On ti, 2016-02-16 at 10:14 +0100, Peter Zijlstra wrote: > On Tue, Feb 16, 2016 at 10:49:36AM +0200, Joonas Lahtinen wrote: > > I originally thought of implementing this more similar to what you > > specify, but then I came across a discussion in the mailing list where > > it was NAKed adding more members to task_struct; > > > > http://comments.gmane.org/gmane.linux.kernel/970273 > > > > Adding proper recursion (the way my initial implementation was going) > > got ugly without modifying task_struct because get_online_cpus() is a > > speed critical code path. > > Yeah, just don't let Linus hear you say that. get_online_cpus() is _not_ > considered performance critical. Oh well, at least changes to it added quite noticeably to the bootup time of a system. > > > So I'm all for fixing the current code in a different way if that will > > then be merged. > > So I'm not sure why you're poking at this horror show to begin with. > ISTR you mentioning a lockdep splat for SKL, but failed to provide > detail. > Quoting my original patch; "See the Bugzilla link for more details. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93294"; The improvement my patch implements is to use lockref for locked reference counting (hotplug code previously rolled its own mutex + atomic combo), which gets rid of the deadlock scenario described and linked in the initial patch. Trace for the scenario; https://bugs.freedesktop.org/attachment.cgi?id=121490 I think using lockref makes it substantially less special, lockref code being a lot more battle-tested in the FS code than the previous cpu_hotplug.lock mess. > Making the hotplug lock _more_ special to fix that is just wrong. Fix > the retarded locking that lead to it. > I do agree that it's still not pretty, but now it does correctly what the previous code was trying to do with custom mutex + atomic. I'm all for fixing the code further, but prior to proceeding there needs to be some sort of an agreement on either making get_online_cpus() slower (which does not seem like a good idea) or adding more members to task_struct. Regards, Joonas > -- Joonas Lahtinen Open Source Technology Center Intel Corporation _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
