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. - 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
