During some recent expedited RCU testing with rcutorture, I noticed RCU stalls when using ftrace_dump() to analyze traces. The issue that since ftrace_dump() can take a long time, any accumulated issue can show up just after the stall is unsupressed. Due to this a stall can be detected and reported.
The problem is something like this: 1. rcu_ftrace_dump_stall_suppress() sets suppression value. 2. ftrace_dump() runs for a long time. 3. rcu_ftrace_dump_stall_unsuppress() clears suppression. 4. Immediately later, stall is detected. Fix this by calling rcu_cpu_stall_reset() when clearing suppression. This resets jiffies_stall to ULONG_MAX, preventing the issue. Tested with gp_exp=true enabled with rcutorture and ftrace_dump() and no stall is reported, with only clean traces in the output. Signed-off-by: Joel Fernandes <[email protected]> --- This is a non-urgent fix and could go in either this or the next merge window (Boqun's call). kernel/rcu/rcu.h | 5 ++++- kernel/rcu/tree_stall.h | 1 + 2 files changed, 5 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 9cf01832a6c3..87fcf528b9ac 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -288,6 +288,7 @@ extern int rcu_cpu_stall_cputime; extern bool rcu_exp_stall_task_details __read_mostly; int rcu_jiffies_till_stall_check(void); int rcu_exp_jiffies_till_stall_check(void); +void rcu_cpu_stall_reset(void); static inline bool rcu_stall_is_suppressed(void) { @@ -302,8 +303,10 @@ do { \ #define rcu_ftrace_dump_stall_unsuppress() \ do { \ - if (rcu_cpu_stall_suppress == 3) \ + if (rcu_cpu_stall_suppress == 3) { \ rcu_cpu_stall_suppress = 0; \ + rcu_cpu_stall_reset(); \ + } \ } while (0) #else /* #endif #ifdef CONFIG_RCU_STALL_COMMON */ diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index b67532cb8770..9a036fc27fed 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -192,6 +192,7 @@ void rcu_cpu_stall_reset(void) WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3); WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX); } +EXPORT_SYMBOL_GPL(rcu_cpu_stall_reset); ////////////////////////////////////////////////////////////////////////////// // -- 2.34.1
