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3970da5c3b
[CORNER CASE] If we have the following file extents layout, btrfs_get_extent() can return a smaller hole during read, and cause unnecessary extra tree searches: item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53 generation 9 type 1 (regular) extent data disk byte 13631488 nr 4096 extent data offset 0 nr 4096 ram 4096 extent compression 0 (none) item 7 key (257 EXTENT_DATA 32768) itemoff 15757 itemsize 53 generation 9 type 1 (regular) extent data disk byte 13635584 nr 4096 extent data offset 0 nr 4096 ram 4096 extent compression 0 (none) In above case, range [0, 4K) and [32K, 36K) are regular extents, and there is a hole in range [4K, 32K), and the fs has "no-holes" feature, meaning the hole will not have a file extent item. [INEFFICIENCY] Assume the system has 4K page size, and we're doing readahead for range [4K, 32K), no large folio yet. btrfs_readahead() for range [4K, 32K) |- btrfs_do_readpage() for folio 4K | |- get_extent_map() for range [4K, 8K) | |- btrfs_get_extent() for range [4K, 8K) | We hit item 6, then for the next item 7. | At this stage we know range [4K, 32K) is a hole. | But our search range is only [4K, 8K), not reaching 32K, thus | we go into not_found: tag, returning a hole em for [4K, 8K). | |- btrfs_do_readpage() for folio 8K | |- get_extent_map() for range [8K, 12K) | |- btrfs_get_extent() for range [8K, 12K) | We hit the same item 6, and then item 7. | But still we goto not_found tag, inserting a new hole em, | which will be merged with previous one. | | [ Repeat the same btrfs_get_extent() calls until the end ] So we're calling btrfs_get_extent() again and again, just for a different part of the same hole range [4K, 32K). [ENHANCEMENT] Make btrfs_do_readpage() to search for a larger extent map if readahead is involved. For btrfs_readahead() we have bio_ctrl::ractl set, and lock extents for the whole readahead range. If we find bio_ctrl::ractl is set, we can use that end range as extent map search end, this allows btrfs_get_extent() to return a much larger hole, thus reduce the need to call btrfs_get_extent() again and again. btrfs_readahead() for range [4K, 32K) |- btrfs_do_readpage() for folio 4K | |- get_extent_map() for range [4K, 32K) | |- btrfs_get_extent() for range [4K, 32K) | We hit item 6, then for the next item 7. | At this stage we know range [4K, 32K) is a hole. | So the hole em for range [4K, 32K) is returned. | |- btrfs_do_readpage() for folio 8K | |- get_extent_map() for range [8K, 32K) | The cached hole em range [4K, 32K) covers the range, | and reuse that em. | | [ Repeat the same btrfs_get_extent() calls until the end ] Now we only call btrfs_get_extent() once for the whole range [4K, 32K), other than the old 8 times. Such change will reduce the overhead of reading large holes a little. For current experimental build (with larger folios) on aarch64, there will be a tiny but consistent ~1% improvement reading a large hole file: Reading a 1GiB sparse file (all hole) using xfs_io, with 64K block size, the result is the time needed to read the whole file, reported from xfs_io. 32 runs, experimental build (with large folios). 64K page size, 4K fs block size. - Avg before: 0.20823 s - Avg after: 0.20635 s - Diff: -0.9% Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Qu Wenruo <wqu@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst