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Date: Fri, 7 Oct 2022 17:19:59 +0000
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 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
+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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