Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled
flat view
From: Peter Zijlstra <peterz@infradead.org>
Date: 2023-01-25 10:50:09
Also in:
kvm, linux-pm, lkml, virtualization
Subsystem:
library code, the rest, tracing · Maintainers:
Andrew Morton, Linus Torvalds, Steven Rostedt, Masami Hiramatsu
On Tue, Jan 24, 2023 at 05:12:14PM +0000, Mark Rutland wrote:
On Tue, Jan 24, 2023 at 03:44:35PM +0100, Peter Zijlstra wrote:quoted
On Mon, Jan 23, 2023 at 05:07:53PM -0500, Steven Rostedt wrote:quoted
Actually, perhaps we can just add this, and all you need to do is create and set CONFIG_NO_RCU_TRACING (or some other name).Elsewhere I've used CONFIG_ARCH_WANTS_NO_INSTR for this.Yes please; if we use CONFIG_ARCH_WANTS_NO_INSTR then arm64 will get this "for free" once we add the missing checks (which I assume we need) in our ftrace_prepare_return().quoted
Anyway, I took it for a spin and it .... doesn't seems to do the job. With my patch the first splat is "RCU not on for: cpuidle_poll_time+0x0/0x70" While with yours I seems to get the endless: "WARNING: suspicious RCU usage" thing. Let me see if I can figure out where it goes side-ways.Hmmm... for WARN_ONCE() don't we need to wake RCU first also? I thought we needed that at least for the printk machinery?
OK, the below seems to work nice for me -- although I'm still on a hacked up printk, but the recursive RCU not watching fail seems to be tamed. Ofc. Paul might have an opinion on this glorious bodge ;-) ---
diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index c303f7a114e9..d48cd92d2364 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h@@ -135,6 +135,21 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip); # define do_ftrace_record_recursion(ip, pip) do { } while (0) #endif +#ifdef CONFIG_ARCH_WANTS_NO_INSTR +# define trace_warn_on_no_rcu(ip) \ + ({ \ + bool __ret = !rcu_is_watching(); \ + if (__ret && !trace_recursion_test(TRACE_RECORD_RECURSION_BIT)) { \ + trace_recursion_set(TRACE_RECORD_RECURSION_BIT); \ + WARN_ONCE(true, "RCU not on for: %pS\n", (void *)ip); \ + trace_recursion_clear(TRACE_RECORD_RECURSION_BIT); \ + } \ + __ret; \ + }) +#else +# define trace_warn_on_no_rcu(ip) false +#endif + /* * Preemption is promised to be disabled when return bit >= 0. */
@@ -144,6 +159,9 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign unsigned int val = READ_ONCE(current->trace_recursion); int bit; + if (trace_warn_on_no_rcu(ip)) + return -1; + bit = trace_get_context_bit() + start; if (unlikely(val & (1 << bit))) { /*
diff --git a/lib/bug.c b/lib/bug.c
index c223a2575b72..0a10643ea168 100644
--- a/lib/bug.c
+++ b/lib/bug.c@@ -47,6 +47,7 @@ #include <linux/sched.h> #include <linux/rculist.h> #include <linux/ftrace.h> +#include <linux/context_tracking.h> extern struct bug_entry __start___bug_table[], __stop___bug_table[];
@@ -153,7 +154,7 @@ struct bug_entry *find_bug(unsigned long bugaddr) return module_find_bug(bugaddr); } -enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs) +static enum bug_trap_type __report_bug(unsigned long bugaddr, struct pt_regs *regs) { struct bug_entry *bug; const char *file;
@@ -209,6 +210,30 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs) return BUG_TRAP_TYPE_BUG; } +enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs) +{ + enum bug_trap_type ret; + bool rcu = false; + +#ifdef CONFIG_CONTEXT_TRACKING_IDLE + /* + * Horrible hack to shut up recursive RCU isn't watching fail since + * lots of the actual reporting also relies on RCU. + */ + if (!rcu_is_watching()) { + rcu = true; + ct_state_inc(RCU_DYNTICKS_IDX); + } +#endif + + ret = __report_bug(bugaddr, regs); + + if (rcu) + ct_state_inc(RCU_DYNTICKS_IDX); + + return ret; +} + static void clear_once_table(struct bug_entry *start, struct bug_entry *end) { struct bug_entry *bug;