Expensive timeout(9) function ?
Bruce Evans
bde at zeta.org.au
Sun Jan 4 23:22:42 PST 2004
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).
Bruce
More information about the freebsd-current
mailing list