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Sender: Alexander Motin <mavbsd@gmail.com>
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Date: Tue, 27 Sep 2011 18:56:13 +0300
From: Alexander Motin <mav@FreeBSD.org>
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To: Adrian Chadd <adrian@freebsd.org>
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Cc: freebsd-current@freebsd.org
Subject: Re: ath / 802.11n performance issues and timer code
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Alexander Motin wrote:
> Adrian Chadd wrote:
>> .. erm, sys/mips/mips/machdep.c:
>>
>> /*
>>  * call platform specific code to halt (until next interrupt) for the idle loop
>>  */
>> void
>> cpu_idle(int busy)
>> {
>>         KASSERT((mips_rd_status() & MIPS_SR_INT_IE) != 0,
>>                 ("interrupts disabled in idle process."));
>>         KASSERT((mips_rd_status() & MIPS_INT_MASK) != 0,
>>                 ("all interrupts masked in idle process."));
>>
>>         if (!busy) {
>>                 critical_enter();
>>                 cpu_idleclock();
>>         }
>>         __asm __volatile ("wait");
>>         if (!busy) {
>>                 cpu_activeclock();
>>                 critical_exit();
>>         }
>> }
>>
>> .. does that look right?
> 
> Yes it does. x86 does the same, but with more details. The general idea
> of the critical section is to block context switch out of idle thread
> until missed time events will be handled inside cpu_activeclock().

I was wrong. That's not good. I have no idea about mips wait instruction
semantics, related to disabling interrupts. In x86 semantics proper
solution is:

	disable_intr();
	if (sched_runnable())
		enable_intr();
	else
		__asm __volatile("sti; hlt");

It makes interrupts enabled atomically with entering sleep state, that
closes race window and prevents entering into sleep after receiving
interrupt.

-- 
Alexander Motin