On Thu, Oct 1, 2026 at 11:29 AM Vineet Gupta <[email protected]> wrote: > > When ftrace graphing is turned on, all tasks in the system missing > return stack page are assigned one. This is done in a simplistic > multi-sweep loop of FTRACE_RETSTACK_ALLOC_SIZE (currently 32) tasks > at a time as follows: > > start_graph_tracing() > do { > alloc_retstack_tasklist > } while (-EAGAIN); > > alloc_retstack_tasklist() > alloc x32 # GFP_KERNEL, may sleep > rcu_read_lock() # preempt off > for_each_process_thread walk N_total, no cond_resched > t->ret_stack = new_page > rcu_read_unlock() # preempt enable but no explicit yield > > Each successive iteration of loop invokes for_each_process_thread() > which doesn't support cursor based resume and always restarts from the > init_task. Thus each successive loop needs to skip the tasks assigned > ret_stack in prior sweeps and thus take longer and longer to find the > candidate 32 tasks. And while the loop end calls rcu unlock, > and re-enables preemption briefly, there is no explicit yield. > > The total number of iterations turn out to be > > N_total * N_null / (2 * FTRACE_RETSTACK_ALLOC_SIZE) > > where N_total is the number of threads on the system and N_null the number > of those whose ret_stack is still NULL. After boot with no fgraph user that > is every thread. > > This is not a tracing-only path. Since commit 4346ba160409 ("fprobe: > Rewrite fprobe on function-graph tracer") fprobe is built on fgraph, > so an ordinary kprobe_multi BPF attach that flips ftrace_graph_active > from 0 to 1 pays the whole cost inside one bpf() syscall. Commit > 2c67dc457bc6 ("tracing: fprobe: optimization for entry only case") > narrows that to return and session probes but does not remove it. > > A host running hundreds of thousands of threads, with the current batch > size of 32, turns the ftrace_graph_active 0 -> 1 transition into a > multi-second, and eventually multi-minute, operation which showed up as > RCU stall and softlockup_panic on heavily loaded Meta Fleet machines > with preemption disabled. > > rcu: INFO: rcu_sched self-detected stall on CPU > rcu: 0-....: (20718 ticks this GP) (t=21000 jiffies g=913 q=8 ncpus=8) > CPU: 0 UID: 0 PID: 160141 Comm: bpftrace > ___slab_alloc+0x549/0xa20 > kmem_cache_alloc_noprof+0x16c/0x340 > register_ftrace_graph+0x3d7/0x6c0 > register_fprobe_ips+0x2d5/0x310 > bpf_kprobe_multi_link_attach+0x218/0x7e0 > __sys_bpf+0x267a/0x27a0 > > Fix this by allocating inline with GFP_NOWAIT instead of pre-allocating > a fixed batch. GFP_NOWAIT does not sleep, so it is legal inside the RCU > read-side critical section. > > A single sweep now serves every task, so the walk becomes O(N) rather > than O(N_total * N_null), and the retry loop is no longer the normal > path. > > scoped_guard(rcu) replaces the open-coded rcu_read_lock() and the > goto unlock, which is what makes returning directly from inside the > critical section legal. > > The pre-allocated array remains as a fallback, although the testing > below never needed it. > > smp_store_release() is a drop-in replacement for the smp_wmb() and > plain store it replaces. > > Measured on a 60-core Sapphire Rapids machine running a PREEMPT_LAZY > kernel (CONFIG_PREEMPT_DYNAMIC=y, mode lazy), timing the write that > drives the ftrace_graph_active 0 -> 1 transition. Threads are spawned > while fgraph is inactive so every one of them has ret_stack == NULL, > and the loop was instrumented to count sweeps: > > threads before after > sweeps time sweeps time > 100000 3126 15.3 s 1 0.019 s > 200000 6253 59.1 s 1 0.047 s > 400000 12503 227.8 s 1 0.092 s > > The sweep count is 1 throughout. Time now scales linearly with the > thread count at roughly 230 ns per task, which is the allocation and > initialisation of one shadow stack. > > Fixes: f201ae2356c7 ("tracing/function-return-tracer: store return stack into > task_struct and allocate it dynamically") > Suggested-by: Peter Zijlstra <[email protected]> > Signed-off-by: Vineet Gupta <[email protected]> > Tested-by: Vineet Gupta <[email protected]> > Cc: [email protected] > --- > kernel/trace/fgraph.c | 44 ++++++++++++++++++++++++++++--------------- > 1 file changed, 29 insertions(+), 15 deletions(-) >
Acked-by: Andrii Nakryiko <[email protected]> [...]
