On Thu, Aug 30, 2012 at 06:07:45PM -0400, Vivek Goyal wrote: > On Wed, Aug 29, 2012 at 10:13:45AM -0700, Kent Overstreet wrote: > > [..] > > > Performance aside, punting submission to per device worker in case of deep > > > stack usage sounds cleaner solution to me. > > > > Agreed, but performance tends to matter in the real world. And either > > way the tricky bits are going to be confined to a few functions, so I > > don't think it matters that much. > > > > If someone wants to code up the workqueue version and test it, they're > > more than welcome... > > Here is one quick and dirty proof of concept patch. It checks for stack > depth and if remaining space is less than 20% of stack size, then it > defers the bio submission to per queue worker. I can't think of any correctness issues. I see some stuff that could be simplified (blk_drain_deferred_bios() is redundant, just make it a wrapper around blk_deffered_bio_work()). Still skeptical about the performance impact, though - frankly, on some of the hardware I've been running bcache on this would be a visible performance regression - probably double digit percentages but I'd have to benchmark it. That kind of of hardware/usage is not normal today, but I've put a lot of work into performance and I don't want to make things worse without good reason. Have you tested/benchmarked it? There's scheduling behaviour, too. We really want the workqueue thread's cpu time to be charged to the process that submitted the bio. (We could use a mechanism like that in other places, too... not like this is a new issue). This is going to be a real issue for users that need strong isolation - for any driver that uses non negligable cpu (i.e. dm crypt), we're breaking that (not that it wasn't broken already, but this makes it worse). I could be convinced, but right now I prefer my solution. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
