6.2 SHOWSTOPPER - em completely unusable on 6.2

[Scott Long]

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