kernel/linux.git/Documentation/kernel-hacking, branch v6.6.132

Documentation: Add document for false sharing

2023-04-10T22:46:11+00:00

When doing performance tuning or debugging performance regressions, more and more cases are found to be related to false sharing [1][2][3], and the situation can be worse for newer platforms with hundreds of CPUs. There are already many commits in current kernel specially for mitigating the performance degradation due to false sharing. False sharing could harm the performance silently without being noticed, due to reasons like: * data members of a big data structure randomly sitting together in one cache line * global data of small size are linked compactly together So it's better to make a simple document about the normal pattern of false sharing, basic ways to mitigate it and call out to developers to pay attention during code-writing. [ Many thanks to Dave Hansen, Ying Huang, Tim Chen, Julie Du and Yu Chen for their contributions ] [1]. https://lore.kernel.org/lkml/20220619150456.GB34471@xsang-OptiPlex-9020/ [2]. https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/ [3]. https://lore.kernel.org/lkml/20230307125538.818862491@linutronix.de/ Signed-off-by: Feng Tang Reviewed-by: Randy Dunlap Reviewed-by: Shakeel Butt Link: https://lore.kernel.org/r/20230407041235.37886-1-feng.tang@intel.com Signed-off-by: Jonathan Corbet

docs: locking: Discourage from calling disable_irq() in atomic

2023-01-11T18:45:26+00:00

Correct the example in the documentation so that disable_irq() is not being called in atomic context. disable_irq() calls sleeping synchronize_irq(), it's not allowed to call them in atomic context. Signed-off-by: Alexander Sverdlin Signed-off-by: Thomas Gleixner Reviewed-by: Manfred Spraul Cc: linux-doc@vger.kernel.org Link: https://lore.kernel.org/lkml/87k02wbs2n.ffs@tglx/ Link: https://lore.kernel.org/r/20221212163715.830315-1-alexander.sverdlin@siemens.com

Merge tag 'timers-core-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2022-12-12T20:52:02+00:00

Pull timer updates from Thomas Gleixner: "Updates for timers, timekeeping and drivers: Core: - The timer_shutdown[_sync]() infrastructure: Tearing down timers can be tedious when there are circular dependencies to other things which need to be torn down. A prime example is timer and workqueue where the timer schedules work and the work arms the timer. What needs to prevented is that pending work which is drained via destroy_workqueue() does not rearm the previously shutdown timer. Nothing in that shutdown sequence relies on the timer being functional. The conclusion was that the semantics of timer_shutdown_sync() should be: - timer is not enqueued - timer callback is not running - timer cannot be rearmed Preventing the rearming of shutdown timers is done by discarding rearm attempts silently. A warning for the case that a rearm attempt of a shutdown timer is detected would not be really helpful because it's entirely unclear how it should be acted upon. The only way to address such a case is to add 'if (in_shutdown)' conditionals all over the place. This is error prone and in most cases of teardown not required all. - The real fix for the bluetooth HCI teardown based on timer_shutdown_sync(). A larger scale conversion to timer_shutdown_sync() is work in progress. - Consolidation of VDSO time namespace helper functions - Small fixes for timer and timerqueue Drivers: - Prevent integer overflow on the XGene-1 TVAL register which causes an never ending interrupt storm. - The usual set of new device tree bindings - Small fixes and improvements all over the place" * tag 'timers-core-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (34 commits) dt-bindings: timer: renesas,cmt: Add r8a779g0 CMT support dt-bindings: timer: renesas,tmu: Add r8a779g0 support clocksource/drivers/arm_arch_timer: Use kstrtobool() instead of strtobool() clocksource/drivers/timer-ti-dm: Fix missing clk_disable_unprepare in dmtimer_systimer_init_clock() clocksource/drivers/timer-ti-dm: Clear settings on probe and free clocksource/drivers/timer-ti-dm: Make timer_get_irq static clocksource/drivers/timer-ti-dm: Fix warning for omap_timer_match clocksource/drivers/arm_arch_timer: Fix XGene-1 TVAL register math error clocksource/drivers/timer-npcm7xx: Enable timer 1 clock before use dt-bindings: timer: nuvoton,npcm7xx-timer: Allow specifying all clocks dt-bindings: timer: rockchip: Add rockchip,rk3128-timer clockevents: Repair kernel-doc for clockevent_delta2ns() clocksource/drivers/ingenic-ost: Define pm functions properly in platform_driver struct clocksource/drivers/sh_cmt: Access registers according to spec vdso/timens: Refactor copy-pasted find_timens_vvar_page() helper into one copy Bluetooth: hci_qca: Fix the teardown problem for real timers: Update the documentation to reflect on the new timer_shutdown() API timers: Provide timer_shutdown[_sync]() timers: Add shutdown mechanism to the internal functions timers: Split [try_to_]del_timer[_sync]() to prepare for shutdown mode ...

timers: Update the documentation to reflect on the new timer_shutdown() API

2022-11-24T14:09:12+00:00

In order to make sure that a timer is not re-armed after it is stopped before freeing, a new shutdown state is added to the timer code. The API timer_shutdown_sync() and timer_shutdown() must be called before the object that holds the timer can be freed. Update the documentation to reflect this new workflow. [ tglx: Updated to the new semantics and updated the zh_CN version ] Signed-off-by: Steven Rostedt (Google) Signed-off-by: Thomas Gleixner Tested-by: Guenter Roeck Reviewed-by: Jacob Keller Reviewed-by: Anna-Maria Behnsen Link: https://lore.kernel.org/r/20221110064147.712934793@goodmis.org Link: https://lore.kernel.org/r/20221123201625.375284489@linutronix.de

Documentation: Replace del_timer/del_timer_sync()

2022-11-24T14:09:11+00:00

Adjust to the new preferred function names. Suggested-by: Steven Rostedt Signed-off-by: Thomas Gleixner Reviewed-by: Jacob Keller Reviewed-by: Anna-Maria Behnsen Link: https://lore.kernel.org/r/20221123201625.075320635@linutronix.de

Documentation: Remove bogus claim about del_timer_sync()

2022-11-24T14:09:10+00:00

del_timer_sync() does not return the number of times it tried to delete the timer which rearms itself. It's clearly documented: The function returns whether it has deactivated a pending timer or not. This part of the documentation is from 2003 where del_timer_sync() really returned the number of deletion attempts for unknown reasons. The code was rewritten in 2005, but the documentation was not updated. Signed-off-by: Thomas Gleixner Reviewed-by: Jacob Keller Reviewed-by: Anna-Maria Behnsen Link: https://lore.kernel.org/r/20221123201624.452282769@linutronix.de

Documentation: Fix spelling mistake in hacking.rst

2022-10-24T17:27:51+00:00

Fix `botton half locks` to `bottom half locks`. Signed-off-by: Mushahid Hussain Link: https://lore.kernel.org/r/20221017112026.88324-1-mushi.shar@gmail.com Signed-off-by: Jonathan Corbet

docs: process: remove outdated submitting-drivers.rst

2022-07-14T21:03:57+00:00

Commit 31b24bee3357 ("docs: add a warning to submitting-drivers.rst") in October 2016 already warns "This (...) should maybe just be deleted, but I'm not quite ready to do that yet". Maybe, six years ago, we were not ready but let us remove old content for the better now and structure and maintain less content in the kernel documentation with a better result. Drop this already outdated document and adjust all textual references. Here is an argument why deleting the content will not remove any useful information to the existing kernel documentation, individually broken down for each section. Section "Allocating Device Numbers" refers to https://www.lanana.org/, and then refers to Documentation/admin-guide/devices.rst. However, the devices.rst clearly states: "The version of this document at lanana.org is no longer maintained." Everything needed for submitting drivers is already stated in devices.rst and the reference to https://www.lanana.org/ is outdated, and should be just deleted. Section "Who To Submit Drivers To" is all about Linux 2.0 - 2.6, before the new release version scheme; the mentioned developers are still around, but actually not the first developers to contact anymore. Section "What Criteria Determine Acceptance" has a few bullet points: Licensing and Copyright is well-covered in process/kernel-license.rst. Interfaces, Code, Portability, Clarity state some obvious things about ensuring kernel code quality. Control suggests to add a MAINTAINERS entry, which is already mentioned in 6.Followthrough.rst: "... added yourself to the MAINTAINERS file..." PM support states a bit about implementing and testing power management of a driver, it remains an open question where to place that in the process documents. Driver developers interested in power management will find the corresponding part on power management in the kernel documentation anyway. In section "What Criteria Do Not Determine Acceptance", the points Vendor and Author states something basic consequence of the kernel being an open-source community software development. Probably no need to mention it nowadays. Section "Resources" lists resources that are also mentioned elsewhere more central. - Linux kernel tree and mailing list is mentioned in many places. - https://lwn.net/Kernel/LDD3/ is mentioned in Documentation/process/kernel-docs.rst. - https://lwn.net/ is mentioned in: - Documentation/process/8.Conclusion.rst - Documentation/process/kernel-docs.rst - https://kernelnewbies.org/ is mentioned in: - Documentation/process/8.Conclusion.rst - Documentation/process/kernel-docs.rst - http://www.linux-usb.org/ is mentioned in Documentation/driver-api/usb/usb.rst - https://landley.net/kdocs/ols/2002/ols2002-pages-545-555.pdf is mentioned in Documentation/process/kernel-docs.rst - https://kernelnewbies.org/KernelJanitors is mentioned in Documentation/process/howto.rst - https://git-scm.com/ is mentioned in - Documentation/process/2.Process.rst - Documentation/process/7.AdvancedTopics.rst - Documentation/process/howto.rst Signed-off-by: Lukas Bulwahn Link: https://lore.kernel.org/r/20220704122537.3407-7-lukas.bulwahn@gmail.com Signed-off-by: Jonathan Corbet

Documentation: kernel-hacking: minor edits for style

2022-04-05T15:53:24+00:00

Rusty's kernel-hacking guides provide important information, however they are written in a narrative style that some readers may interpret as off-putting. Since the goal is to make kernel documentation accessible to as many new developers as possible, it's best to avoid the turns of phrase that require a specific cultural context to properly understand. Signed-off-by: Konstantin Ryabitsev Signed-off-by: Jonathan Corbet

docs: fix typo in Documentation/kernel-hacking/locking.rst

2022-01-27T18:22:33+00:00

Change copy_from_user*( to copy_from_user() . Signed-off-by: Takahiro Itazuri Link: https://lore.kernel.org/r/20220124081447.34066-1-itazur@amazon.com Signed-off-by: Jonathan Corbet