Peter Jeremy wrote:
On Wed, 2006-Sep-27 10:32:49 -0600, Scott Long wrote:
My theory here is that something in the kernel, likely VM/VFS, is
holding the Giant lock for an inordinate amount of time.
In the past (RELENG_5) I've had major problems with syncer delaying
interrupt threads for long periods (I've seen 8msec). See
http://lists.freebsd.org/pipermail/freebsd-stable/2005-February/012346.html
I'm not sure if this is still a problem (but I am still having some
problems which may be caused by excessive interrupt and will be doing
some debugging as I get time).
I have a few ideas on tracking down the root cause, but they are pretty
pretty painful and slow.
In my case, I was fairly certain that the problem I was seeing was
excessive interrupt latency for my driver. The approach I took was to
capture TSC, IRQ number and curproc address in lapic_handle_intr(),
atpic_handle_intr() and at the beginning of my interrupt handler into
a ring buffer. I'd dump the ring buffer into a file using a userland
tool and then post-process the file looking for oddities. In my case,
there was a _very_ high correlation between long latencies and syncer.
If anyone's interested in this approach, I can provide the relevant
code diffs.
Yes, I was thinking about the syncer too, but the timeouts for ethernet
interfaces are measured in seconds, not milliseconds.
2. Add INTR_FAST shims to the usb and ichsmb drivers so that neither
uses an ithread.
The problem I ran into with this approach was that my interrupt
handler needs to use psignal(9) - which requires PROC_LOCK() which
(AFAIK) isn't allowed in an INTR_FAST handler.
You can define a very simple INTR_FAST handler that just disables the
interrupt at the device and then schedules a taskqueue to do the real
work. This is what I did for if_em, actually.
It would be useful if our interrupt subsystem allowed both INTR_FAST
and normal interrupt handlers to be defined. If an INTR_FAST handler
is defined then it gets executed and defines whether its associated
interrupt thread handler needs to be triggered. If there's no
INTR_FAST handler then the interrupt thread is always triggered.
This was an SoC2006 project, and I believe it will be committed fairly soon.
Scott
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