Messages in this thread | | | Date | Sun, 23 May 2004 20:38:14 +0100 | From | Russell King <> | Subject | Re: scheduler: IRQs disabled over context switches |
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On Sun, May 23, 2004 at 11:59:01AM -0700, Davide Libenzi wrote: > On Sun, 23 May 2004, Russell King wrote: > > > The 2.6.6 scheduler disables IRQs across context switches, which is > > bad news for IRQ latency on ARM - to the point where 16550A FIFO > > UARTs to overrun. > > > > I'm considering defining prepare_arch_switch & co as follows on ARM, > > so that we release IRQs over the call to context_switch(). > > > > #define prepare_arch_switch(rq,next) \ > > do { \ > > spin_lock(&(next)->switch_lock); \ > > spin_unlock_irq(&(rq)->lock); \ > > } while (0) > > #define finish_arch_switch(rq,prev) \ > > spin_unlock(&(prev)->switch_lock) > > #define task_running(rq,p) \ > > ((rq)->curr == (p) || spin_is_locked(&(p)->switch_lock)) > > > > The question is... why are we keeping IRQs disabled over context_switch() > > in the first case? Looking at the code, the only thing which is touched > > outside of the two tasks is rq->prev_mm. Since runqueues are CPU- > > specific and we're holding at least one spinlock, I think the above > > is preempt safe and SMP safe. > > Other archs already do the above.
Not quite - look harder. They use spin_unlock_irq in finish_arch_switch rather than prepare_arch_switch.
Ralf Baechle mentioned that he thinks the above on MIPS causes some rare but weird problems, though he wasn't able to give anything specific at the time.
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/ 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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