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SubjectRe: [patch] Real-Time Preemption, -RT-2.6.10-rc1-mm3-V0.7.25-0
Ingo Molnar wrote:

>i have released the -V0.7.25-0 Real-Time Preemption patch, which can be
>downloaded from the usual place:
>this release includes fixes, new features and latency improvements.
>the biggest change is the threading of the lone remaining non-threaded
>external interrupt: the timer interrupt (IRQ#0).
>It was not threaded until now because unlike device interrupts the timer
>interrupt is quite deeply attached to Linux's architecture, driving
>process timing, profiling and the scheduler. I separated these into
>stuff that can be done from a thread context and stuff that needs to
>execute directly.
>Fortunately most of the expensive and latency-generating stuff could be
>pushed into the irq thread. As a side-effect this enabled the turning of
>rtc_lock and xtime_lock into a mutex. Also, it removed some ugly hacks
>from rtc.c and should improve worst-case latencies.
>the other bigger change is the reworking of the .config space. Instead
>of the deep hierarchy of hard-to-understand technical PREEMPT options,
>there's now a flattened out choice of 4 preemption models:
> ( ) No Forced Preemption (Server)
> ( ) Voluntary Kernel Preemption (Desktop)
> ( ) Preemptible Kernel (Low-Latency Desktop)
> (X) Complete Preemption (Real-Time)
>and updated help texts. Plus for the preemption models where it can be
>freely turned off, softirq and hardirq threading is an additional
>the third bigger change is the reworking of the tracer to make it easier
>to drive it from user-space. There are 3 runtime flags now:
> /proc/sys/kernel/trace_enabled (default: 1)
> /proc/sys/kernel/trace_user_triggered (default: 0)
> /proc/sys/kernel/trace_freerunning (default: 0)
>semantics of user-triggered tracing: if enabled then any active tracing
>of wakeup and/or critical sections stops and userspace drives start/stop
>events via gettimeofday(0,1)/gettimeofday(0,0). The latter saves the
>current trace into /proc/latency_trace, the former clears the trace
>buffer and starts tracing anew. Doing another gettimeofday(0,1) on an
>already running tracer clears the trace and restarts it without saving
>the current trace into /proc/latency_trace. Doing a gettimeofday(0,0) on
>an already stopped tracer has no effect (i.e. /proc/latency_trace wont
>be saved a second time). The tracer does timing for userspace
>automatically the same way it does it for the built-in timing
>mechanisms, and it can be configured via the preempt_max_latency and
>preempt_tresh tunables.
>also, wakeup-timing, irq-off and preempt-off critical section timing can
>now be done at once again, the /proc/sys/kernel/preempt_wakeup_timing
>flag switches between the modes. (default: 1)
>other changes since -V0.7.24:
> - debug feature: added the RTC-debug patch sanitized by K.R. Foley,
> plus further cleanups.
> - added upstream fix for kobject related crash, pointed out by Shane
> Shrybman.
> - cleanup: Kconfig help text fixes from Amit Shah
> - latency improvement: on UP-IOAPIC, when redirecting an interrupt, do
> not ack the APIC. This is the method used for direct interrupts and
> on UP it might as well work out fine. It is certainly faster than
> masking/unmasking, making UP-IOAPIC the fastest PIC mode again.
> - livelock fix: the timer-irq threading unearthed a seqlock related
> livelock scenario, which triggered in do_gettimeoffset() big-time.
> The solution is to serialize seqlock readers with writers _iff_ the
> seqlock status is 'invalid'. This is a rare event, but when it
> happens it saves the day.
> - debugging helper: the /proc/sys/kernel/debug_direct_keyboard flag
> (default: 0) will hack the keyboard IRQ into being direct. NOTE: the
> keyboard in this mode should only be used to access SysRq
> functionality that is not possible via the threaded keyboard handler.
> The direct keyboard IRQ can crash the system.
> - new kernel profiling features: added profile=preempt to profile
> whether interrupts hit the kernel in preemptible mode or in a
> critical section. Added /proc/sys/kernel/prof_pid to profile a
> specific PID only. (default: -1, meaning all tasks profiled) Added
> /proc/irq/prof_cpu_mask back.
> - robustness improvement: do not report atomicity-warnings during
> kernel oopses - it's more important to get the oops out to the
> console.
>to create a -V0.7.25-0 tree from scratch, the patching order is:
> Ingo
Got 2 times a hard lock up with this one. Happened while i was typing
something and downloading both after 15-20 minutes.
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