On Tue, 22 Dec 2015, One Thousand Gnomes wrote: > On Tue, 22 Dec 2015 12:18:44 +1100 Finn Thain > <fthain@xxxxxxxxxxxxxxxxxxx> wrote: > > > Because of the rudimentary design of the chip, it is necessary to poll > > the SCSI bus signals during PIO and this tends to hog the CPU. The > > driver will accept new commands while others execute, and this causes > > a soft lockup because the workqueue item will not terminate until the > > issue queue is emptied. > > > > When exercising dmx3191d using sequential IO from dd, the driver is > > sent 512 KiB WRITE commands and 128 KiB READs. For a PIO transfer, the > > rate is is only about 300 KiB/s, so these are long-running commands. > > And although PDMA may run at several MiB/s, interrupts are disabled > > for the duration of the transfer. > > > > Fix the unresponsiveness and soft lockup issues by calling > > cond_resched() after each command is completed and by limiting > > max_sectors for drivers that don't implement real DMA. > > Is there a reason for not doing some limiting in the DMA case too. A > 512K write command even with DMA on a low end 68K box introduces a > second of latency before another I/O can be scheduled ? The DMA case is the atari_scsi case. I'd like to think that atari_scsi would have only the latency issues that might be expected from any SCSI-2 host adapter driver. Unlike PDMA, interrupts are not disabled for these DMA transfers. Note that this patch isn't really relevant to DMA, because the main loop iterates only when done == 0, that is, !hostdata->dmalen. -- > > Alan -- 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
