Date:   Fri, 7 Oct 2022 13:31:23 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rushikesh.s.kadam@...el.com,
        urezki@...il.com, neeraj.iitr10@...il.com, frederic@...nel.org,
        paulmck@...nel.org, rostedt@...dmis.org, youssefesmat@...gle.com,
        surenb@...gle.com
Subject: Re: [PATCH v7 10/11] scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()



> On Oct 7, 2022, at 1:19 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> On Fri, Oct 07, 2022 at 03:18:26AM +0000, Joel Fernandes wrote:
>>> On Tue, Oct 04, 2022 at 02:41:56AM +0000, Joel Fernandes (Google) wrote:
>>> From: Uladzislau Rezki <urezki@...il.com>
>>> 
>>> Slow boot time is seen on KVM running typical Linux distributions due to
>>> SCSI layer calling call_rcu(). Recent changes to save power may be
>>> causing this slowness. Using call_rcu_flush() fixes the issue and brings
>>> the boot time back to what it originally was. Convert it.
>>> 
>>> Signed-off-by: Uladzislau Rezki <urezki@...il.com>
>>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> 
>> And I successfully setup Debian on KVM and verified that this fixes it, so
>> now I have a nice reproducible rig for my
>> 'lazy-callback-doing-a-wakeup-detector' (I wrote half the detector thanks to
>> ideas from Steve, and will finish the other half tomorrow or so).
>> 
>> Tested-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> Looks like I can catch Vlad's issue with the below patch. Thoughts? Does this
> look reasonable for mainline? (I think so as it is self-contained and the
> debug option is default off, and could be useful down the line).
> 
> [    6.887033 ] rcu: *****************************************************
> [    6.891242 ] rcu: RCU: A wake up has been detected from a lazy callback!
> [    6.895377 ] rcu: The callback name is: scsi_eh_inc_host_failed
> [    6.899084 ] rcu: The task it woke up is: scsi_eh_1 (61)
> [    6.902405 ] rcu: This could cause performance issues! Check the stack.
> [    6.906532 ] rcu: *****************************************************
> 
> 
> [   17.127128 ] rcu: *****************************************************
> [   17.131397 ] rcu: RCU: A wake up has been detected from a lazy callback!
> [   17.135703 ] rcu: The callback name is: scsi_eh_inc_host_failed
> [   17.139485 ] rcu: The task it woke up is: scsi_eh_1 (61)
> [   17.142828 ] rcu: This could cause performance issues! Check the stack.
> [   17.146962 ] rcu: *****************************************************
> 
> And thanks to Steve for the binary search code.
> 
> One thing I found is I have to ignore kworkers because there are times when a
> work item is queued from a callback and those callbacks don't seem to
> contribute to performance issues. So I am filtering these:
> 
> [   38.631724 ] rcu: The callback name is: thread_stack_free_rcu
> [   38.635317 ] rcu: The task it woke up is: kworker/3:2 (143)
> 
> [   39.649332 ] rcu: The callback name is: delayed_put_task_struct
> [   39.653037 ] rcu: The task it woke up is: kworker/0:1 (40)
> 
> ---8<-----------------------
> 
> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> Subject: [PATCH] lazy wake debug
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
> kernel/rcu/Kconfig      |   7 ++
> kernel/rcu/lazy-debug.h | 149 ++++++++++++++++++++++++++++++++++++++++
> kernel/rcu/tree.c       |   9 +++
> 3 files changed, 165 insertions(+)
> create mode 100644 kernel/rcu/lazy-debug.h
> 
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index edd632e68497..08c06f739187 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -322,4 +322,11 @@ config RCU_LAZY
>      To save power, batch RCU callbacks and flush after delay, memory
>      pressure or callback list growing too big.
> 
> +config RCU_LAZY_DEBUG
> +    bool "RCU callback lazy invocation debugging"
> +    depends on RCU_LAZY
> +    default n
> +    help
> +      Debugging to catch issues caused by delayed RCU callbacks.
> +
> endmenu # "RCU Subsystem"
> diff --git a/kernel/rcu/lazy-debug.h b/kernel/rcu/lazy-debug.h
> new file mode 100644
> index 000000000000..fc1cc1cb89f0
> --- /dev/null
> +++ b/kernel/rcu/lazy-debug.h
> @@ -0,0 +1,149 @@
> +#include <linux/string.h>
> +#include <linux/spinlock.h>
> +
> +#ifdef CONFIG_RCU_LAZY_DEBUG
> +#include <linux/preempt.h>
> +#include <trace/events/sched.h>
> +
> +static DEFINE_PER_CPU(bool, rcu_lazy_cb_exec) = false;
> +static DEFINE_PER_CPU(void *, rcu_lazy_ip) = NULL;
> +
> +static DEFINE_RAW_SPINLOCK(lazy_funcs_lock);
> +
> +#define FUNC_SIZE 1024

And I know this array can overflow in the future so I will add checks for that in the code if we are going with this patch.

 - Joel 


> +static unsigned long lazy_funcs[FUNC_SIZE];
> +static int nr_funcs;
> +
> +static void __find_func(unsigned long ip, int *B, int *E, int *N)
> +{
> +    unsigned long *p;
> +    int b, e, n;
> +
> +    b = n = 0;
> +    e = nr_funcs - 1;
> +
> +    while (b <= e) {
> +        n = (b + e) / 2;
> +        p = &lazy_funcs[n];
> +        if (ip > *p) {
> +            b = n + 1;
> +        } else if (ip < *p) {
> +            e = n - 1;
> +        } else
> +            break;
> +    }
> +
> +    *B = b;
> +    *E = e;
> +    *N = n;
> +
> +    return;
> +}
> +
> +static bool lazy_func_exists(void* ip_ptr)
> +{
> +    int b, e, n;
> +    unsigned long flags;
> +    unsigned long ip = (unsigned long)ip_ptr;
> +
> +    raw_spin_lock_irqsave(&lazy_funcs_lock, flags);
> +    __find_func(ip, &b, &e, &n);
> +    raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> +
> +    return b <= e;
> +}
> +
> +static int lazy_func_add(void* ip_ptr)
> +{
> +    int b, e, n;
> +    unsigned long flags;
> +    unsigned long ip = (unsigned long)ip_ptr;
> +
> +    raw_spin_lock_irqsave(&lazy_funcs_lock, flags);
> +    if (nr_funcs >= FUNC_SIZE) {
> +        raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> +        return -1;
> +    }
> +
> +    __find_func(ip, &b, &e, &n);
> +
> +    if (b > e) {
> +        if (n != nr_funcs)
> +            memmove(&lazy_funcs[n+1], &lazy_funcs[n],
> +                (sizeof(*lazy_funcs) * (nr_funcs - n)));
> +
> +        lazy_funcs[n] = ip;
> +        nr_funcs++;
> +    }
> +
> +    raw_spin_unlock_irqrestore(&lazy_funcs_lock, flags);
> +    return 0;
> +}
> +
> +static void rcu_set_lazy_context(void *ip_ptr)
> +{
> +    bool *flag = this_cpu_ptr(&rcu_lazy_cb_exec);
> +    *flag = lazy_func_exists(ip_ptr);
> +
> +    if (*flag) {
> +        *this_cpu_ptr(&rcu_lazy_ip) = ip_ptr;
> +    } else {
> +        *this_cpu_ptr(&rcu_lazy_ip) = NULL;
> +    }
> +}
> +
> +static void rcu_reset_lazy_context(void)
> +{
> +    bool *flag = this_cpu_ptr(&rcu_lazy_cb_exec);
> +    *flag = false;
> +}
> +
> +static bool rcu_is_lazy_context(void)
> +{
> +    return *(this_cpu_ptr(&rcu_lazy_cb_exec));
> +}
> +
> +static void
> +probe_waking(void *ignore, struct task_struct *p)
> +{
> +    if (WARN_ON(!in_nmi() && !in_hardirq() && rcu_is_lazy_context())) {
> +        pr_err("*****************************************************\n");
> +        pr_err("RCU: A wake up has been detected from a lazy callback!\n");
> +        pr_err("The callback name is: %ps\n", *this_cpu_ptr(&rcu_lazy_ip));
> +        pr_err("The task it woke up is: %s (%d)\n", p->comm, p->pid);
> +        pr_err("This could cause performance issues! Check the stack.\n");
> +        pr_err("*****************************************************\n");
> +    }
> +}
> +
> +static void rcu_lazy_debug_init(void)
> +{
> +    int ret;
> +    pr_info("RCU Lazy CB debugging is turned on, system may be slow.\n");
> +
> +    ret = register_trace_sched_waking(probe_waking, NULL);
> +    if (ret)
> +        pr_info("RCU: Lazy debug ched_waking probe could not be registered.");
> +}
> +
> +#else
> +
> +static int lazy_func_add(void* ip_ptr)
> +{
> +    return -1;
> +}
> +
> +
> +static void rcu_set_lazy_context(void *ip_ptr)
> +{
> +}
> +
> +static void rcu_reset_lazy_context(void)
> +{
> +}
> +
> +static void rcu_lazy_debug_init(void)
> +{
> +}
> +
> +#endif
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c20544c4aa29..ad8d4e52ae92 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -67,6 +67,7 @@
> 
> #include "tree.h"
> #include "rcu.h"
> +#include "lazy-debug.h"
> 
> #ifdef MODULE_PARAM_PREFIX
> #undef MODULE_PARAM_PREFIX
> @@ -2245,7 +2246,10 @@ static void rcu_do_batch(struct rcu_data *rdp)
> 
>        f = rhp->func;
>        WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
> +
> +        rcu_set_lazy_context(f);
>        f(rhp);
> +        rcu_reset_lazy_context();
> 
>        rcu_lock_release(&rcu_callback_map);
> 
> @@ -2770,6 +2774,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy)
>    }
> 
>    check_cb_ovld(rdp);
> +
> +    if (lazy)
> +        lazy_func_add(func);
> +
>    if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy))
>        return; // Enqueued onto ->nocb_bypass, so just leave.
>    // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> @@ -4805,6 +4813,7 @@ void __init rcu_init(void)
>    rcu_early_boot_tests();
> 
>    kfree_rcu_batch_init();
> +    rcu_lazy_debug_init();
>    rcu_bootup_announce();
>    sanitize_kthread_prio();
>    rcu_init_geometry();
> -- 
> 2.38.0.rc1.362.ged0d419d3c-goog
> 

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