + * Use a physical idle state instead of busy polling so long as + * its target residency is below the residency threshold, its + * exit latency is not greater than the predicted idle duration, + * and the next timer doesn't expire soon. */ if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) && + s->target_residency_ns < RESIDENCY_THRESHOLD_NS && s->target_residency_ns <= data->next_timer_ns && s->exit_latency_ns <= predicted_ns) { predicted_ns = s->target_residency_ns;