On 05/27/2009 11:26 AM, FUJITA Tomonori wrote: > On Wed, 27 May 2009 10:54:41 +0300 > Boaz Harrosh <bharrosh@xxxxxxxxxxx> wrote: > >> On 05/27/2009 04:36 AM, FUJITA Tomonori wrote: >>> On Tue, 26 May 2009 19:05:05 +0300 >>> Boaz Harrosh <bharrosh@xxxxxxxxxxx> wrote: >>> >>>> On 05/26/2009 06:31 PM, FUJITA Tomonori wrote: >>>>> Can we just fix some drivers not to do the DMA with the sense buffer in >>>>> scsi_cmnd? IIRC, there are only five or six drivers that do such. >>>> This is not so. >>>> All drivers that go through scsi_eh_prep_cmnd() will eventually DMA through >>>> the regular read path. Including all the drivers that do nothing and let >>>> scsi-ml do the REQUEST_SENSE >>>> >>>> Actually I have exact numbers, from the last time I did all that >>> Hmm, we discussed this before, I think. >>> >> Sure we did I sent these patches. to summarize, 3 types of drivers: >> 1. Only memcpy into sense_buffer - 60% >> 2. Use scsi_eh_prep_cmnd and DMA read into sense. >> 2.1 Do nothing and scsi-ml does scsi_eh_prep_cmnd - 30% >> 3. Prepare DMA descriptors for sense_buffer before execution - 10% >> >>> scsi-ml uses scsi_eh_prep_cmnd only via scsi_send_eh_cmnd(). There are >>> some users of scsi_send_eh_cmnd in scsi-ml but only scsi_request_sense >>> does the DMA in the sense_buffer of scsi_cmnd. >>> >> Also drivers use scsi_eh_prep_cmnd at interrupt time and proceed to >> DMA into the sense_buffer. >> >>> Only scsi_error_handler() uses scsi_request_sense() and >>> scsi_send_eh_cmnd() works synchronously. So scsi-ml can easily avoid >>> the the DMA in the sense_buffer of scsi_cmnd if we have one sense >>> buffer per scsi_host. >> Not so. As James explained then, once you have a CHECK_CONDITION return, the >> Q-per-host is frozen, yes. But as soon as you send the REQUEST_SENSE the >> target Q is unfrozen again and all in-flight commands can error, much before >> the REQUEST_SENSE returns. > > Hmm, I'm not sure what you mean. > > Why is 'all in-flight commands can error' a problem? The sense_buffer > per host is used by only scsi_eh kernel thread. I agree, then the current situation has a problem. Target has command A && B in Q. - A returns CHECK_CONDITION, scsi_eh thread kicks in, sends a REQUEST_SENSE. - Immediately command B returns with CHECK_CONDITION, Target Q is frozen again. - message is queued for scsi_eh thread but that one is stuck waiting for the first REQUEST_SENSE to return, and the second-REQUEST_SENSE is never sent, target Q is frozen forever. I guess all the drivers that support target queueing do not depend on scsi_eh thread to issue the REQUEST_SENSE command. As I said, there are very few drivers that do nothing and let scsi_eh take care of REQUEST_SENSE. This will not however solve these drivers that might need many concurrent sense buffers. Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
