Date:	Sat, 14 Feb 2015 15:29:37 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Josh Triplett <josh@...htriplett.org>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christoph Hellwig <hch@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	scsi <linux-scsi@...r.kernel.org>
Subject: Re: scsi: Implement per-cpu logging buffer

On 02/13/2015 04:45 PM, Josh Triplett wrote:
> On Fri, Feb 13, 2015 at 09:48:36AM +0100, Hannes Reinecke wrote:
>> On 02/12/2015 06:18 PM, Josh Triplett wrote:
>>> On Thu, Feb 12, 2015 at 02:29:35PM +0100, Hannes Reinecke wrote:
>>>> On 02/12/2015 01:36 PM, Geert Uytterhoeven wrote:
>>>>> On Wed, Feb 11, 2015 at 8:16 PM, Linux Kernel Mailing List
>>>>> <linux-kernel@...r.kernel.org> wrote:
>>>>>> Gitweb:     http://git.kernel.org/linus/;a=commit;h=ded85c193a391a84076d5c6a7a5668fe164a490e
>>>>>> Commit:     ded85c193a391a84076d5c6a7a5668fe164a490e
>>>>>> Parent:     b0a93d96b2814c725161f91a4e35d0c29ec0f95b
>>>>>> Refname:    refs/heads/master
>>>>>> Author:     Hannes Reinecke <hare@...e.de>
>>>>>> AuthorDate: Thu Jan 8 07:43:42 2015 +0100
>>>>>> Committer:  Christoph Hellwig <hch@....de>
>>>>>> CommitDate: Fri Jan 9 15:44:28 2015 +0100
>>>>>>
>>>>>>     scsi: Implement per-cpu logging buffer
>>>>>>
>>>>>>     Implement a per-cpu buffer for formatting messages to avoid line breaks
>>>>>>     up under high load.  This patch implements scmd_printk() and
>>>>>>     sdev_prefix_printk() using the per-cpu buffer and makes sdev_printk() a
>>>>>>     wrapper for sdev_prefix_printk().
>>>>>>
>>>>>>     Tested-by: Robert Elliott <elliott@...com>
>>>>>>     Reviewed-by: Robert Elliott <elliott@...com>
>>>>>>     Signed-off-by: Hannes Reinecke <hare@...e.de>
>>>>>>     Signed-off-by: Christoph Hellwig <hch@....de>
>>>>>
>>>>>> --- /dev/null
>>>>>> +++ b/drivers/scsi/scsi_logging.c
>>>>>
>>>>>> +#define SCSI_LOG_SPOOLSIZE 4096
>>>>>> +#define SCSI_LOG_BUFSIZE 128
>>>>>> +
>>>>>> +#if (SCSI_LOG_SPOOLSIZE / SCSI_LOG_BUFSIZE) > BITS_PER_LONG
>>>>>> +#warning SCSI logging bitmask too large
>>>>>> +#endif
>>>>>> +
>>>>>> +struct scsi_log_buf {
>>>>>> +       char buffer[SCSI_LOG_SPOOLSIZE];
>>>>>> +       unsigned long map;
>>>>>> +};
>>>>>> +
>>>>>> +static DEFINE_PER_CPU(struct scsi_log_buf, scsi_format_log);
>>>>>
>>>>> Do we really need a static 4 KiB per-CPU buffer?
>>>>>
>>>>> bloat-o-meter:
>>>>>
>>>>> add/remove: 183/94 grow/shrink: 314/211 up/down: 33467/-21291 (12176)
>>>>> function                                     old     new   delta
>>>>> scsi_format_log                                -    4100   +4100
>>>>> handle_mm_fault                             1794    2750    +956
>>>>> scsi_log_print_sense_hdr                       -     774    +774
>>>>> proc_keys_show                                 -     770    +770
>>>>>
>>>> Define 'need'.
>>>> We don't absolutely 'need' it. (Configure it out and it's gone).
>>>>
>>>> But when we want to avoid several logging messages coming in from
>>>> various CPUs overwriting each other and _not_ introduce additional
>>>> latency by locking a single buffer, then yes.
>>>>
>>>> We can possibly reduce it to, say, 1KiB or even lower by imposing
>>>> stricter rules on the logging functions.
>>>> But I don't see a way around the per-CPU buffer.
>>>
>>> It seems very odd to introduce a mechanism like this specifically for
>>> SCSI, rather than introducing a generic per-CPU buffered-print mechanism
>>> in printk, controlled by a config option.  That option could then
>>> automatically go away when !SMP, or !PRINTK, or if users don't actually
>>> care about message ordering.
>>>
>> But then we ran afoul with the printk purists.
>>
>> Thing is, if we were to use per-CPU buffers for printk() out of
>> necessity we have to queue these buffers for writing out.
>> So there is a time window during which the message already is in the
>> per-CPU buffer but still not printed out as printk() is currently
>> writing out one of the other per-CPU buffers.
>>
>> If there is a consensus that such a delayed printk() is useful and a
>> valid use case then yes, sure I can give it a go.
>>
>> Personally I think printk() currently has an unfortunate double
>> purpose: on the one hand it should print out emergency messages
>> immediate so that they'll be visible if the system crashes. On the
>> other hand it is used as a general logging facility, where frankly
>> most of the subsystems simple do not care at all if and when the
>> message are printed.
>> Splitting that off would indeed be a good idea, as then we can have
>> the ultra-fast, go to console now printk() thingie, and another 'hey
>> I don't care, just wanted to let you know something happened'
>> delayed logging output.
>>
>> But I certainly will not attempt to implement this without a broader
>> consensus. Typically patching printk is a good way of getting flamed.
> 
> I'm not suggesting that you change printk(); I'm just suggesting that
> you drop the scsi_* prefixes from the buffered logging mechanism you've
> already created, and make it a generic buffered logging mechanism that
> other subsystems can use if desired.  More importantly, though, whether
> you keep it SCSI-specific or not, please consider making it
> configurable, dependent on SMP and PRINTK, and fall through to just
> printk if configured out.
> 
Ah, right. _that_ is easy to accomplish.
And sounds even as it would have a chance of succeeding.

I'll be updating the patch.

@jejb, hch: should I do a new patch or update the existing one?

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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