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

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Date:	Tue, 26 Aug 2014 16:23:07 +0200
From:	Hannes Reinecke <hare@...e.de>
To:	dgilbert@...erlog.com,
	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
CC:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-scsi@...r.kernel.org, yrl.pp-manager.tt@...achi.com,
	linux-kernel@...r.kernel.org,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [RFC PATCH -logging 00/10] scsi/constants: Output continuous
 error messages on trace

On 08/22/2014 09:54 PM, Douglas Gilbert wrote:
> On 14-08-12 11:13 PM, Yoshihiro YUNOMAE wrote:
>> Hi Douglas,
>>
>> Thank you for your comment.
>>
>> (2014/08/08 22:07), 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
>>>> traceevents
>>>> 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).
>>>>
>>>>   (*1) https://lkml.org/lkml/2014/5/20/742
>>>>
>>>> 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
>>>> tracers.
>>>>
>>>> This patch set is based on Hannes' logging branch:
>>>> http://git.kernel.org/cgit/linux/kernel/git/hare/scsi-devel.git/log/?h=logging
>>>>
>>>>
>>>>
>>>> [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?
>>
>> I have already sent the idea using local buffer on this February,
>> but it was rejected by James (*1). By using stack region for local
>> buffer, stack overflow can occur. So, I implemented this feature
>> to atomically output an error message with device information.
>>
>> (*1) https://lkml.org/lkml/2014/5/20/742
>
> Hi,
> In the "_str" variants that I referred to, the caller provides
> the buffer and its length. The responsibility of the
> implementation of those "_str" variants is to use that
> buffer, not exceed it, and make sure that it is null terminated
> on return.
>
> Can't see any inherent threat to the stack size there, and if
> there is then that is just bad design by the caller.
>
> The advantage of the "_str" variants is that the caller can
> supply context and print/log a more useful message, perhaps
> including the caller's __func__ . That message could include
> sense information (perhaps truncated to 128 bytes, say),
> and be output as a single unit.
>
> IMO too many log messages are multi-line and in a noisy,
> misbehaving system those messages can be interleaved
> with other "noise" making them difficult to decipher.
>
Precisely, and that is what my patchset is trying to address.

I'm currently porting it to core-for-3.18, will be posting it
for review once it's done.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@...e.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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