> On Jan 10, 2023, at 1:07 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
>>> On Dec 21, 2022, at 1:56 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>>>
>>> On Mon, Dec 19, 2022 at 02:29:08PM -0600, Robert Elliott wrote:
>>> Convert the functions that temporarily suppress RCU CPU
>>> stall reporting:
>>> rcu_sysrq_start()
>>> rcu_sysrq_end()
>>>
>>> to more generic functions that may be called by functions
>>> other than the SysRq handler:
>>> rcu_suppress_start()
>>> rcu_suppress_end()
>>>
>>> Covert the underlying variable:
>>> rcu_cpu_stall_suppress
>>>
>>> to an atomic variable so multiple threads can manipulate it at the
>>> same time, incrementing it in start() and decrementing in end().
>>>
>>> Expose that in sysfs in a read-only
>>> /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress_dyn
>
> Robert,
>
> What is the use of exposing this read-only node?
>
> I find it hard to believe anyone or any tool would be reading it during a scenario where stalls are required to be dynamically suppressed.
>
> Or maybe I missed the point of this patch as I am late to the party.
I was hasty to not be clear — I have no problem with the idea of dynamic suppression, just missing the point about the read only infomercial via sysfs node. More kernel memory wasted in sysfs however small :-).
Thanks,
- J
>
> Thanks,
>
> - J
>
>
>>>
>>> Keep
>>> /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress
>>> as writeable by userspace, but OR that into the equations rather than
>>> directly manipulate the atomic value.
>>>
>>> Signed-off-by: Robert Elliott <elliott@xxxxxxx>
>>
>> I really like the idea of making the suppressing and unsuppressing of
>> RCU CPU stall warnings SMP-safe, thank you! Yes, as far as I know,
>> there have been no problems due to this, but accidents waiting to happen
>> and all that.
>>
>> Some comments and questions below.
>>
>>> ---
>>> .../RCU/Design/Requirements/Requirements.rst | 6 +++---
>>> Documentation/RCU/stallwarn.rst | 19 +++++++++++++++----
>>> arch/parisc/kernel/process.c | 2 +-
>>> drivers/tty/sysrq.c | 4 ++--
>>> include/linux/rcupdate.h | 8 ++++----
>>> kernel/rcu/rcu.h | 11 ++++++-----
>>> kernel/rcu/tree_stall.h | 18 ++++++++++--------
>>> kernel/rcu/update.c | 11 ++++++++++-
>>> 8 files changed, 51 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
>>> index 49387d823619..5083490bb6fc 100644
>>> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
>>> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
>>> @@ -1639,9 +1639,9 @@ against mishaps and misuse:
>>> ``rcupdate.rcu_cpu_stall_suppress`` to suppress the splats. This
>>> kernel parameter may also be set via ``sysfs``. Furthermore, RCU CPU
>>> stall warnings are counter-productive during sysrq dumps and during
>>> - panics. RCU therefore supplies the rcu_sysrq_start() and
>>> - rcu_sysrq_end() API members to be called before and after long
>>> - sysrq dumps. RCU also supplies the rcu_panic() notifier that is
>>> + panics. RCU therefore supplies the rcu_suppress_start() and
>>> + rcu_suppress_end() API members to be called before and after long
>>> + CPU usage. RCU also supplies the rcu_panic() notifier that is
>>> automatically invoked at the beginning of a panic to suppress further
>>> RCU CPU stall warnings.
>>>
>>> diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
>>> index e38c587067fc..9e6fba72f56d 100644
>>> --- a/Documentation/RCU/stallwarn.rst
>>> +++ b/Documentation/RCU/stallwarn.rst
>>> @@ -135,13 +135,24 @@ see include/trace/events/rcu.h.
>>> Fine-Tuning the RCU CPU Stall Detector
>>> ======================================
>>>
>>> -The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
>>> -CPU stall detector, which detects conditions that unduly delay RCU grace
>>> -periods. This module parameter enables CPU stall detection by default,
>>> -but may be overridden via boot-time parameter or at runtime via sysfs.
>>> +RCU's CPU stall detector detects conditions that unduly delay RCU grace
>>> +periods. CPU stall detection is enabled by default, but may be overridden
>>> +via boot-time parameter or at runtime via sysfs (accessible via
>>> +/sys/module/rcupdate/parameters).
>>> +
>>> +The rcupdate.rcu_cpu_stall_suppress module parameter disables RCU's
>>> +CPU stall detector.
>>> +
>>> +/sys/module/rcu_cpu_stall_suppress_dyn reports an internal semaphore
>>
>> Actually an atomically updated variable as opposed to a semaphore,
>> correct? Replacing "an internal semaphore" with something like "a
>> variable" would be fine from my viewpoint.
>>
>>> +used by the RCU's CPU stall detector. Functions holding a CPU for a
>>> +long time (e.g., sysrq printouts) increment this value before use
>>> +and decrement it when done, so the value reports the number of
>>> +functions currently disabling stalls.
>>> +
>>> The stall detector's idea of what constitutes "unduly delayed" is
>>> controlled by a set of kernel configuration variables and cpp macros:
>>>
>>> +
>>> CONFIG_RCU_CPU_STALL_TIMEOUT
>>> ----------------------------
>>
>> And thank you for updating the documentation!
>>
>>> diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
>>> index c4f8374c7018..038378fe7bfa 100644
>>> --- a/arch/parisc/kernel/process.c
>>> +++ b/arch/parisc/kernel/process.c
>>> @@ -126,7 +126,7 @@ void machine_power_off(void)
>>> "Please power this system off now.");
>>>
>>> /* prevent soft lockup/stalled CPU messages for endless loop. */
>>> - rcu_sysrq_start();
>>> + rcu_suppress_start();
>>> lockup_detector_soft_poweroff();
>>> for (;;);
>>> }
>>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>>> index d2b2720db6ca..81ab63a473a7 100644
>>> --- a/drivers/tty/sysrq.c
>>> +++ b/drivers/tty/sysrq.c
>>> @@ -579,7 +579,7 @@ void __handle_sysrq(int key, bool check_mask)
>>> orig_suppress_printk = suppress_printk;
>>> suppress_printk = 0;
>>>
>>> - rcu_sysrq_start();
>>> + rcu_suppress_start();
>>> rcu_read_lock();
>>> /*
>>> * Raise the apparent loglevel to maximum so that the sysrq header
>>> @@ -623,7 +623,7 @@ void __handle_sysrq(int key, bool check_mask)
>>> console_loglevel = orig_log_level;
>>> }
>>> rcu_read_unlock();
>>> - rcu_sysrq_end();
>>> + rcu_suppress_end();
>>>
>>> suppress_printk = orig_suppress_printk;
>>> }
>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>> index 03abf883a281..c0d8a4aae7ff 100644
>>> --- a/include/linux/rcupdate.h
>>> +++ b/include/linux/rcupdate.h
>>> @@ -131,11 +131,11 @@ static inline void rcu_init_tasks_generic(void) { }
>>> #endif
>>>
>>> #ifdef CONFIG_RCU_STALL_COMMON
>>> -void rcu_sysrq_start(void);
>>> -void rcu_sysrq_end(void);
>>> +void rcu_suppress_start(void);
>>> +void rcu_suppress_end(void);
>>> #else /* #ifdef CONFIG_RCU_STALL_COMMON */
>>> -static inline void rcu_sysrq_start(void) { }
>>> -static inline void rcu_sysrq_end(void) { }
>>> +static inline void rcu_suppress_start(void) { }
>>> +static inline void rcu_suppress_end(void) { }
>>> #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
>>>
>>> #if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK))
>>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>>> index c5aa934de59b..a658955a1174 100644
>>> --- a/kernel/rcu/rcu.h
>>> +++ b/kernel/rcu/rcu.h
>>> @@ -224,24 +224,25 @@ extern int rcu_cpu_stall_ftrace_dump;
>>> extern int rcu_cpu_stall_suppress;
>>> extern int rcu_cpu_stall_timeout;
>>> extern int rcu_exp_cpu_stall_timeout;
>>> +extern atomic_t rcu_cpu_stall_suppress_dyn;
>>> int rcu_jiffies_till_stall_check(void);
>>> int rcu_exp_jiffies_till_stall_check(void);
>>>
>>> static inline bool rcu_stall_is_suppressed(void)
>>> {
>>> - return rcu_stall_is_suppressed_at_boot() || rcu_cpu_stall_suppress;
>>> + return rcu_stall_is_suppressed_at_boot() ||
>>> + rcu_cpu_stall_suppress ||
>>> + atomic_read(&rcu_cpu_stall_suppress_dyn);
>>> }
>>>
>>> #define rcu_ftrace_dump_stall_suppress() \
>>> do { \
>>> - if (!rcu_cpu_stall_suppress) \
>>> - rcu_cpu_stall_suppress = 3; \
>>
>> One thing we are losing here is the ability to see what is suppressing
>> the current stall, for example, from a crash dump. Maybe that is OK,
>> as I haven't needed to debug that sort of thing.
>>
>> Thoughts from those who have had this debugging experience?
>>
>>> + atomic_inc(&rcu_cpu_stall_suppress_dyn); \
>>> } while (0)
>>>
>>> #define rcu_ftrace_dump_stall_unsuppress() \
>>> do { \
>>> - if (rcu_cpu_stall_suppress == 3) \
>>> - rcu_cpu_stall_suppress = 0; \
>>> + atomic_dec(&rcu_cpu_stall_suppress_dyn); \
>>> } while (0)
>>>
>>> #else /* #endif #ifdef CONFIG_RCU_STALL_COMMON */
>>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>>> index 5653560573e2..260748bc5bc8 100644
>>> --- a/kernel/rcu/tree_stall.h
>>> +++ b/kernel/rcu/tree_stall.h
>>> @@ -103,23 +103,25 @@ bool rcu_gp_might_be_stalled(void)
>>> return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
>>> }
>>>
>>> -/* Don't do RCU CPU stall warnings during long sysrq printouts. */
>>> -void rcu_sysrq_start(void)
>>> +/* Don't do RCU CPU stall warnings during functions holding the CPU
>>> + * for a long period of time such as long sysrq printouts.
>>> + */
>>> +void rcu_suppress_start(void)
>>> {
>>> - if (!rcu_cpu_stall_suppress)
>>> - rcu_cpu_stall_suppress = 2;
>>
>> And the same point here.
>>
>>> + atomic_inc(&rcu_cpu_stall_suppress_dyn);
>>> }
>>> +EXPORT_SYMBOL_GPL(rcu_suppress_start);
>>
>> If this is being exported to modules, the question of who suppressed
>> the CPU stalls might at some point become more urgent.
>>
>> If the problem was reproducible, I would simply attach a BPF program to
>> rcu_suppress_start() and rcu_suppress_end() counting the stack traces of
>> all callers to these functions. This might or might not make everyone
>> happy, though.
>>
>>> -void rcu_sysrq_end(void)
>>> +void rcu_suppress_end(void)
>>> {
>>> - if (rcu_cpu_stall_suppress == 2)
>>> - rcu_cpu_stall_suppress = 0;
>>> + atomic_dec(&rcu_cpu_stall_suppress_dyn);
>>> }
>>> +EXPORT_SYMBOL_GPL(rcu_suppress_end);
>>>
>>> /* Don't print RCU CPU stall warnings during a kernel panic. */
>>> static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
>>> {
>>> - rcu_cpu_stall_suppress = 1;
>>> + atomic_inc(&rcu_cpu_stall_suppress_dyn);
>>> return NOTIFY_DONE;
>>> }
>>>
>>> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>>> index f5e6a2f95a2a..ceee9d777384 100644
>>> --- a/kernel/rcu/update.c
>>> +++ b/kernel/rcu/update.c
>>> @@ -501,11 +501,18 @@ EXPORT_SYMBOL_GPL(rcutorture_sched_setaffinity);
>>> #ifdef CONFIG_RCU_STALL_COMMON
>>> int rcu_cpu_stall_ftrace_dump __read_mostly;
>>> module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
>>> -int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings.
>>> +
>>> +int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings from sysfs
>>> EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
>>> module_param(rcu_cpu_stall_suppress, int, 0644);
>>> +
>>> +atomic_t rcu_cpu_stall_suppress_dyn __read_mostly; // !0 = suppress stall warnings from functions
>>> +EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress_dyn);
>>> +module_param_named(rcu_cpu_stall_suppress_dyn, rcu_cpu_stall_suppress_dyn.counter, int, 0444);
>>
>> I am not seeing a valid use case for specifying an initial
>> value on the kernel command line. Or does this somehow prevent
>> rcupdate.rcu_cpu_stall_suppress_dyn from being specified on the kernel
>> command line?
>>
>> If something like rcupdate.rcu_cpu_stall_suppress_dyn=3 can be specified
>> (incorrectly, in my current view) on the kernel command line, maybe
>> something as shown below would help?
>>
>>> +
>>> int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
>>> module_param(rcu_cpu_stall_timeout, int, 0644);
>>> +
>>> int rcu_exp_cpu_stall_timeout __read_mostly = CONFIG_RCU_EXP_CPU_STALL_TIMEOUT;
>>> module_param(rcu_exp_cpu_stall_timeout, int, 0644);
>>> #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
>>> @@ -616,6 +623,8 @@ void __init rcupdate_announce_bootup_oddness(void)
>>> pr_info("\tAll grace periods are expedited (rcu_expedited).\n");
>>> if (rcu_cpu_stall_suppress)
>>> pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n");
>>> + if (atomic_read(&rcu_cpu_stall_suppress_dyn))
>>> + pr_info("\tDynamic RCU CPU stall warnings suppressed (rcu_cpu_stall_suppress_dyn).\n");
>>
>> Should this instead be a WARN_ON() placed somewhere so that it won't
>> mess up the printing of the other parameters?
>>
>> Or maybe have this code set it to back to zero, with the message
>> indicating that this is being done?
>>
>>> if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT)
>>> pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout);
>>> rcu_tasks_bootup_oddness();
>>> --
>>> 2.38.1
>>>
