RCU stall on 6.1-rc4 (and some previous releases) related to ftrace

[Guilherme G. Piccoli]

Hi folks, I've noticed some RCU stalls when enabling ftrace filtering in
6.1-rc4 (also 6.1-rc3 and I guess I've seen this before, but only in
6.0+). Here is the full dmesg: https://termbin.com/4xo6 , see below for
a small snippet of the stall [0].

I've briefly talked to Steve on IRC, and he mentioned that my kernel
seemed to have PREEMPT_VOLUNTARY=y (and indeed, this is the case - see
the full config here: https://termbin.com/t48d), saying that maybe
adding a cond_resched() in the loop would help.

So, I've cooked a small hack (see the patch attached) and it seems to
work. Steve: lemme know if you want to send it (since it's your idea and
maybe my hack is not covering all cases), or if you prefer, I can work a
commit message and send myself.

Any other advice / tests required, please lemme me know and I'll be glad
in contributing.

Cheers,


Guilherme


[0]
rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 1-....
} 6 jiffies s: 1289 root: 0x2/.
rcu: blocking rcu_node structures (internal RCU debug):
Sending NMI from CPU 4 to CPUs 1:
NMI backtrace for cpu 1
[...]
RIP: 0010:find_kallsyms_symbol+0x85/0x1a0
[...]
Call Trace:
 <TASK>
 ? get_refcyc_per_delivery.constprop.0+0x200/0x200 [amdgpu]
 module_address_lookup+0x63/0xc0
 ? get_refcyc_per_delivery.constprop.0+0x200/0x200 [amdgpu]
 kallsyms_lookup_buildid+0xb6/0x130
 ftrace_match_record+0x43/0xf0
 ? match_records+0x210/0x3b0
 ? __kmem_cache_alloc_node+0x165/0x260
 match_records+0x13a/0x3b0
 ftrace_process_regex.isra.0+0x101/0x120
 ftrace_filter_write+0x57/0x90
[...]
rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: { 1-....
} 25 jiffies s: 1289 root: 0x2/.
rcu: blocking rcu_node structures (internal RCU debug):
Sending NMI from CPU 4 to CPUs 1:
[...]
RIP: 0010:find_kallsyms_symbol+0x8f/0x1a0
[...]
Call Trace:
 <TASK>
 ? dcn21_dmcu_create+0xd0/0xd0 [amdgpu]
 module_address_lookup+0x63/0xc0
 ? dcn21_dmcu_create+0xd0/0xd0 [amdgpu]
 kallsyms_lookup_buildid+0xb6/0x130
 ftrace_match_record+0x43/0xf0
 ? match_records+0x210/0x3b0
 ? __kmem_cache_alloc_node+0x165/0x260
 match_records+0x13a/0x3b0
 ftrace_process_regex.isra.0+0x101/0x120
 ftrace_filter_write+0x57/0x90
[...]From 45fda369a49d0e59689e4a6b9babc12598265825 Mon Sep 17 00:00:00 2001
From: "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx>
Date: Thu, 10 Nov 2022 16:42:17 -0300
Subject: [PATCH] ftrace/hack: Add cond_resched() to prevent RCU stall

Suggestion from Steve on IRC
---
 kernel/trace/ftrace.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 7dc023641bf1..8c2c04b2eb52 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4184,6 +4184,7 @@ match_records(struct ftrace_hash *hash, char *func, int len, char *mod)
 		if (rec->flags & FTRACE_FL_DISABLED)
 			continue;
 
+		cond_resched();
 		if (ftrace_match_record(rec, &func_g, mod_match, exclude_mod)) {
 			ret = enter_record(hash, rec, clear_filter);
 			if (ret < 0) {
-- 
2.38.0