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@xxxxxxxxxxxxxxx> wrote: >>>>>> Gitweb: http://git.kernel.org/linus/;a=commit;h=ded85c193a391a84076d5c6a7a5668fe164a490e >>>>>> Commit: ded85c193a391a84076d5c6a7a5668fe164a490e >>>>>> Parent: b0a93d96b2814c725161f91a4e35d0c29ec0f95b >>>>>> Refname: refs/heads/master >>>>>> Author: Hannes Reinecke <hare@xxxxxxx> >>>>>> AuthorDate: Thu Jan 8 07:43:42 2015 +0100 >>>>>> Committer: Christoph Hellwig <hch@xxxxxx> >>>>>> 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@xxxxxx> >>>>>> Reviewed-by: Robert Elliott <elliott@xxxxxx> >>>>>> Signed-off-by: Hannes Reinecke <hare@xxxxxxx> >>>>>> Signed-off-by: Christoph Hellwig <hch@xxxxxx> >>>>> >>>>>> --- /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@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
