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SubjectRe: RCU related performance regression in 3.3
On Fri, May 04, 2012 at 11:41:13PM +0200, Pascal Chapperon wrote:
> Le 04/05/2012 17:04, Paul E. McKenney a écrit :
> >On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote:
> >>Le 01/05/2012 17:45, Paul E. McKenney a écrit :
> >>
> >>>Here is my RCU_FAST_NO_HZ patch stack on top of v3.4-rc4.
> >>>
> >>>Or you can pull branch fnh.2012.05.01a from:
> >>>
> >>> git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> >>>
> >>> Thanx, Paul
> >>>
> >>I applied your global patch on top of v3.4-rc4. But the slowdown is
> >>worse than before : boot sequence took 80s instead 20-30s (12s for
> >>initramfs instead of 2s).
> >>
> >>I'll send you rcu tracing log in a second mail.
> >
> >Hmmm... Well, I guess I am glad that I finally did something that
> >had an effect, but I sure wish that the effect had been in the other
> >direction!
> >
> >Just to make sure I understand: the difference between the 20-30s and
> >the 80s is exactly the patch I sent you?
> >
> > Thanx, Paul
> >
> >
> Yes. Exactly same kernel config as in previous results, I applied
> your patch against v3.4-rc4, and sorry, the result is exactly what I
> said;
> I saw that your global patch was quite huge, and addresses things which
> are not directly related with the initial patch (commit
> 7cb92499000e3c86dae653077b1465458a039ef6); maybe a side effect?
>
> However, I'm ready to try this patch on my smaller laptop which
> supports well CONFIG_FAST_NO_HZ=y and systemd, if you think it can
> help ?
>
> Another thought: this issue as nothing to do with i7 Hyper-threading
> capacities ? (as I test core2duo, Pentium ulv in same conditions and I
> don't encountered any slowdown ?)

Well, one possibility is that your setup starts the jiffies counter
at some interesting value. The attached patch (also against v3.4-rc4)
applies a bit more paranoia to the initialization to handle this
and other possibilities.

Thanx, Paul

------------------------------------------------------------------------
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 3370997..1480900 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -292,6 +292,8 @@ TRACE_EVENT(rcu_dyntick,
* "More callbacks": Still more callbacks, try again to clear them out.
* "Callbacks drained": All callbacks processed, off to dyntick idle!
* "Timer": Timer fired to cause CPU to continue processing callbacks.
+ * "Demigrate": Timer fired on wrong CPU, woke up correct CPU.
+ * "Cleanup after idle": Idle exited, timer canceled.
*/
TRACE_EVENT(rcu_prep_idle,

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d..403306b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1829,6 +1829,8 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp->qlen++;
if (lazy)
rdp->qlen_lazy++;
+ else
+ rcu_idle_count_callbacks_posted();

if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index cdd1be0..36ca28e 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -471,6 +471,7 @@ static void __cpuinit rcu_prepare_kthreads(int cpu);
static void rcu_prepare_for_idle_init(int cpu);
static void rcu_cleanup_after_idle(int cpu);
static void rcu_prepare_for_idle(int cpu);
+static void rcu_idle_count_callbacks_posted(void);
static void print_cpu_stall_info_begin(void);
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
static void print_cpu_stall_info_end(void);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c023464..ccbdc72 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1938,6 +1938,14 @@ static void rcu_prepare_for_idle(int cpu)
{
}

+/*
+ * Don't bother keeping a running count of the number of RCU callbacks
+ * posted because CONFIG_RCU_FAST_NO_HZ=n.
+ */
+static void rcu_idle_count_callbacks_posted(void)
+{
+}
+
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */

/*
@@ -1978,11 +1986,20 @@ static void rcu_prepare_for_idle(int cpu)
#define RCU_IDLE_GP_DELAY 6 /* Roughly one grace period. */
#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */

+/* Loop counter for rcu_prepare_for_idle(). */
static DEFINE_PER_CPU(int, rcu_dyntick_drain);
+/* If rcu_dyntick_holdoff==jiffies, don't try to enter dyntick-idle mode. */
static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
-static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
-static ktime_t rcu_idle_gp_wait; /* If some non-lazy callbacks. */
-static ktime_t rcu_idle_lazy_gp_wait; /* If only lazy callbacks. */
+/* Timer to awaken the CPU if it enters dyntick-idle mode with callbacks. */
+static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
+/* Scheduled expiry time for rcu_idle_gp_timer to allow reposting. */
+static DEFINE_PER_CPU(unsigned long, rcu_idle_gp_timer_expires);
+/* Enable special processing on first attempt to enter dyntick-idle mode. */
+static DEFINE_PER_CPU(bool, rcu_idle_first_pass);
+/* Running count of non-lazy callbacks posted, never decremented. */
+static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted);
+/* Snapshot of rcu_nonlazy_posted to detect meaningful exits from idle. */
+static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted_snap);

/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@@ -1995,6 +2012,8 @@ static ktime_t rcu_idle_lazy_gp_wait; /* If only lazy callbacks. */
*/
int rcu_needs_cpu(int cpu)
{
+ /* Flag a new idle sojourn to the idle-entry state machine. */
+ per_cpu(rcu_idle_first_pass, cpu) = 1;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu))
return 0;
@@ -2045,16 +2064,39 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
}

/*
+ * Handler for smp_call_function_single(). The only point of this
+ * handler is to wake the CPU up, so the handler does only tracing.
+ */
+void rcu_idle_demigrate(void *unused)
+{
+ trace_rcu_prep_idle("Demigrate");
+}
+
+/*
* Timer handler used to force CPU to start pushing its remaining RCU
* callbacks in the case where it entered dyntick-idle mode with callbacks
* pending. The hander doesn't really need to do anything because the
* real work is done upon re-entry to idle, or by the next scheduling-clock
* interrupt should idle not be re-entered.
+ *
+ * One special case: the timer gets migrated without awakening the CPU
+ * on which the timer was scheduled on. In this case, we must wake up
+ * that CPU. We do so with smp_call_function_single().
*/
-static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
+static void rcu_idle_gp_timer_func(unsigned long cpu_in)
{
+ int cpu = (int)cpu_in;
+
trace_rcu_prep_idle("Timer");
- return HRTIMER_NORESTART;
+ if (cpu == smp_processor_id()) {
+ WARN_ON_ONCE(1); /* Getting here can hang the system... */
+ } else {
+ preempt_disable();
+ if (cpu_online(cpu))
+ smp_call_function_single(cpu, rcu_idle_demigrate,
+ NULL, 0);
+ preempt_enable();
+ }
}

/*
@@ -2062,19 +2104,11 @@ static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
*/
static void rcu_prepare_for_idle_init(int cpu)
{
- static int firsttime = 1;
- struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
-
- hrtimer_init(hrtp, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
- hrtp->function = rcu_idle_gp_timer_func;
- if (firsttime) {
- unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
-
- rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
- upj = jiffies_to_usecs(RCU_IDLE_LAZY_GP_DELAY);
- rcu_idle_lazy_gp_wait = ns_to_ktime(upj * (u64)1000);
- firsttime = 0;
- }
+ per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
+ setup_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ rcu_idle_gp_timer_func, cpu);
+ per_cpu(rcu_idle_gp_timer_expires, cpu) = jiffies - 1;
+ per_cpu(rcu_idle_first_pass, cpu) = 1;
}

/*
@@ -2084,7 +2118,8 @@ static void rcu_prepare_for_idle_init(int cpu)
*/
static void rcu_cleanup_after_idle(int cpu)
{
- hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
+ del_timer(&per_cpu(rcu_idle_gp_timer, cpu));
+ trace_rcu_prep_idle("Cleanup after idle");
}

/*
@@ -2108,6 +2143,29 @@ static void rcu_cleanup_after_idle(int cpu)
*/
static void rcu_prepare_for_idle(int cpu)
{
+ struct timer_list *tp;
+
+ /*
+ * If this is an idle re-entry, for example, due to use of
+ * RCU_NONIDLE() or the new idle-loop tracing API within the idle
+ * loop, then don't take any state-machine actions, unless the
+ * momentary exit from idle queued additional non-lazy callbacks.
+ * Instead, repost the rcu_idle_gp_timer if this CPU has callbacks
+ * pending.
+ */
+ if (!per_cpu(rcu_idle_first_pass, cpu) &&
+ (per_cpu(rcu_nonlazy_posted, cpu) ==
+ per_cpu(rcu_nonlazy_posted_snap, cpu))) {
+ if (rcu_cpu_has_callbacks(cpu)) {
+ tp = &per_cpu(rcu_idle_gp_timer, cpu);
+ mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
+ }
+ return;
+ }
+ per_cpu(rcu_idle_first_pass, cpu) = 0;
+ per_cpu(rcu_nonlazy_posted_snap, cpu) =
+ per_cpu(rcu_nonlazy_posted, cpu) - 1;
+
/*
* If there are no callbacks on this CPU, enter dyntick-idle mode.
* Also reset state to avoid prejudicing later attempts.
@@ -2140,11 +2198,15 @@ static void rcu_prepare_for_idle(int cpu)
per_cpu(rcu_dyntick_drain, cpu) = 0;
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
if (rcu_cpu_has_nonlazy_callbacks(cpu))
- hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
- rcu_idle_gp_wait, HRTIMER_MODE_REL);
+ per_cpu(rcu_idle_gp_timer_expires, cpu) =
+ jiffies + RCU_IDLE_GP_DELAY;
else
- hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
- rcu_idle_lazy_gp_wait, HRTIMER_MODE_REL);
+ per_cpu(rcu_idle_gp_timer_expires, cpu) =
+ jiffies + RCU_IDLE_LAZY_GP_DELAY;
+ tp = &per_cpu(rcu_idle_gp_timer, cpu);
+ mod_timer_pinned(tp, per_cpu(rcu_idle_gp_timer_expires, cpu));
+ per_cpu(rcu_nonlazy_posted_snap, cpu) =
+ per_cpu(rcu_nonlazy_posted, cpu);
return; /* Nothing more to do immediately. */
} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* We have hit the limit, so time to give up. */
@@ -2184,6 +2246,19 @@ static void rcu_prepare_for_idle(int cpu)
trace_rcu_prep_idle("Callbacks drained");
}

+/*
+ * Keep a running count of the number of non-lazy callbacks posted
+ * on this CPU. This running counter (which is never decremented) allows
+ * rcu_prepare_for_idle() to detect when something out of the idle loop
+ * posts a callback, even if an equal number of callbacks are invoked.
+ * Of course, callbacks should only be posted from within a trace event
+ * designed to be called from idle or from within RCU_NONIDLE().
+ */
+static void rcu_idle_count_callbacks_posted(void)
+{
+ __this_cpu_add(rcu_nonlazy_posted, 1);
+}
+
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */

#ifdef CONFIG_RCU_CPU_STALL_INFO
@@ -2192,14 +2267,12 @@ static void rcu_prepare_for_idle(int cpu)

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
- struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
+ struct timer_list *tltp = &per_cpu(rcu_idle_gp_timer, cpu);

- sprintf(cp, "drain=%d %c timer=%lld",
+ sprintf(cp, "drain=%d %c timer=%lu",
per_cpu(rcu_dyntick_drain, cpu),
per_cpu(rcu_dyntick_holdoff, cpu) == jiffies ? 'H' : '.',
- hrtimer_active(hrtp)
- ? ktime_to_us(hrtimer_get_remaining(hrtp))
- : -1);
+ timer_pending(tltp) ? tltp->expires - jiffies : -1);
}

#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
--
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