On Thu, Aug 13, 2009 at 11:02 PM, Microbit_Ubuntu<microbit@xxxxxxxxxxxxxxxxxxxxxx> wrote: > Hi Mohammed, > > On Thu, 2009-08-13 at 15:59 +0300, Mohammed Gamal wrote: >> Hi All, >> I know that kernel preemption exists in order to allow high-priority >> processes to interrupt the kernel if the kernel executes on relatively >> long code paths in order to improve latency times and process >> responsiveness. However, I am curious to know when and where the >> kernel gets preempted and at what code paths. I searched LXR for >> instances where preempt_schedule() gets called for instance but it >> seems it never gets explicitly called anywhere in the kernel (please >> CCIW). So, when exactly does the kernel get preempted? A pointer to >> the actual code path where that happens is appreciated. >> >> Regards, >> Mohammed >> >> -- >> To unsubscribe from this list: send an email with >> "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx >> Please read the FAQ at http://kernelnewbies.org/FAQ >> > > Because I had the indexer/search from Eclipse on into my embedded target kernel > anyway, I did a quick scan for "preempt_schedule". > > With 2.6.29.4 kernel I get 35 matches : > > - 4 in system.map > - 1 in module.symvers > - 3 in sched.c > Line 4681 : asmlinkage void __sched preempt_schedule(void) > Line 4712 : asmlinkage void __sched preempt_schedule_irq(void) > and just the exported symbol > - 2 in kprobes.c > the only relevant is : > static struct kprobe_blackpoint kprobe_blacklist[] = { > {"preempt_schedule",}, > {NULL} /* Terminator */ > - 2 in preempt.h > the relevant is macro : > #define preempt_check_resched() \ > do { \ > if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \ > preempt_schedule(); \ > } while (0) > - 2 in Documentation /ftrace.txt > Line 811 & 1043 > - one for each entry in /arch/* .. eg. in arch/arm/kernel/entry-armv.S : > #ifdef CONFIG_PREEMPT > svc_preempt: > mov r8, lr > 1: bl preempt_schedule_irq @ irq en/disable is done inside > ldr r0, [tsk, #TI_FLAGS] @ get new tasks TI_FLAGS > tst r0, #_TIF_NEED_RESCHED > moveq pc, r8 @ go again > b 1b > #endif > > Perhaps that'll give a hand. > > HTH > -- > Best regards, > Kris > > > > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx > Please read the FAQ at http://kernelnewbies.org/FAQ > > Hi All, The search on LXR revealed the same thing, basically preempt_schedule_irq() is called explicitly only within an entry.S routine in each architecture (it was resume_kernel on ia32 IIRC), which does make some sense. As far as I understood, Michi's answer explains why and when the kernel *can* get preempted, however what I really want to know is when and where kernel preemption is *triggered*. Please correct me if I did misunderstand anything. Regards, Mohammed -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
