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Date: Fri, 22 Feb 2008 13:09:16 -0800 From: Max Krasnyansky <maxk@...lcomm.com> To: mingo@...e.hu Cc: linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl, pj@....com, Max Krasnyansky <maxk@...lcomm.com> Subject: [PATCH sched-devel 5/7] cpuisol: Documentation updates Documented sysfs interface as suggested by Andrew Morton. Added general documentation that describes how to configure and use CPU isolation features. Signed-off-by: Max Krasnyansky <maxk@...lcomm.com> --- Documentation/ABI/testing/sysfs-devices-system-cpu | 41 +++++++ Documentation/cpu-isolation.txt | 113 ++++++++++++++++++++ 2 files changed, 154 insertions(+), 0 deletions(-) diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu new file mode 100644 index 0000000..32dde5b --- /dev/null +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu @@ -0,0 +1,41 @@ +What: /sys/devices/system/cpu/... +Date: Feb. 2008 +KernelVersion: 2.6.24 +Contact: LKML <linux-kernel@...r.kernel.org> +Description: + +The /sys/devices/system/cpu tree provides information about all cpu's +known to the running kernel. + +Following files are created for each cpu. 'N' is the cpu number. + +/sys/devices/system/cpu/cpuN/ + online (0644) On-line attribute. Indicates whether the cpu is on-line. + The cpu can be brought off-line by writing '0' into + this file. Similarly it can be brought back on-line + by writing '1' into this file. This attribute is + not available for the cpu's that cannot be brought + off-line. Typically cpu0. For more information see + Documentation/cpu-hotplug.txt + + isolated (0644) Isolation attribute. Indicates whether the cpu + is isolated. + The cpu can be isolated by writing '1' into this + file. Similarly it can be un-isolated by writing + '0' into this file. In order to isolate the cpu it + must first be brought off-line. This attribute is + not available for the cpu's that cannot be brought + off-line. Typically cpu0. + Note this attribute is present only if "CPU isolation" + is enabled. For more information see + Documentation/cpu-isolation.txt + + cpufreq (0755) Frequency scaling state. + For more info see + Documentation/cpu-freq/... + + cache (0755) Cache information. FIXME + + cpuidle (0755) Idle state information. FIXME + + topology (0755) Topology information. FIXME diff --git a/Documentation/cpu-isolation.txt b/Documentation/cpu-isolation.txt new file mode 100644 index 0000000..b9ca425 --- /dev/null +++ b/Documentation/cpu-isolation.txt @@ -0,0 +1,113 @@ +CPU isolation support in Linux(tm) Kernel + +Maintainers: + +Scheduler and scheduler domain bits: + Ingo Molnar <mingo@...e.hu> + +General framework, irq and workqueue isolation: + Max Krasnyanskiy <maxk@...lcomm.com> + +ChangeLog: +- Initial version. Feb 2008, MaxK + +Introduction +------------ + +The primary idea behind CPU isolation is the ability to use some CPU cores +as a dedicated engines for running user-space code with minimal kernel +overhead/intervention, think of it as an SPE in the Cell processor. For +example CPU isolation allows for running CPU intensive(100%) RT task +on one of the processors without adversely affecting or being affected +by the other system activities. With the current (as of early 2008) +multi-core CPU trend we may see more and more applications that explore +this capability: real-time gaming engines, simulators, hard real-time +apps, etc. + +Current CPU isolation support consists of the following features: + +1. Isolated CPU(s) are excluded from the scheduler load balancing logic. + Applications must explicitly bind threads in order to run on those + CPU(s). + +2. By default interrupts are not routed to the isolated CPU(s). + Users must route interrupts (if any) to those CPU(s) explicitly. + +3. Kernel avoids any activity on the isolated CPU(s) as much as possible. + This includes workqueues, per CPU threads, etc. Please note that + this feature is optional and is disabled by default. + +Kernel configuration options +---------------------------- + +Following options need to be enabled in order to use CPU isolation + CONFIG_CPUISOL Top-level config option. Enables general + CPU isolation framework and enables features + #1 and #2 described above. + + CONFIG_CPUISOL_WORKQUEUE These options provide deeper isolation + CONFIG_CPUISOL_STOPMACHINE from various kernel subsystems. They implement + CONFIG_CPUISOL_... feature #3 described above. + See Kconfig help for more information on each + individual option. + +How to isolate a CPU +-------------------- + +There are two ways for isolating a CPU + +Kernel boot command line: + isolcpus=n0,n1,... + + This option enables isolation for all CPU(s) specified in the comma + separated list. + Example: + isolcpus=1,5 + +Sysfs interface: + In order to isolate a CPU through this mechanism it must first be + brought off-line. Hence the command sequence looks like: + echo 0 > /sys/devices/system/cpu/cpuN/online + echo 1 > /sys/devices/system/cpu/cpuN/isolated + echo 1 > /sys/devices/system/cpu/cpuN/online + + +Kernel interfaces +----------------- + +cpu_isolated_map - Bitmap of all isolated CPU(s). + This bitmap is updated either by isolcpus= command + line option or by writing to sysfs attributes. In + the latter case isolation bit can be updated only + when the corresponding CPU is off-line. + +cpu_isolated(N) - Use that function to check whether CPU N is isolated. + + +User-space application interfaces +--------------------------------- + +From the user-space application perspective isolated CPU(s) are no +different from any other CPU in the system. Once the required CPU(s) +have been isolated applications can use standard POSIX apis to bind its +threads to those CPU(s): + sched_setaffinity + sched_getaffinity + pthread_getaffinity_np + pthread_setaffinity_np + +One thing to keep in mind though is that Linux kernel heavily uses per-CPU +data structures and mechanisms. For example if a thread running on an +isolated CPU makes a system call, that system call will be serviced +on the same CPU. Some system calls trigger timers, softirq and other +kernel activities. Those will likely also run on the same CPU where +system call was originally serviced. If this behavior is undesirable +threads running on the isolated CPU(s) should avoid using certain system +calls and instead delegate that to the threads running on non-isolated +CPU(s). For example you probably do not want to write to the filesystem +or the console. Generally, time, mutex and pthread related system calls +are perfectly safe. + +If you intend to use CPU isolation for real-time applications please +check out rt wiki at + http://rt.wiki.kernel.org/index.php/Main_Page -- 1.5.4.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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