pmdalmsensors: query only the requested chip during metric fetch

[Ali-Nasrolahi]

PMDA lmsensors: Reduce redundant sensor probing during metric fetches

Patch Description

This patch optimizes the lmsensors PMDA by avoiding redundant full-system sensor scans when serving metric fetch
requests. Currently, every metric fetch executes sensors -j which probes all sensor chips, even though only a
single metric value is ultimately returned. As a result, a PCP fetch involving multiple metrics repeatedly re-reads the
same hardware. This patch records the originating lm_sensors chip for each metric during PMDA initialization and uses
that information to query only the chip associated with the requested metric.

In other words, instead of executing sensors -j, we run sensors -j <chip> which reduces the amount of work that is
needed.

Additionally, the cost with current implementation grows with:

• the number of sensor chips present,
• the latency of individual sensor backends,
• and the number of metrics requested by PCP clients.

Related Issue

While investigating why the PMDA was occasionally timing out, I found that one sensor backend on my system requires
roughly half a second to respond. Because the PMDA executes a full sensors -j invocation for every metric, the latency
combined across all requested metrics, which then exceeded the PMCD request timeout and causing all metrics in the
request to fail. Since only one metric value is needed for each callback, querying only the corresponding sensor chip
avoids repeatedly waiting on unrelated hardware.

For the context, my environment is Fedora 44 with PCP 7.1.5:

❯ rpm -q pcp pcp-pmda-lmsensors
pcp-7.1.5-3.fc44.x86_64
pcp-pmda-lmsensors-7.1.5-3.fc44.x86_64

Warm Regards,
Ali :)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository YAML (base), Repository UI (inherited), Organization UI (inherited)

Review profile: CHILL

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Run ID: 9599868b-f7cc-4a3b-b1a3-69ca2fc72a83

📥 Commits

Reviewing files that changed from the base of the PR and between 0c6ddfc and 49cf76b.

📒 Files selected for processing (1)

• src/pmdas/lmsensors/pmdalmsensors.python

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved hardware sensor readings by associating each metric with its originating sensor chip.
  • Ensured sensor values are retrieved from the correct chip when multiple chips expose similar metrics.

Walkthrough

lmsensors_get now accepts an optional chip filter, records chip ownership for generated sensor names, and uses that mapping during per-metric fetches.

Changes

lm_sensors chip scoping

Layer / File(s)	Summary
Track chips and filter acquisition src/pmdas/lmsensors/pmdalmsensors.python	Adds the sensorchips mapping, supports an optional chip argument, filters sensors -j acquisition, and records each metric’s originating chip.
Fetch metrics from mapped chips src/pmdas/lmsensors/pmdalmsensors.python	Passes the mapped chip when fetching each requested sensor instance.

Poem

A rabbit watched the sensors glow,
Chips now guide each metric’s flow.
Names remember where they start,
Fetches seek their matching part.
“Hop!” says Bun, “the readings align!” 🐇

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: fetching metrics from only the requested chip.
Description check	✅ Passed	The description is directly related to the change and explains the optimization in detail.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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