On some CPUs, there is a small window in the idle task which might cause a large latency to wakeup a process. http://www.linux-mips.org/archives/linux-mips/2005-11/msg00114.html This can be avoided on some CPUs which can use xxx_wait_irqoff(), but still there are many CPUs out of luck. And now we have dyntick/tickless kernel. On tickless kernel the problem might become more serious. We cannot know the worst latency time. Theoretically a task can lose wakeup-event forever. Of course "nowait" kernel option will help, but are there any other good solutions? Just an idea: If we put an WAIT in hazard area of the MTC0 which enables interrupts, can we accomplish something like atomic-test-and-wait operation? void r4k_wait_bulletproof(void) { local_irq_disable(); if (!need_resched()) __asm__( " .set push \n" " .set mips3 \n" " .set noat \n" " .align 4 \n" /* avoid stall on wait */ " mfc0 $1, $12 \n" " ori $1, 1 \n" " mtc0 $1, $12 \n" " wait \n" " xori $1, 1 \n" " mtc0 $1, $12 \n" " .set pop \n"); local_irq_enable(); } If this work as expected? Comments from pipeline gurus are welcome ;) --- Atsushi Nemoto
