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. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (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
