Date: Sat, 28 Mar 2020 18:51:57 -0400 From: Mark Johnston <markj@freebsd.org> To: Eric Joyner <erj@freebsd.org> Cc: freebsd-net@freebsd.org, Hans Petter Selasky <hps@selasky.org>, John Baldwin <jhb@freebsd.org>, shurd <shurd@freebsd.org>, Drew Gallatin <gallatin@netflix.com>, Gleb Smirnoff <glebius@freebsd.org> Subject: Re: Issue with epoch_drain_callbacks and unloading iavf(4) [using iflib] Message-ID: <20200328225150.GA82767@raichu> In-Reply-To: <CAKdFRZjdiz_axuweksNUHis7jPKXHqOmhQg%2BQWzpVnsKY%2Bcrmg@mail.gmail.com> References: <CAKdFRZjxp=mTkUzFU8qsacP86OQOC9vCDCQ%2BO2iF7svRRGDK8w@mail.gmail.com> <0e2e97f2-df75-3c6f-9bdd-e8c2ab7bf79e@selasky.org> <CAKdFRZi3UoRuz=OXnBG=NVcJe605x9OwrLmdCyD98mDeTpbf0Q@mail.gmail.com> <a6523ed6-9d61-d1b4-5822-5787cf5c0e43@selasky.org> <20200130030911.GA15281@spy> <CA%2Bb0zg-1CQ81dsNGv_O3ebLLko6Piei0A1NCPZUT5JH8EOyntw@mail.gmail.com> <CA%2Bb0zg809EGMS1Ngr38BSb1yNpDqxbCnAv9eC%2BcDwbMQ5t%2BqXQ@mail.gmail.com> <20200212222219.GE83892@raichu> <CAKdFRZjdiz_axuweksNUHis7jPKXHqOmhQg%2BQWzpVnsKY%2Bcrmg@mail.gmail.com>
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On Wed, Mar 11, 2020 at 04:32:40PM -0700, Eric Joyner wrote: > Mark, > > I did get some time to get back and retry this; however your second patch > still doesn't solve the problem. Looking into it a bit, it looks like the > kldunload process isn't hitting the code you've changed; it's hanging in > epoch_wait_preempt() in if_detach_internal(), which is immediately before > epoch_drain_callbacks(). > > I did a kernel dump while it was hanging, and this is the backtrace for the > kldunload process: I see. I think the callback can be made much simpler and avoid the problematic sched_bind() calls. I wrote a patch that allows waiting threads to lend scheduling priority to a preempted thread blocked in an epoch section, based on some code I wrote to implement preemptible SMR sections. If waiting for a running thread, the callback just spins. This might be enough to solve your problem, I posted the two lightly tested patches here: https://reviews.freebsd.org/D24214 https://reviews.freebsd.org/D24215 If we hit a situation where a reader is preempted and then its CPU is hogged by a high-priority kernel thread, this still won't be enough, but I suspect it'll solve your case. Would you be able to test?
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