Messages in this thread | | | Subject | Re: [PATCH] use x86 cpu park to speedup smp_init in kexec situation | From | David Woodhouse <> | Date | Wed, 08 Dec 2021 20:35:00 +0000 |
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On Wed, 2021-12-08 at 11:03 -0800, Paul E. McKenney wrote: > On Wed, Dec 08, 2021 at 06:32:15PM +0000, David Woodhouse wrote: > > On Wed, 2021-12-08 at 09:35 -0800, Paul E. McKenney wrote: > > > On Wed, Dec 08, 2021 at 04:57:07PM +0000, David Woodhouse wrote: > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index ef8d36f580fc..544198c674f2 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -4246,11 +4246,11 @@ void rcu_cpu_starting(unsigned int cpu) > > > > > > > > rnp = rdp->mynode; > > > > mask = rdp->grpmask; > > > > + raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > > > > If I am not too confused this morning, this can result in confusing > > > lockdep splats because lockdep needs RCU to be watching the CPU > > > acquiring the lock. See the rcu_lockdep_current_cpu_online() > > > function and is callers, with emphasis on lockdep_rcu_suspicious() > > > and rcu_read_lock_held_common(). > > > > Hm, OK. And it is the very act of setting rnp->ofl_seq & 1 which > > triggers that, yes? > > Prevents that from triggering, but if I recall correctly, yes.
OK, thanks.
> > Ok, thanks. My initial hack of sticking my own spinlock around the > > whole thing was also working for testing, but now I'm trying to clean > > it up so I can post something for merging. > > Sounds good! > > You know, maybe it would be way easier to just create a new spinlock and > use arch_spin_lock() to acquire it and arch_spin_unlock() to release it, > bypassing lockdep for that one lock. Then proceed as in your initial > patch.
Hm. So... (summarising a little from IRC for the peanut gallery and our own subsequent recollection) I had a play with doing an atomic 'acquire' for rnp->ofl_seq which is basically "spin until you can use cmpxchg() to atomically increment it to an odd number".
http://david.woodhou.se/acquire-ofl-seq.patch
But *every* call to that 'acquire_ofl_seq() is paired with locking rcu_state.ofl_lock, and *every* release is paired with unlocking rcu_state.ofl_lock.
So I don't think I want a *new* lock; I think I want to use arch_spin_lock on rcu_state.ofl_lock and expand it slightly (as in my previous attempt to cover the modifications of rnp->ofl_seq.
Will throw that together and see what breaks...
> > > Though I are having some difficulty remembering why that wait loop in > > > rcu_gp_init() needs to be there. I am going to try removing it and > > > seeing if rcutorture will be kind enough to remind me. ;-) > > > > > > And it will of course be necessary to upgrade rcutorture to test > > > concurrent CPU-online operations. Will there be some sort of > > > start-CPU-online function, or should I instead expect to need to > > > provide multiple kthreads for onlining and an additional kthread > > > for offliing? > > > > This is just at *boot* time, not runtime hotplug/unplug. We observed > > that we spend quite a lot of time on a 96-way 2-socket Skylake system > > just sending INIT to each CPU in turn, then waiting for it to be fully > > online, then moving on to the next one. Hence doing them all in > > parallel, which reduces the AP bringup time from about 300ms to 30ms. > > > > https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/parallel-5.16 > > > > Nice win!!! > > And I do understand that you are only worried about boot speed, but > adequate stress-testing of this will require run-time exercising of this. > Yes, 30ms is fast, but you have other overheads when repeatedly rebooting, > and so doing runtime tests will find bugs faster.
Absolutely!
> > > Huh. I take it that concurrent online and offline is future work? > > > Or does that need to work initially? > > > > Concurrent *online* (at boot) is the whole point. Those last two > > commits currently in the branch linked above are the "oh crap, *that* > > part doesn't work if you really let it happen concurrently, so let's > > serialize them" hacks. In particular, the RCU one is > > https://git.infradead.org/users/dwmw2/linux.git/commitdiff/5f4b77c9459c > > > > > > And now I'm trying to come up with something a little less hackish :) > > Understood! I am just trying to work out a decent validation plan for > this. Let's just say that changes in this area have not traditionally > been boring. ;-) > > > > More to the point, what are you using to stress-test this capability? > > > > Just boot. With lots of CPUs (and vCPUs in qemu, but even with a nice > > fast parallel CPU bringup, Linux then spends the next 16 seconds > > printing silly pr_info messages about KVM features so it isn't the most > > exciting overall result right now) > > > > I confess I haven't actually tested runtime hotplug/unplug again > > recently. I should do that ;) > > The rcutorture TREE03 scenario is rather aggressive about this. > From the root of a recent Linux-kernel source tree: > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 1h configs "TREE03" --trust-make > > Or, if you have a 64-CPU system: > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 1h configs "4*TREE03" --trust-make > > The latter would be a semi-credible smoke test for this sort of change.
Thanks.
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