[kconfig]: Set THP default to madvise instead of always

[prabhataravind]

What I did

Set the kernel compile-time default for Transparent Huge Pages (THP) from always to madvise by adding CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y in config.local/featureset-sonic/config

Why I did it

With CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS=y, khugepaged continuously promotes 4KB pages to 2MB huge pages in the background. On memory-constrained network switches this has several negative effects:

• Memory fragmentation: Once promoted, huge pages cannot be partially reclaimed. Even when applications free most of their memory, scattered live objects pin entire 2MB pages, preventing the kernel from reclaiming unused memory.
• Allocator inefficiency: Userspace allocators (e.g., jemalloc) use MADV_DONTNEED to return memory to the kernel, but this is ineffective on sub-ranges of huge pages, causing a persistent gap between application-level and kernel-level memory accounting.
• Process exit latency: Tearing down large THP-backed address spaces under memory pressure can cause threads to block in do_exit(), triggering hung task watchdog timeouts.

Setting the default to madvise ensures THP is only used when applications explicitly opt in via madvise(MADV_HUGEPAGE). This is the recommended setting for latency-sensitive and embedded networking workloads (consistent with guidance by other linux distributions for similar environments).

How I verified it

• Built the kernel with updated kconfig and booted on a SONiC switch
• Confirmed: cat /sys/kernel/mm/transparent_hugepage/enabled is set as follows:
  always [madvise] never

Details if related

[prabhataravind]

/azp run Azure.sonic-linux-kernel

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Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[mssonicbld]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[saiarcot895]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

Updates SONiC’s kernel config fragment to change the compile-time default mode for Transparent Huge Pages (THP) from always to madvise, so THP is only used when applications explicitly opt in via madvise(MADV_HUGEPAGE).

Changes:

• Add CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y to the SONiC featureset config fragment.
• Document the intent inline with a short comment.

[prabhataravind]

@prsunny for viz

[prsunny]

lgtm, @saiarcot895 , review/signoff

[paulmenzel]

This is the recommended setting for latency-sensitive and embedded networking workloads (consistent with guidance by other linux distributions for similar environments).

Could you please elaborate, and add references?