AIC94XX discovery timeout problem details...

AIC94XX discovery timeout problem details...

[malahal]

I chased the time out problem and found that the PORTE_BYTES_DMAED port
event must be responded with a call to lldd_port_formed() which will
update PHY_IS_UP and port-links fields in DDB 0. As there is a single
thread handling PHY/port events as well as discovery, we really can't
handle PORTE_BYTES_DMAED event until the discovery is complete. This
results in SCSI commands timing out in the discovery thread no matter
what I do!  This problem may be unique to Vitesse expander, read the PS
section for details.

Tried using two threads, one for events and the other for discovery.
That avoided the timeout problems, but the discovery thread would die
after few iterations due to the event thread and the discovery thread
racing each other for setting up and tearing down of sysfs objects.

I tried calling lldd_port_formed() with appropriate phy_mask from the
notify_port_event() itself. That worked fine. It is just a hack for now!

Any comments, suggestions?

Thanks, Malahal.
PS: I instrumented the code to use only a single PHY out of 4-phy wide
port by prematurely returning from sas_form_port() for all PHYs except
one. In other words, I ignored the PORTE_BYTES_DMAED event for all PHYs
but one.  There were SCSI timeouts every time I did that except if the
PHY I was using is phy3.  When I swapped cables, the PHY that worked
changed too. I believe, it goes with a specific Vitesse expander PHY. Is
it possible that the Vitesse expander uses a specific PHY to respond to
some SCSI requests even though the adapter uses a different PHY while
sending the request?  Is this behavior of communicating on two different
PHYs for a single SCSI command is allowed in the spec? Also note that
the discovery commands (SMP) just work fine.

Re: AIC94XX discovery timeout problem details...

[Luben Tuikov]

--- malahal@us.ibm.com wrote:
> I chased the time out problem and found that the PORTE_BYTES_DMAED port
> event must be responded with a call to lldd_port_formed() which will
> update PHY_IS_UP and port-links fields in DDB 0. As there is a single
> thread handling PHY/port events as well as discovery, we really can't
> handle PORTE_BYTES_DMAED event until the discovery is complete. This
> results in SCSI commands timing out in the discovery thread no matter
> what I do!  This problem may be unique to Vitesse expander, read the PS
> section for details.
> 
> Tried using two threads, one for events and the other for discovery.

Malahal,

If you go back in the archives, and take a look at the SAS Stack
as I submitted it last year, you'll notice that this is exactly how
the code is: there is a separate event thread and a separate
discovery thread.

That is, from inception, my code has always had two separate threads,
one for events and one for discovery.

I wasn't aware that the code had been changed such that a single
thread handles events and discovery.  This is a very naive approach,
and a regression over the original code.

> That avoided the timeout problems, but the discovery thread would die
> after few iterations due to the event thread and the discovery thread
> racing each other for setting up and tearing down of sysfs objects.

Indeed, the situation presented in such circumstances is tricky.
These "races" have been dealt with in my (original) code.  Currently,
I don't experience any problems with my SAS Stack, as I maintain it.

If any of my original comments are still left in the code or the README
files, that would give you a hint of how such "races" are handled.

> I tried calling lldd_port_formed() with appropriate phy_mask from the
> notify_port_event() itself. That worked fine. It is just a hack for now!

This and your postscript below, tell me that you're coming around full
circle to how the original code works.  I'd suggest that you take a look
at the original code.

> Any comments, suggestions?

Good luck,
     Luben

> Thanks, Malahal.
> PS: I instrumented the code to use only a single PHY out of 4-phy wide
> port by prematurely returning from sas_form_port() for all PHYs except
> one. In other words, I ignored the PORTE_BYTES_DMAED event for all PHYs
> but one.  There were SCSI timeouts every time I did that except if the
> PHY I was using is phy3.  When I swapped cables, the PHY that worked
> changed too. I believe, it goes with a specific Vitesse expander PHY. Is
> it possible that the Vitesse expander uses a specific PHY to respond to
> some SCSI requests even though the adapter uses a different PHY while
> sending the request?  Is this behavior of communicating on two different
> PHYs for a single SCSI command is allowed in the spec?
> Also note that
> the discovery commands (SMP) just work fine.
> -
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> 

Re: AIC94XX discovery timeout problem details...

[malahal]

I am planning to process port/phy events in two phases. Phase-I sets up
asd_sas_port, asd_sas_phy lists and calls the LLDD with a correct
phy_mask. Phase-II is done in the thread context by queuing to the scsi
work queue that involves setting up sysfs objects.  Still there would be
just one thread setting up/tearing down sysfs objects that should avoid
any races.

Would appreciate any suggestions or issues with this approach.

Thanks, Malahal.

Luben Tuikov [ltuikov@yahoo.com] wrote:
> --- malahal@us.ibm.com wrote:
> > I chased the time out problem and found that the PORTE_BYTES_DMAED port
> > event must be responded with a call to lldd_port_formed() which will
> > update PHY_IS_UP and port-links fields in DDB 0. As there is a single
> > thread handling PHY/port events as well as discovery, we really can't
> > handle PORTE_BYTES_DMAED event until the discovery is complete. This
> > results in SCSI commands timing out in the discovery thread no matter
> > what I do!  This problem may be unique to Vitesse expander, read the PS
> > section for details.
> > 
> > Tried using two threads, one for events and the other for discovery.
> 
> Malahal,
> 
> If you go back in the archives, and take a look at the SAS Stack
> as I submitted it last year, you'll notice that this is exactly how
> the code is: there is a separate event thread and a separate
> discovery thread.
> 
> That is, from inception, my code has always had two separate threads,
> one for events and one for discovery.
> 
> I wasn't aware that the code had been changed such that a single
> thread handles events and discovery.  This is a very naive approach,
> and a regression over the original code.
> 
> > That avoided the timeout problems, but the discovery thread would die
> > after few iterations due to the event thread and the discovery thread
> > racing each other for setting up and tearing down of sysfs objects.
> 
> Indeed, the situation presented in such circumstances is tricky.
> These "races" have been dealt with in my (original) code.  Currently,
> I don't experience any problems with my SAS Stack, as I maintain it.
> 
> If any of my original comments are still left in the code or the README
> files, that would give you a hint of how such "races" are handled.
> 
> > I tried calling lldd_port_formed() with appropriate phy_mask from the
> > notify_port_event() itself. That worked fine. It is just a hack for now!

Re: AIC94XX discovery timeout problem details...

[James Bottomley]

On Tue, 2006-09-19 at 13:59 -0700, malahal@us.ibm.com wrote:
> I am planning to process port/phy events in two phases. Phase-I sets up
> asd_sas_port, asd_sas_phy lists and calls the LLDD with a correct
> phy_mask. Phase-II is done in the thread context by queuing to the scsi
> work queue that involves setting up sysfs objects.  Still there would be
> just one thread setting up/tearing down sysfs objects that should avoid
> any races.

Erm ... just a minute, I don't understand what the problem is.  The
reason you can handle both in a single event thread is because the
PORTE_BYTES_DMAED event either triggers discovery or wide port formation
(if it's the discovery of another phy in the port).

It sounds like the scenario you have is that discovery is kicked off
before the wide port is formed (I actually see this a bit in my LSI
expander as well).  This shouldn't be an issue: we're perfectly entitled
to do discovery on a single phy if we so choose and not form the wide
port until after discovery is completed.

We really need to debug this.  In your PS you wrote:

> PS: I instrumented the code to use only a single PHY out of 4-phy wide
> port by prematurely returning from sas_form_port() for all PHYs except
> one. In other words, I ignored the PORTE_BYTES_DMAED event for all PHYs
> but one.  There were SCSI timeouts every time I did that except if the
> PHY I was using is phy3.  When I swapped cables, the PHY that worked
> changed too. I believe, it goes with a specific Vitesse expander PHY. Is
> it possible that the Vitesse expander uses a specific PHY to respond to
> some SCSI requests even though the adapter uses a different PHY while
> sending the request?  Is this behavior of communicating on two different
> PHYs for a single SCSI command is allowed in the spec? Also note that
> the discovery commands (SMP) just work fine.

However, SAS specs require us to open connections for SMP and receive
the responses through the connection we opened.  The target is
specifically prohibited from responding outside of this protocol
boundary.  The theory behind this is that the Host controls port
formation ... until we open multiple connections between the source and
target ports, the actual connection remains narrow.  All of this should
support the current discovery model.

I suppose the only way to confirm what's going on would be with a SAS
analyser.  Does the host send multiple OPEN's even before the
PORTE_BYTES_DMAED is responded to?

James

Re: AIC94XX discovery timeout problem details...

[malahal]

James, Jack Hammer from Adaptec gave us SAS analyser output.  All SMP
commands work fine, but SSP commands are sent on a PHY that is up and
updated (in the DDB0). In response, the expander tries to open
connection on a different PHY that is up but not yet updated in DDB0.
The HBA sends OPEN_REJECT(retry) and the Vitesse expander retries the
same path forever!

Thanks, Malahal.

James Bottomley [James.Bottomley@SteelEye.com] wrote:
> However, SAS specs require us to open connections for SMP and receive
> the responses through the connection we opened.  The target is
> specifically prohibited from responding outside of this protocol
> boundary.  The theory behind this is that the Host controls port
> formation ... until we open multiple connections between the source and
> target ports, the actual connection remains narrow.  All of this should
> support the current discovery model.
> 
> I suppose the only way to confirm what's going on would be with a SAS
> analyser.  Does the host send multiple OPEN's even before the
> PORTE_BYTES_DMAED is responded to?
> 
> James
> 

Re: AIC94XX discovery timeout problem details...

[Luben Tuikov]

--- malahal@us.ibm.com wrote:
> I am planning to process port/phy events in two phases. Phase-I sets up
> asd_sas_port, asd_sas_phy lists and calls the LLDD with a correct
> phy_mask. Phase-II is done in the thread context by queuing to the scsi
> work queue that involves setting up sysfs objects.  Still there would be
> just one thread setting up/tearing down sysfs objects that should avoid
> any races.
> 
> Would appreciate any suggestions or issues with this approach.

Malahal,

Your persev erance is commen dable.  Unfortunately this approach is too
simplistic and naive, no offence intended.  That is, such an approach
seems to be the first thing that "pops" into someone's head on how
to do it and a more elaborate analysis of the protocol is warranted
when coding such concepts.  That is, you want to handle the absolute
general case of _more than one type of event_. (This is not intuitive.)

You should take a look at the "patches" submitted by Bottomley and Hellwig
in the areas of event infra, event processing, and discovery, since
I posted the SAS Stack:
  - The Priority Queue Without Duplication Implementation (sas_event.c)
for queueing infinitely many events in finite storage, sorted by arrival
time, where insertion is O(1) and ordered removal is O(1) has
been completely removed.  For some reason they called it "long queue"
in the subject line of their message or in the body, cannot remember.
  - Threaded event processing has been completely removed.
  - Threaded discovery has been removed for the more naive and simplistic,
do-it-all-now-at-once way.

Add to this the fact that some things shouldn't be done "right now",
even though they happened "right now", but should be done "way later"
because you want other things which depend on those things to ...
This isn't intuitive, but is the result of careful analysis of the protocol.

That is, since my original posting of the SAS Stack, Bottomley's refusal
to accept it into the kernel has resulted in what you have right now.

But look on the bright side: just take a look at the mailing list at how
many people are now employed by this?  Five, six people?  So from another
angle, Bottomley's refusal to accept what is the beginning of an enterprise
level SAS Stack written by one person, and his subsequent "patches" has
now created jobs for at least 5 people, and keeps another 1 or 2 busy with
something to do.

Good luck! (Because you're going to need it.)
   Luben
P.S. I even wrote a proposal for a presentation at OLS this year ('06) to
describe the design and architectural decisions of the SAS Stack and SAS LLDD,
as well as talk about SATL and future SCSI directions and while this proposal
was initialy approved, then later it was silently dropped.  The details are in
the archives of linux-scsi.  Good luck!

> Thanks, Malahal.
> 
> Luben Tuikov [ltuikov@yahoo.com] wrote:
> > --- malahal@us.ibm.com wrote:
> > > I chased the time out problem and found that the PORTE_BYTES_DMAED port
> > > event must be responded with a call to lldd_port_formed() which will
> > > update PHY_IS_UP and port-links fields in DDB 0. As there is a single
> > > thread handling PHY/port events as well as discovery, we really can't
> > > handle PORTE_BYTES_DMAED event until the discovery is complete. This
> > > results in SCSI commands timing out in the discovery thread no matter
> > > what I do!  This problem may be unique to Vitesse expander, read the PS
> > > section for details.
> > > 
> > > Tried using two threads, one for events and the other for discovery.
> > 
> > Malahal,
> > 
> > If you go back in the archives, and take a look at the SAS Stack
> > as I submitted it last year, you'll notice that this is exactly how
> > the code is: there is a separate event thread and a separate
> > discovery thread.
> > 
> > That is, from inception, my code has always had two separate threads,
> > one for events and one for discovery.
> > 
> > I wasn't aware that the code had been changed such that a single
> > thread handles events and discovery.  This is a very naive approach,
> > and a regression over the original code.
> > 
> > > That avoided the timeout problems, but the discovery thread would die
> > > after few iterations due to the event thread and the discovery thread
> > > racing each other for setting up and tearing down of sysfs objects.
> > 
> > Indeed, the situation presented in such circumstances is tricky.
> > These "races" have been dealt with in my (original) code.  Currently,
> > I don't experience any problems with my SAS Stack, as I maintain it.
> > 
> > If any of my original comments are still left in the code or the README
> > files, that would give you a hint of how such "races" are handled.
> > 
> > > I tried calling lldd_port_formed() with appropriate phy_mask from the
> > > notify_port_event() itself. That worked fine. It is just a hack for now!
> -
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