JDK-8310233: Fix THP detection on Linux

[tstuefe]

Today, if we use UseTransparentHugePages, we assume that the static hugepage detection we do is valid for THPs:

• that THPs use the page size (in hotspot used as "default large page size") found in /proc/memlimit "Hugepagesize")
• that THPs are enabled if that page size is >0.

Both assumptions are incorrect:

• whether THPs are enabled should be checked at /sys/kernel/mm/transparent_hugepage/enabled, which is a tri-state value ("always", "madvise", "never"). THPs are available for the first two states.
• The page size employed by khugepaged is set in /sys/kernel/mm/transparent_hugepage/hpage_pmd_size. It can differ from the default page size used for static hugepages. For example, we could configure a system such that it uses 1G static hugepages, but the THP page size would still be 2M.

---

About the patch:

This is a limited, minimally invasive patch to fix THP detection. The patch aims to be easy to downport. There is more work to do, which I will do in subsequent RFEs.

Functionally, for static (non-THP) pages nothing changes. THP-mode now correctly detects THP support in the OS, and uses the correct page size (see examples below).

---

Example 1: System has THPs disabled, but static hugepages (1g, 2m) configured:

thomas@starfish $ cat /sys/kernel/mm/transparent_hugepage/enabled
always madvise [never]
thomas@starfish $ cat /proc/meminfo | grep Hugepage
Hugepagesize:    1048576 kB

Without patch, we incorrectly assume THPs are enabled, and that THP page size is 1G (!), which we then proceed and use as heap page size, causing the heap size to be rounded up from 512m -> 1G. But - even though it is printed as "1G page backed" in log output - in reality it will still 4K-page-backed: the madvise(2) we use to set the THP page size will be ignored by the system, since THPs are disabled.

thomas@starfish $ ./images/jdk/bin/java -Xmx512m -XX:+UseLargePages -XX:+UseTransparentHugePages -Xlog:pagesize -version
[0.001s][info][pagesize] Using the default large page size: 1G
[0.001s][info][pagesize] Usable page sizes: 4k, 2M, 1G
...
[0.016s][info][pagesize] Heap:  min=1G max=1G base=0x00000000c0000000 size=1G page_size=1G

With patch, we correctly refuse to use large pages (and we log more info):

thomas@starfish $ ./images/jdk/bin/java -Xmx512m -XX:+UseLargePages -XX:+UseTransparentHugePages -Xlog:pagesize -version
[0.001s][info][pagesize] Static hugepage support: 2M, 1G (default)
[0.001s][info][pagesize]   default pagesize: 1G
[0.001s][info][pagesize] Transparent hugepage (THP) support:
[0.001s][info][pagesize]   mode: never
[0.001s][warning][pagesize] UseLargePages disabled, no large pages configured and available on the system.

---

Example 2: System has THPs enabled, but THP page size is just 2M, whereas the system uses a static default hugepage size of 1G:

thomas@starfish $ cat /sys/kernel/mm/transparent_hugepage/enabled
always [madvise] never
thomas@starfish $ cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size 
2097152
thomas@starfish $ cat /proc/meminfo | grep Hugepage
Hugepagesize:    1048576 kB

Without patch, THP page size is not correctly recognized as 2M. Instead, we again use 1G as page size for the heap:

thomas@starfish $ ./images/jdk/bin/java -Xmx512m -XX:+UseLargePages -XX:+UseTransparentHugePages -Xlog:pagesize -version
[0.001s][info][pagesize] Using the default large page size: 1G
[0.001s][info][pagesize] Usable page sizes: 4k, 2M, 1G
...
[0.010s][info][pagesize] Heap:  min=1G max=1G base=0x00000000c0000000 size=1G page_size=1G

With patch, we correctly identify the THP page size as 2M, and use that for the heap:

thomas@starfish $ ./images/jdk/bin/java -Xmx512m -XX:+UseLargePages -XX:+UseTransparentHugePages -Xlog:pagesize -version
[0.001s][info][pagesize] Static hugepage support: 2M, 1G (default)
[0.001s][info][pagesize]   default pagesize: 1G
[0.001s][info][pagesize] Transparent hugepage (THP) support:
[0.001s][info][pagesize]   mode: madvise
[0.001s][info][pagesize]   pagesize: 2M
[0.001s][info][pagesize] Large page support enabled. Usable page sizes: 4k, 2M
[0.001s][info][pagesize]  Default: 2M
...
[0.010s][info][pagesize] Heap:  min=8M max=512M base=0x00000000e0000000 size=512M page_size=2M

---

Tests: GHAs all green. Local experiments on x64 Linux on machines with 1G pages succeeded.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8310233: Fix THP detection on Linux (Bug - P4)

Reviewers

• Johan Sjölen (@jdksjolen - Reviewer) ⚠️ Review applies to 69c39c8a
• David Holmes (@dholmes-ora - Reviewer)

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[jdksjolen]

Hi Thomas,

Thanks for these changes. I've some suggestions for the code, but the conceptual change is good.

[openjdk]

⚠️ @tstuefe This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

[tstuefe]

@jdksjolen Thanks a lot for your review!

See inline remarks. Most of your code suggestions are good, but this patch just moved the static hugepage detection parts out of os_linux.cpp, and left them (mostly) alone otherwise, to avoid adding regressions. I'll keep your input in mind for the next round of cleanups.

Wrt NonInterleavingLogStream, I decided to not do that. It is not needed, since we are single-threaded, and it without it I may still be lucky to get it integrated cleanly to at least 17.

I added a new repro case. Please take a look!

Thank you.

[jdksjolen]

Some of the methods seems to leak file descriptors, other than that tests looks good.

[tstuefe]

Thanks @jdksjolen! I changed the test accordingly.

[jdksjolen]

Thank you Thomas!

These changes look good to me now, I'm approving this.

[openjdk]

@tstuefe This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8310233: Fix THP detection on Linux

Reviewed-by: jsjolen, dholmes

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 127 new commits pushed to the master branch:

• 81c4e8f: 8304006: jlink should create the jimage file in the native endian for the target platform
• e8f66bf: 8310949: RISC-V: Initialize UseUnalignedAccesses
• 19691fa: 6361826: (reflect) provide method for mapping strings to class object for primitive types
• c84866a: 8310551: vmTestbase/nsk/jdb/interrupt/interrupt001/interrupt001.java timed out due to missing prompt
• 0d2196f: 8311992: Test java/lang/Thread/virtual/JfrEvents::testVirtualThreadPinned failed
• f3b96f6: 8311862: RISC-V: small improvements to shift immediate instructions
• a63f865: 8311946: add support for libgraal specific jtreg tests
• 167d1c1: 8311986: Disable runtime/os/TestTracePageSizes.java for ShenandoahGC
• 7539cc0: 8303134: JFR: Missing stack trace during chunk rotation stress
• d1fa1a8: 8311825: Duplicate qualified enum constants not detected
• ... and 117 more: https://git.openjdk.org/jdk/compare/33011ea19bb29e88ce18a138a8fa8b34f8c97407...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

[tstuefe]

Gentle ping, second review needed.

[tstuefe]

Reviewer notes:
scan_default_hugepagesize is just a code move, it used to live in scan_default_large_page_size in os_linux.cpp. It is responsible for reading the default static hugepage size from /proc/meminfo. This code is mostly untouched to reduce chance for regressions (though the code could be tightened and cleaned for sure).

[tstuefe]

May I have a second review?

[tstuefe]

@jdksjolen was kind enough to put this through Oracle's CI, all good.

[dholmes-ora]

Not an area I am familiar with but scanning through the code this seems reasonable. A few minor comments below.

Thanks.

[tstuefe]

Not an area I am familiar with but scanning through the code this seems reasonable. A few minor comments below.

Thanks.

Many thanks, @dholmes-ora! Remarks follow.

[tstuefe]

@dholmes-ora : I massaged in your feedback.

[dholmes-ora]

Nothing further from me.

Thanks.

[tstuefe]

Thanks, @dholmes-ora !

/integrate

[openjdk]

Going to push as commit 37ca902.
Since your change was applied there have been 127 commits pushed to the master branch:

• 81c4e8f: 8304006: jlink should create the jimage file in the native endian for the target platform
• e8f66bf: 8310949: RISC-V: Initialize UseUnalignedAccesses
• 19691fa: 6361826: (reflect) provide method for mapping strings to class object for primitive types
• c84866a: 8310551: vmTestbase/nsk/jdb/interrupt/interrupt001/interrupt001.java timed out due to missing prompt
• 0d2196f: 8311992: Test java/lang/Thread/virtual/JfrEvents::testVirtualThreadPinned failed
• f3b96f6: 8311862: RISC-V: small improvements to shift immediate instructions
• a63f865: 8311946: add support for libgraal specific jtreg tests
• 167d1c1: 8311986: Disable runtime/os/TestTracePageSizes.java for ShenandoahGC
• 7539cc0: 8303134: JFR: Missing stack trace during chunk rotation stress
• d1fa1a8: 8311825: Duplicate qualified enum constants not detected
• ... and 117 more: https://git.openjdk.org/jdk/compare/33011ea19bb29e88ce18a138a8fa8b34f8c97407...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tstuefe Pushed as commit 37ca902.

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