Messages in this thread |  | | Date | Thu, 9 Apr 2026 21:15:50 +0200 | | From | Vasily Gorbik <> | | Subject | Re: BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition |
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On Thu, Apr 09, 2026 at 10:22:00AM -0700, Paul E. McKenney wrote: > On Thu, Apr 09, 2026 at 03:08:45PM +0200, Vasily Gorbik wrote: > > Commit 61bbcfb50514 ("srcu: Push srcu_node allocation to GP when > > non-preemptible") defers srcu_node tree allocation when called under > > raw spinlock, putting SRCU through ~6 transitional grace periods > > (SRCU_SIZE_ALLOC to SRCU_SIZE_BIG). During this transition srcu_gp_end() > > uses mask = ~0, which makes srcu_schedule_cbs_snp() call queue_work_on() > > for every possible CPU. Since rcu_gp_wq is WQ_PERCPU, work targets > > per-CPU pools directly - pools for not-online CPUs have no workers, > > work accumulates, workqueue lockup detector fires. > > > > Before 61bbcfb50514, GFP_ATOMIC allocation went straight to > > SRCU_SIZE_BIG, the mask = ~0 path was never reached. > > > > Affects systems with convert_to_big active (auto when nr_cpu_ids >= 128) > > and possible CPUs > online CPUs. Hit on s390 LPAR (76 online, 400 possible), > > where possible CPUs > online CPUs is the usual case. > > Also reproducible on x86 KVM --smp 16,maxcpus=255 (CONFIG_NR_CPUS=256) > > or simply -smp 1,maxcpus=2 with srcutree.convert_to_big=1 > > or --smp 16,maxcpus=64 with srcutree.big_cpu_lim=32 (CONFIG_NR_CPUS=64) > > > > s390 log (76 online CPUs, 400 possible, all pools 76-399 stuck): > > > > BUG: workqueue lockup - pool cpus=76 node=0 flags=0x4 nice=0 stuck for 1842s! > > BUG: workqueue lockup - pool cpus=77 node=0 flags=0x4 nice=0 stuck for 1842s! > > ... > > BUG: workqueue lockup - pool cpus=399 node=0 flags=0x4 nice=0 stuck for 1842s! > > Showing busy workqueues and worker pools: > > workqueue rcu_gp: flags=0x108 > > pwq 306: cpus=76 node=0 flags=0x4 nice=0 active=3 refcnt=4 > > pending: 3*srcu_invoke_callbacks > > pwq 310: cpus=77 node=0 flags=0x4 nice=0 active=3 refcnt=4 > > pending: 3*srcu_invoke_callbacks > > ... > > pwq 1598: cpus=399 node=0 flags=0x4 nice=0 active=3 refcnt=4 > > pending: 3*srcu_invoke_callbacks > > > > Not sure if replacing mask = ~0 with something derived from > > cpu_online_mask would be racy in that context. > > > > [1] https://lore.kernel.org/rcu/acRho9L4zA2MRuxc@tardis.local > > [2] https://lore.kernel.org/rcu/fe28d664-3872-40f6-83c6-818627ad5b7d@paulmck-laptop > > This was a pre-existing bug, but the change made it much more likely > to happen.
Yes, indeed.
> Does the alleged (and untested) fix below do the trick? The theory is > that if a given CPU has ever been fully online, it has workqueues set up. > Directly checking whether a CPU is currently online is vulnerable to a CPU > piling up lots of SRCU callbacks, then going offline. So we do need to > be prepared to invoke SRCU callbacks for CPUs that are currently offline.
Yes, tested on s390 LPAR (76 online, 400 possible) as well as on x86 KVM with --smp 16,maxcpus=255 and CONFIG_NR_CPUS=256 no more workqueue lockup in both cases.
Thank you!
Tested-by: Vasily Gorbik <gor@linux.ibm.com>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index 0d01cd8c4b4a7..e68ee7f69e1fc 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -898,7 +898,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp > int cpu; > > for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) { > - if (!(mask & (1UL << (cpu - snp->grplo)))) > + if (!(mask & (1UL << (cpu - snp->grplo))) || !rcu_cpu_beenfullyonline(cpu)) > continue; > srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay); > }
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