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Date: Thu, 30 Aug 2012 18:43:59 -0700 From: Kent Overstreet <koverstreet@...gle.com> To: Vivek Goyal <vgoyal@...hat.com> Cc: Mikulas Patocka <mpatocka@...hat.com>, linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org, dm-devel@...hat.com, tj@...nel.org, bharrosh@...asas.com, Jens Axboe <axboe@...nel.dk> Subject: Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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