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Date: Fri, 16 Sep 2022 22:19:14 +0000
From: Joel Fernandes <joel@...lfernandes.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
"Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH rcu/next 2/3] rcu: Move trace_rcu_callback() before
bypassing
On 9/16/2022 7:09 AM, Frederic Weisbecker wrote:
> On Thu, Sep 15, 2022 at 12:14:18AM +0000, Joel Fernandes (Google) wrote:
>> If any CB is queued into the bypass list, then trace_rcu_callback() does
>> not show it. This makes it not clear when a callback was actually
>> queued, as you only end up getting a trace_rcu_invoke_callback() trace.
>> Fix it by moving trace_rcu_callback() before
>> trace_rcu_nocb_try_bypass().
>>
>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> ---
>> kernel/rcu/tree.c | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 5ec97e3f7468..9fe581be8696 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -2809,10 +2809,7 @@ void call_rcu(struct rcu_head *head, rcu_callback_t
>> func)
>> }
>>
>> check_cb_ovld(rdp);
>> - if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
>> - return; // Enqueued onto ->nocb_bypass, so just leave.
>> - // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired
>> ->nocb_lock.
>> - rcu_segcblist_enqueue(&rdp->cblist, head);
>> +
>> if (__is_kvfree_rcu_offset((unsigned long)func))
>> trace_rcu_kvfree_callback(rcu_state.name, head,
>> (unsigned long)func,
>> @@ -2821,6 +2818,11 @@ void call_rcu(struct rcu_head *head, rcu_callback_t
>> func)
>> trace_rcu_callback(rcu_state.name, head,
>> rcu_segcblist_n_cbs(&rdp->cblist));
>>
>> + if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
>> + return; // Enqueued onto ->nocb_bypass, so just leave.
>> + // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired
>> ->nocb_lock.
>> + rcu_segcblist_enqueue(&rdp->cblist, head);
>> +
>> trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
>>
>> /* Go handle any RCU core processing required. */
>
> Two subtle changes induced here:
>
> * rcu_segcblist_n_cbs() is now read lockless. It's just tracing so no huge
> deal
> but still, if this races with callbacks invocation, we may on some rare
> occasion
> read stale numbers on traces while enqueuing (think about rcu_top for
> example)
Actually I disagree with this point now. Changes to the number of callbacks
in the main ->cblist can be lockless. It uses atomic on CONFIG_RCU_NOCB. On
non CONFIG_RCU_NOCB, CB cannot be queued as interrupts will be disabled.
Also, in rcu_do_batch(), the count is manipulated after calling
rcu_nocb_lock_irqsave(rdp, flags);
> * trace_rcu_callback() will now show the number of callbacks _before_
> enqueuing
> instead of _after_. Not sure if it matters, but sometimes tools rely on
> trace
> events.
Yeah this is fixable and good point. So how about the below?
---8<-----------------------
From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Subject: [PATCH] rcu: Call trace_rcu_callback() also for bypass queuing
If any CB is queued into the bypass list, then trace_rcu_callback() does
not show it. This makes it not clear when a callback was actually
queued, as you only end up getting a trace_rcu_invoke_callback() trace.
Fix it by calling the tracing function even for bypass queue.
Also, while at it, optimize the tracing so that rcu_state is not
accessed here if tracing is disabled, because that's useless if we are
not tracing. A quick inspection of the generated assembler shows that
rcu_state is accessed even if the jump label for the tracepoint is
disabled.
__trace_rcu_callback:
movq 8(%rdi), %rcx
movq rcu_state+3640(%rip), %rax
movq %rdi, %rdx
cmpq $4095, %rcx
ja .L3100
movq 192(%rsi), %r8
1:jmp .L3101 # objtool NOPs this
.pushsection __jump_table, "aw"
.balign 8
.long 1b - .
.long .L3101 - .
.quad __tracepoint_rcu_kvfree_callback+8 + 2 - .
.popsection
With this change, the jump label check which is NOOPed is moved to the
beginning of the function.
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
kernel/rcu/tree.c | 31 +++++++++++++++++++++++--------
1 file changed, 23 insertions(+), 8 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5ec97e3f7468..b64df55f7f55 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2728,6 +2728,23 @@ static void check_cb_ovld(struct rcu_data *rdp)
raw_spin_unlock_rcu_node(rnp);
}
+/*
+ * Trace RCU callback helper, call after enqueuing callback.
+ * The ->cblist must be locked when called.
+ */
+void __trace_rcu_callback(struct rcu_head *head,
+ struct rcu_data *rdp)
+{
+ if (trace_rcu_kvfree_callback_enabled() &&
+ __is_kvfree_rcu_offset((unsigned long)head->func))
+ trace_rcu_kvfree_callback(rcu_state.name, head,
+ (unsigned long)head->func,
+ rcu_segcblist_n_cbs(&rdp->cblist));
+ else if (trace_rcu_callback_enabled())
+ trace_rcu_callback(rcu_state.name, head,
+ rcu_segcblist_n_cbs(&rdp->cblist));
+}
+
/**
* call_rcu() - Queue an RCU callback for invocation after a grace period.
* @head: structure to be used for queueing the RCU updates.
@@ -2809,17 +2826,15 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
}
check_cb_ovld(rdp);
- if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
+
+ if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags)) {
+ __trace_rcu_callback(head, rdp);
return; // Enqueued onto ->nocb_bypass, so just leave.
+ }
+
// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
rcu_segcblist_enqueue(&rdp->cblist, head);
- if (__is_kvfree_rcu_offset((unsigned long)func))
- trace_rcu_kvfree_callback(rcu_state.name, head,
- (unsigned long)func,
- rcu_segcblist_n_cbs(&rdp->cblist));
- else
- trace_rcu_callback(rcu_state.name, head,
- rcu_segcblist_n_cbs(&rdp->cblist));
+ __trace_rcu_callback(head, rdp);
trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
--
2.37.3.968.ga6b4b080e4-goog
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