2017-12-07 18:29 UTC+01:00, Paul E. McKenney <paul...@linux.vnet.ibm.com>: > On Thu, Dec 07, 2017 at 05:14:54PM +0100, Frederic Weisbecker wrote: >> 2017-12-04 18:16 UTC+01:00, Paul E. McKenney >> <paul...@linux.vnet.ibm.com>: >> > On Mon, Dec 04, 2017 at 04:53:15PM +0100, Frederic Weisbecker wrote: >> >> 2017-12-02 20:24 UTC+01:00, Paul E. McKenney >> >> I would prefer to keep it. It's useful for automated boot testing >> >> based on configs such as 0-day or -tip test machines. But I'm likely >> >> to migrate it to isolcpus implementation. Maybe something along the >> >> lines of CONFIG_CPU_ISOLATION_ALL. >> > >> > How about instead allowing something like "nohz_full=1-" specify that >> > all CPUs other than CPU 0 should be nohz_full CPUs? That would shrink >> > the code by eliminating CONFIG_NO_HZ_FULL_ALL while still allowing >> > easy automation of that particular scenario. >> > >> > (Right now, the boot code complains about "nohz_full=1-", which means >> > that whatever is generating the boot parameters needs to know how many >> > CPUs there really are, which as you say can be a pain.) >> >> Yes but automated boot testing is rather based on configs than boot >> options. It's much easier. I think that's how Wu Fengguang lab works, >> and -tip automated tests as well. > > So you have gotten bug reports from them? Because I see splats from > rcutorture testing rather frequently. This thing is in no way a subtle > low-probability bug. ;-)
Nope I haven't got anything from them. So far you're the only reproducer I know :) >> >> >> Did you have any nohz_full= or isolcpus= boot options? >> >> > >> >> > Replacing CONFIG_NO_HZ_FULL_ALL=y with nohz_full=1-7 works, that >> >> > is CONFIG_NO_HZ_FULL=y, CONFIG_NO_HZ_FULL_ALL=n, and nohz_full=1-7 >> >> > on an eight-CPU test. >> >> > >> >> > But it is relatively easy to test. Running the rcutorture TREE04 >> >> > scenario on a four-socket x86 gets me RCU CPU stall warnings within >> >> > a few minutes more than half the time. ;-) >> >> >> >> Indeed I managed to trigger something. If it's the same thing I should >> >> be able to track down the root cause. >> >> >> >> [ 123.907557] ??? Writer stall state RTWS_STUTTER(8) g160 c160 f0x0 >> >> ->state 0x1 cpu 2 >> >> [ 123.915184] rcu_torture_wri S 0 111 2 0x80080000 >> >> [ 123.920673] Call Trace: >> >> [ 123.923096] ? __schedule+0x2bf/0xbb0 >> >> [ 123.926715] ? _raw_spin_unlock_irqrestore+0x59/0x70 >> >> [ 123.931657] schedule+0x3c/0x90 >> >> [ 123.934777] schedule_timeout+0x1e1/0x560 >> > >> > It might well be the same thing, as this schedule_timeout() does look >> > familiar. I have some diagnostic patches in -rcu, please see below >> > for the overall effect. >> >> I fear I can hit that even without any nohz_full CPU as well. > > Indeed, I do hit that with my TREE01 scenario, which does not set > CONFIG_NO_HZ_FULL. But it is much less frequent. The good news is that > I have finally figured out a way to extract information from this thing > without suppressing it. At the moment it appears to be a rather strange > deadlock involving CPU hotplug, timers, and RCU. > > But that is a completely different bug from the ones for which I have > the two patches in my tree. > > Anyway, I will keep those two patches because I cannot have the > corresponding bugs possibly hiding RCU bugs in my testing. If you > put some other fix in place, I will drop those two patches in favor of > your fix. Ok. I'm a bit troubled by this bug, I hate to push a fix for a bug I don't understand nor can reproduce. But having CONFIG_NO_HZ_FULL select CONFIG_CPU_ISOLATION is already a fix for sanity that I need to push. So I think I'm going to take your patch anyway and rewrite the changelog to take all that into account. Thanks Paul!
