On Sat, Feb 14, 2015 at 03:29:37PM +0100, Hannes Reinecke wrote: > 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. Thank you, that's *greatly* appreciated. - Josh Triplett -- 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
