cpuidle: menu: Fix high wakeup latency on modern platforms
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Changes in v2 from the cover letter
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Patches in v2 No applies recorded yet
| # | Subject and files | Size | Replies | Applied |
|---|---|---|---|---|
| 1/1 | cpuidle: menu: Use min() to prevent deep C-states when tick is stoppeddrivers/ | +8 | 1 reply |
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Cover letter [PATCH v2 0/1] cpuidle: menu: Fix high wakeup latency on modern platforms
From: Ionut Nechita <[email protected]>
Hi,
This v2 patch addresses high wakeup latency in the menu cpuidle governor on modern platforms with high C-state exit latencies.
Changes in v2:
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Based on Christian Loehle's feedback, I've simplified the approach to use min(predicted_ns, data->next_timer_ns) instead of the 25% safety margin from v1.
The simpler approach is cleaner and achieves the same goal: preventing the governor from selecting excessively deep C-states when the prediction suggests a short idle period but next_timer_ns is large (e.g., 10ms).
I will test both approaches (simple min vs 25% margin) and provide detailed comparison data including:
- C-state residency tables
- Usage statistics
- Idle miss counts (above/below)
- Actual latency measurements
Thank you Christian for the valuable feedback and for pointing out that the simpler approach may be sufficient.
Background:
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On Intel server platforms from 2022 onwards (Sapphire Rapids, Granite Rapids), we observe excessive wakeup latencies (~150us) in network- sensitive workloads when using the menu governor with NOHZ_FULL enabled.
The issue stems from the governor using next_timer_ns directly when the tick is already stopped and predicted_ns < TICK_NSEC. This causes selection of very deep package C-states (PC6) even when the prediction suggests a much shorter idle duration.
On platforms with high C-state exit latencies (Intel SPR: 190us for C6, or systems with large C-state gaps like C2 36us → C3 700us with 350us exit latency), this results in significant wakeup penalties.
Testing:
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Initial testing on Sapphire Rapids shows 5x latency reduction (151us → ~30us). I will provide comprehensive test results comparing baseline, simple min(), and the 25% margin approach.
Ionut Nechita (1):
cpuidle: menu: Use min() to prevent deep C-states when tick is stopped
drivers/cpuidle/governors/menu.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)
Signature · 1 line
2.52.0