Re: [RFC PATCH -logging 00/10] scsi/constants: Output continuous error messages on trace

From: Douglas Gilbert
Date: Wed Aug 27 2014 - 10:48:26 EST

On 14-08-27 10:23 AM, Hannes Reinecke wrote:
On 08/08/2014 03:07 PM, Douglas Gilbert wrote:
On 14-08-08 01:50 PM, Yoshihiro YUNOMAE wrote:
Hi All,

This patch set introduces new traceevents in order to output
continuous error
messages. Current SCSI printk messages in upstream kernel can be
divided by and
mixed with other messages. Even if each error message has its
device id,
sometimes we can easily be lost in mixed logs because the
message's device id
is separated from it's body. To avoid it, I'd like to use
traceevents to
store error messages into the ftrace or perf buuffer, because
are atomically commited to the buffer.

In this patch set, all printk messages are removed based on a local
discussion with Hannes, but I think printk messages should be kept
because all
users don't enable traceevents and rsyslog can store as files.

However, if printk of logging branch is kept, the messages are
duplicate and
it can induce stack overflow by using local buffer(*1).


So, my suggestion is follows:

1) printk
Keeps current implemntation of upstream kernel.
The messages are divided and can be mixed, but all users can check
the error
messages without any settings.

2) traceevents
To get the complete messages, we can use ftrace or perf (or
something on them).
Users can always understand correct messages, but they need to set
up the

This patch set is based on Hannes' logging branch:

[1/10] ~ [6/10]: just cleanup for logging branch
[7/10] ~ [10/10]: introduce new traceevents

Any comments are welcome!

In sg3_utils there are now string yielding equivalents
to the sense buffer "print" functions. They take a form
like this:
char * get_sense_str(const unsigned char * sense_buffer,
int sb_len, int blen, char * b);

So this just does the hard work of decoding the sense buffer
(or saying it is invalid) the result of which it places in
buffer 'b'. And 'b' is returned (so this function can be in
the arguments of a driver's printing function).

Adding such string functions would give other parts of the
SCSI subsystem the capability of tailoring their own
messages that include sense data information.

Existing sense buffer "print" function could be kept and
implemented using the newer "_str" variants. Would that
be worth the trouble?

Hmm. Probably not.

I would rather go the approach we've been taking with the
VPD pages, and do _not_ decode any sense code data
(except from the usual sense key/ASC/ASCQ values, of course).
Instead we should rather ensure that we can get to the raw
sense code values from userspace so that we can interpret
it later with userspace tools.

My plan for updating scsi logging is:

- move all lone 'printk' statements into dev_printk() variants
and ensure they are printed in one line to avoid breaking
logging statements up under high load
- Update scsi_trace to use the functionality from constants.c
- Convert the current scsi_logging mechanism over to tracepoints.

The first bit is mostly done; I'll be sending the patchset for review.
The hard part is the third bit; would be really grand if we can
model this with the existing scsi_logging_level interface intact.
But not sure if that's possible nor if it's desirable.

Btw, _now_ would be a good chance to send an update of constants.c
with latest SPC bits ...

From constants.c:
* Updated to SPC-4 T10/1713-D Rev 36g, D. Gilbert 20130701

Not much has changed between then and now (37a). There is a
lot pending for SPC-5 plus all the ZBC stuff, little of which
has hit the drafts yet. WRITE ATOMIC and a few new asc/ascq
codes is about all that I have noticed. Even when new asc/ascq
strings are approved, it needs the SPC4/5 technical editor to
allocate numbers for them.

So even if the timing is good from the kernel POV, not much
is available. The next T10 meeting is in a few weeks, lets
see what happens there.

Doug Gilbert

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