On Sun, May 26, 2013 at 1:01 AM, Hopsing K <hopsi...@gmail.com> wrote: > I'm running UML in Linux 3.2 (Debian) in SKAS0 mode. I compiled UML with > defconfig. > Now I try to poked in the source to figure out how Preemption is done > in UML. I came so far to understand that there is a ptrace-ing parent and > for each UML process a userspace-slave process on the host: > arch/um/os-Linux/skas/process.c:userspace(). This routine also has the jump > to > interrupt_end() which calls schedule(). I saw that SIGALRM handler is > installed > for that ptrace-ing parent and the userspace() loop catches SIGALRM for the > traced slave. However I didnt see it being called. Only the ptrace-ing > parent > gets SIGALRM and the signal handler arch/um/os-Linux/signal.c:hard_handler() > is called directly. With my understanding there must be some connection > between > hard_handler() and the call to interrupt_end() inside the userspace() loop, > but I > cannot find any, I cannot figure out how Preemption is initiated when the > SIGALRM arrives. > I wonder: Is all kernel code implementing cooperative threading ? Is there > no preemption when in kernel space? > If a userspace process runs a loop "while(1);" and therefore never calls out > to the kernel, how can another process been sheduled? The ptrace-ing > parent's > userspace() loop will never get control back. > I can also see a variable userspace_pid[0] that I assume is the active > slave-process and that there is only one such process running at any time, > the others are stopped? > I'm really curious how Preemption is done internally in UML, any insight > would > be welcome. > Some question at the side: can you call longjmp from within a signal > handler? My first assumption was that UML uses longjmp from within > the SIGALRM handler to implement preemption, butseems not to . > -- Thanks Konrad
UML installs a SIGVTALRM handler which feeds the TIMER_IRQ. In the generic TIMER_IRQ handler Linux runs scheduler_tick() at some point. Thanks, //richard ------------------------------------------------------------------------------ Try New Relic Now & We'll Send You this Cool Shirt New Relic is the only SaaS-based application performance monitoring service that delivers powerful full stack analytics. Optimize and monitor your browser, app, & servers with just a few lines of code. Try New Relic and get this awesome Nerd Life shirt! http://p.sf.net/sfu/newrelic_d2d_may _______________________________________________ User-mode-linux-devel mailing list User-mode-linux-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel
