On Tue, Nov 26, 2019 at 10:29:18AM +0100, Michal Hocko wrote: > On Mon 25-11-19 10:54:53, Roman Gushchin wrote: > > Christian reported a warning like the following obtained during running some > > KVM-related tests on s390: > > > > WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x50/0x58 > > Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_na> > > CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66 > > Hardware name: IBM 2964 NC9 712 (LPAR) > > Workqueue: events sysfs_slab_remove_workfn > > Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58) > > R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3 > > Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 0036008100000000 > > 0000001f00000000 0035008100000000 0000001fb3573ab8 0000000000000000 > > 0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb3573b00 > > 0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e009263cd0 > > Krnl Code: 0000001529746842: f0a0000407fe srp 4(11,%r0),2046,0 > > 0000001529746848: 47000700 bc 0,1792 > > #000000152974684c: a7f40001 brc 15,152974684e > > >0000001529746850: a7f4fff2 brc 15,1529746834 > > 0000001529746854: 0707 bcr 0,%r7 > > 0000001529746856: 0707 bcr 0,%r7 > > 0000001529746858: eb8ff0580024 stmg %r8,%r15,88(%r15) > > 000000152974685e: a738ffff lhi %r3,-1 > > Call Trace: > > ([<000003e009263d00>] 0x3e009263d00) > > [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70 > > [<0000001529b04882>] kobject_put+0xaa/0xe8 > > [<000000152918cf28>] process_one_work+0x1e8/0x428 > > [<000000152918d1b0>] worker_thread+0x48/0x460 > > [<00000015291942c6>] kthread+0x126/0x160 > > [<0000001529b22344>] ret_from_fork+0x28/0x30 > > [<0000001529b2234c>] kernel_thread_starter+0x0/0x10 > > Last Breaking-Event-Address: > > [<000000152974684c>] percpu_ref_exit+0x4c/0x58 > > ---[ end trace b035e7da5788eb09 ]--- > > > > The problem occurs because kmem_cache_destroy() is called immediately > > after deleting of a memcg, so it races with the memcg kmem_cache > > deactivation. > > > > flush_memcg_workqueue() at the beginning of kmem_cache_destroy() > > is supposed to guarantee that all deactivation processes are finished, > > but failed to do so. It waits for an rcu grace period, after which all > > children kmem_caches should be deactivated. During the deactivation > > percpu_ref_kill() is called for non root kmem_cache refcounters, > > but it requires yet another rcu grace period to finish the transition > > to the atomic (dead) state. > > > > So in a rare case when not all children kmem_caches are destroyed > > at the moment when the root kmem_cache is about to be gone, we need > > to wait another rcu grace period before destroying the root > > kmem_cache. > > Could you explain how rare this really is please? It seems that we don't destroy root kmem_caches with enabled memcg accounting that often, but maybe I'm biased here. > I still have to wrap > my head around the overall logic here. It looks quite fragile to me TBH. > I am worried that is relies on implementation detail of the PCP ref > counters too much. It is definitely very complicated and fragile, but I hope it won't remain in this state for long. The new slab controller, which I'm working on, eliminates all this logic all together and generally simplifies things a lot. Simple because there will be no need to create and destroy per-memcg kmem_caches. Thanks!
