Expensive timeout(9) function ?
John Baldwin
jhb at FreeBSD.org
Tue Jan 6 07:21:26 PST 2004
On 05-Jan-2004 Bruce Evans wrote:
> On Sun, 4 Jan 2004, Bjoern A. Zeeb wrote:
>
>> what reports do you expect with the
>>
>> "Expensive timeout(9) function"
>>
>> message ?
>
> What you reported (function names and timeout time) is interesting.
>
> Why do we see it ?
>
> Kernel bugs :-).
>
>> Expensive timeout(9) function: 0xc04885a0(0) 1.024846430 s [1]
>> Expensive timeout(9) function: 0xc04885a0(0) 1.024846430 s [1]
>> Expensive timeout(9) function: 0xc04b3940(0) 0.008629758 s [2]
>> Expensive timeout(9) function: 0xc04b39a0(0) 0.004333781 s [2]
>> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3]
>> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3]
>> Expensive timeout(9) function: 0xc04f71f0(0) 0.027004551 s [3]
>>
>> [1] sys/kern/kern_synch.c:loadav()
>> [2] sys/kern/uipc_domain.c:pfslowtimo()
>> [3] sys/netinet/ip_fw2.c:ipfw_tick()
>
> [1] is easiest to understand. loadav() is obviously broken since it uses
> sleep locks. Apparently it sometimes sleeps for more than 1 second
> altogether! There is a check for sleeping in timeouts under DIAGNOSTIC.
> I would expect to complaints from this too if you just used DIAGNOSTIC
> to get the above.
>
> [3] ipfw_tick() is obviously broken in the same way. This is from
> blind conversion of splimp() to a sleep lock. Mutexes work quite
> differently from spl's. A quick fix for timeout routines that only
> lock things once might be to use mtx_trylock() and not do anything in
> the timeout routine (except re-arm the timeout, perhaps with a smaller
> interval) if the mutex cannot be acquired immediately. This depends
> on the exact timing of timeout routines not being critical (not that
> we have exact timing -- the above shows all timeouts being delayed by
> a factor of at least 100 (1 second instead of 1/100 seconds)). This
> should work expecially well in loadav() -- loadav() intentionally adds
> jitter to the interval. This might have worked in schedcpu() too
> (schedcpu() was converted to a thread).
Ugh, loadav() needs to move to a thread, too, then. Perhaps loadav()
and schedcpu() can share a thread by having the schedcpu thread just
run loadav() occasionally.
--
John Baldwin <jhb at FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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