From owner-freebsd-mips@FreeBSD.ORG  Tue Sep 27 03:36:27 2011
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Date: Tue, 27 Sep 2011 11:36:26 +0800
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From: Adrian Chadd <adrian@freebsd.org>
To: John Baldwin <jhb@freebsd.org>, attilio@freebsd.org
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Cc: freebsd-current@freebsd.org, freebsd-mips@freebsd.org
Subject: Re: ath / 802.11n performance issues and timer code
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.. and as a follow up (and cc'ing attillo and freebsd-mips, in case
it's relevant to other platforms and there's a MIPS specific thing to
fix):

* 2128: mi_switch to idle
* 2129: kern_clocksource.c:762 - ie, cpu_idleclock() has been called
* 2130: the ath interrupt comes in
* 2134: it's skipped for now as the idle thread is in a critical section
* 2136: kern_clocksource.c:266 - ie, getnextcpuevent(), inside cpu_idleclock().

What I bet is happening is this race between the critical section +
cpu_idleclock() and the ath0 interrupt:

* idle gets scheduled
* critical_enter() is called in the mips cpu_idle() routine
* the ath interrupt comes in here and gets handled, but since we're in
a critical section, it won't preempt things
* the cpu_idleclock() code completes without releasing the preemption,
and the only thing that wakes up from that wait is the next interrupt
(clock, arge0, etc.)




Adrian