From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kent Overstreet Subject: Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers Date: Thu, 30 Aug 2012 18:43:59 -0700 Message-ID: <20120831014359.GB15218@moria.home.lan> References: <1346175456-1572-1-git-send-email-koverstreet@google.com> <1346175456-1572-10-git-send-email-koverstreet@google.com> <20120829165006.GB20312@google.com> <20120829170711.GC12504@redhat.com> <20120829171345.GC20312@google.com> <20120830220745.GI27257@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20120830220745.GI27257-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Sender: linux-bcache-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Vivek Goyal Cc: Mikulas Patocka , linux-bcache-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, dm-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, bharrosh-C4P08NqkoRlBDgjK7y7TUQ@public.gmane.org, Jens Axboe List-Id: linux-bcache@vger.kernel.org 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.