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df180b1a4c
The SMMU CMDQ lock is highly contentious when there are multiple CPUs
issuing commands and the queue is nearly full.
The lock has the following states:
- 0: Unlocked
- >0: Shared lock held with count
- INT_MIN+N: Exclusive lock held, where N is the # of shared waiters
- INT_MIN: Exclusive lock held, no shared waiters
When multiple CPUs are polling for space in the queue, they attempt to
grab the exclusive lock to update the cons pointer from the hardware. If
they fail to get the lock, they will spin until either the cons pointer
is updated by another CPU.
The current code allows the possibility of shared lock starvation
if there is a constant stream of CPUs trying to grab the exclusive lock.
This leads to severe latency issues and soft lockups.
Consider the following scenario where CPU1's attempt to acquire the
shared lock is starved by CPU2 and CPU0 contending for the exclusive
lock.
CPU0 (exclusive) | CPU1 (shared) | CPU2 (exclusive) | `cmdq->lock`
--------------------------------------------------------------------------
trylock() //takes | | | 0
| shared_lock() | | INT_MIN
| fetch_inc() | | INT_MIN
| no return | | INT_MIN + 1
| spins // VAL >= 0 | | INT_MIN + 1
unlock() | spins... | | INT_MIN + 1
set_release(0) | spins... | | 0 see[NOTE]
(done) | (sees 0) | trylock() // takes | 0
| *exits loop* | cmpxchg(0, INT_MIN) | 0
| | *cuts in* | INT_MIN
| cmpxchg(0, 1) | | INT_MIN
| fails // != 0 | | INT_MIN
| spins // VAL >= 0 | | INT_MIN
| *starved* | | INT_MIN
[NOTE] The current code resets the exclusive lock to 0 regardless of the
state of the lock. This causes two problems:
1. It opens the possibility of back-to-back exclusive locks and the
downstream effect of starving shared lock.
2. The count of shared lock waiters are lost.
To mitigate this, we release the exclusive lock by only clearing the sign
bit while retaining the shared lock waiter count as a way to avoid
starving the shared lock waiters.
Also deleted cmpxchg loop while trying to acquire the shared lock as it
is not needed. The waiters can see the positive lock count and proceed
immediately after the exclusive lock is released.
Exclusive lock is not starved in that submitters will try exclusive lock
first when new spaces become available.
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Alexander Grest <Alexander.Grest@microsoft.com>
Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
Signed-off-by: Will Deacon <will@kernel.org>
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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