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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Joel Fernandes <joelaf@google.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	rostedt <rostedt@goodmis.org>, Namhyung Kim <namhyung@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Tom Zanussi <tom.zanussi@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>, fweisbec <fweisbec@gmail.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	kbuild test robot <fengguang.wu@intel.com>,
	baohong liu <baohong.liu@intel.com>,
	vedang patel <vedang.patel@intel.com>,
	kernel-team <kernel-team@lge.com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can
Date: Thu, 26 Apr 2018 11:13:16 -0400 (EDT)	[thread overview]
Message-ID: <2099399401.1995.1524755596613.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CAJWu+oq7dqn4PEySyrrETaGNg4TsJvHobu=UdMEves+V3fgVZQ@mail.gmail.com>

----- On Apr 25, 2018, at 7:13 PM, Joel Fernandes joelaf@google.com wrote:

> Hi Mathieu,
> 
> On Wed, Apr 25, 2018 at 2:40 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Apr 25, 2018, at 5:27 PM, Joel Fernandes joelaf@google.com wrote:
>>
>>> On Tue, Apr 24, 2018 at 9:20 PM, Paul E. McKenney
>>> <paulmck@linux.vnet.ibm.com> wrote:
>>> [..]
>>>>> >
>>>>> > Sounds good, thanks.
>>>>> >
>>>>> > Also I found the reason for my boot issue. It was because the
>>>>> > init_srcu_struct in the prototype was being done in an initcall.
>>>>> > Instead if I do it in start_kernel before the tracepoint is used, it
>>>>> > fixes it (although I don't know if this is dangerous to do like this
>>>>> > but I can get it to boot atleast.. Let me know if this isn't the
>>>>> > right way to do it, or if something else could go wrong)
>>>>> >
>>>>> > diff --git a/init/main.c b/init/main.c
>>>>> > index 34823072ef9e..ecc88319c6da 100644
>>>>> > --- a/init/main.c
>>>>> > +++ b/init/main.c
>>>>> > @@ -631,6 +631,7 @@ asmlinkage __visible void __init start_kernel(void)
>>>>> >         WARN(!irqs_disabled(), "Interrupts were enabled early\n");
>>>>> >         early_boot_irqs_disabled = false;
>>>>> >
>>>>> > +       init_srcu_struct(&tracepoint_srcu);
>>>>> >         lockdep_init_early();
>>>>> >
>>>>> >         local_irq_enable();
>>>>> > --
>>>>> >
>>>>> > I benchmarked it and the performance also looks quite good compared
>>>>> > to the rcu tracepoint version.
>>>>> >
>>>>> > If you, Paul and other think doing the init_srcu_struct like this
>>>>> > should be Ok, then I can try to work more on your srcu prototype and
>>>>> > roll into my series and post them in the next RFC series (or let me
>>>>> > know if you wanted to work your srcu stuff in a separate series..).
>>>>>
>>>>> That is definitely not what I was expecting, but let's see if it works
>>>>> anyway...  ;-)
>>>>>
>>>>> But first, I was instead expecting something like this:
>>>>>
>>>>> DEFINE_SRCU(tracepoint_srcu);
>>>>>
>>>>> With this approach, some of the initialization happens at compile time
>>>>> and the rest happens at the first call_srcu().
>>>>>
>>>>> This will work -only- if the first call_srcu() doesn't happen until after
>>>>> workqueue_init_early() has been invoked.  Which I believe must have been
>>>>> the case in your testing, because otherwise it looks like __call_srcu()
>>>>> would have complained bitterly.
>>>>>
>>>>> On the other hand, if you need to invoke call_srcu() before the call
>>>>> to workqueue_init_early(), then you need the patch that I am beating
>>>>> into shape.  Plus you would need to use DEFINE_SRCU() and to avoid
>>>>> invoking init_srcu_struct().
>>>>
>>>> And here is the patch.  I do not intend to send it upstream unless it
>>>> actually proves necessary, and it appears that current SRCU does what
>>>> you need.
>>>>
>>>> You would only need this patch if you wanted to invoke call_srcu()
>>>> before workqueue_init_early() was called, which does not seem likely.
>>>
>>> Cool. So I was chatting with Paul and just to update everyone as well,
>>> I tried the DEFINE_SRCU instead of the late init_srcu_struct call and
>>> can make it past boot too (thanks Paul!). Also I don't see a reason we
>>> need the RCU callback to execute early and its fine if it runs later.
>>>
>>> Also, I was thinking of introducing a separate trace_*event*_srcu API
>>> as a replacement to the _rcuidle API. Then I can make use of it for my
>>> tracepoints, and then later can use it for the other tracepoints
>>> needing _rcuidle. After that we can finally get rid of the _rcuidle
>>> API if there are no other users of it. This is just a rough plan, but
>>> let me know if there's any issue with this plan that you can think
>>> off.
>>> IMO, I believe its simpler if the caller worries about whether it can
>>> tolerate if tracepoint probes can block or not, than making it a
>>> property of the tracepoint. That would also simplify the patch to
>>> introduce srcu and keep the tracepoint creation API simple and less
>>> confusing, but let me know if I'm missing something about this.
>>
>> One problem with your approach is that you can have multiple callers
>> for the same tracepoint name, where some could be non-preemptible and
>> others blocking. Also, there is then no clear way for the callback
> 
> Shouldn't it be responsibility of the caller to make sure it calls
> correct API? So if you're wanting to allow probes to block, then you'd
> call trace*blocking, if not then you don't. So the caller side can
> just always do the right thing. That's a caller side issue.

The issue there is that tracepoint.c has APIs both for instrumentation
and for registration of probe providers (callbacks). I want tracepoint.c
to provide guarantees that it won't connect incompatible probes and
callsites together.

> 
>>
>> Regarding the name, I'm OK with having something along the lines of
>> trace_*event*_blocking or such. Please don't use "srcu" or other naming
>> that is explicitly tied to the underlying mechanism used internally
>> however: what we want to convey is that this specific tracepoint probe
> 
> Problem is that _blocking isn't the right word either. In my IRQ trace
> point case, it will look something like this then:
> 
> local_irq_disable();
> // IRQs are now off.
> trace_irq_disable_blocking(..);
> 
> This wouldn't make sense. What we really want is to use the SRCU
> implementation so that its low overhead...
> 
> So it would be something like:
> 
> local_irq_disable();
> // IRQs are now off.
> trace_irq_disable_srcu(..);
> 
> I also Ok if, as Paul was saying in his last email, that just for
> _rcuidle, we use SRCU so that we don't have to do the rcu_enter_irq
> stuff. Or we kill the _rcuidle API completely and use _srcu for those
> users instead. We already have 1 implementation specific name anyway
> (rcuidle), we're just replacing it with another one. If in the future,
> if we want to change that name we can always do so (Also if you will,
> correcting the existing already bad naming is a different problem and
> we're not making it any worse tbh).

Using SRCU rather than the sched-rcu tracepoint synchronization in your
use-case it caused by a limitation of sched-rcu: it cannot be efficiently
used within idle code. So you don't care about the "can_sleep" property
of SRCU. You could event mix SRCU and sched-rcu callsites for the same
probe name, and it would be perfectly valid.

So even though both "can_sleep" and "rcuidle" caller variants would end
up using SRCU under the hood, each can have its own caller API, e.g.:

* trace_<event>() -> only non-sleeping probes can register to those.
                     Uses sched-rcu under the hood.

* trace_<event>_can_sleep() -> both sleeping and non-sleeping probes can
                               register to those. Uses SRCU under the hood.

* trace_<event>_rcuidle() -> only non-sleeping probes can register to those,
                             uses SRCU under the hood.

> 
>> can be preempted and block. The underlying implementation could move to
>> a different RCU flavor brand in the future, and it should not impact
>> users of the tracepoint APIs.
>>
>> In order to ensure that probes that may block only register themselves
>> to tracepoints that allow blocking, we should introduce new tracepoint
>> declaration/definition *and* registration APIs also contain the
>> "BLOCKING/blocking" keywords (or such), so we can ensure that a
>> tracepoint probe being registered to a "blocking" tracepoint is indeed
>> allowed to block.
> 
> I feel this problem you're describing is slightly out of the scope of
> the issues we're talking about, I think. Even right now, someone can
> write a callback that blocks and then bad things will happen. If I
> understand correctly, all callbacks right now will execute in a
> preempt disabled section because of rcu_read_lock_sched. So we already
> have a problem (without the SRCU changes) that if a callback blocks,
> then we'll have hard to diagnose sleeping while atomic issues. Sorry
> if I missed your point.

The current situation is that no callback whatsoever can sleep. If we
introduce an API allowing some callbacks to sleep, I want to make sure
we don't end up registering sleepable callbacks to non-preemptible callsites.
Considering that the callback can be provided by a kernel module whereas the
callsite is within the kernel, having this kind of correctness validation
within tracepoint.c appears important.

Thanks!

Mathieu


> 
> thanks,
> 
> - Joel

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

  reply	other threads:[~2018-04-26 15:13 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-17  4:07 [RFC v4 0/4] Centralize and unify usage of preempt/irq tracepoints Joel Fernandes
2018-04-17  4:07 ` [RFC v4 1/4] tracepoint: Add API to not do lockdep checks during RCU ops Joel Fernandes
2018-04-17  4:07 ` [RFC v4 2/4] softirq: reorder trace_softirqs_on to prevent lockdep splat Joel Fernandes
2018-04-17  4:07 ` [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can Joel Fernandes
2018-04-18  9:02   ` Masami Hiramatsu
2018-04-19  5:43     ` Namhyung Kim
2018-04-20  7:07       ` Joel Fernandes
2018-04-23  1:14         ` Joel Fernandes
2018-04-23  3:19           ` Paul E. McKenney
2018-04-23 14:31             ` Mathieu Desnoyers
2018-04-23 14:53               ` Steven Rostedt
2018-04-23 14:59                 ` Mathieu Desnoyers
2018-04-23 15:12                   ` Paul E. McKenney
2018-04-23 16:18                   ` Steven Rostedt
2018-04-23 17:12                     ` Mathieu Desnoyers
2018-04-23 17:24                       ` Joel Fernandes
2018-04-23 21:22                       ` Steven Rostedt
2018-04-24 15:56                         ` Paul E. McKenney
2018-04-24 16:01                           ` Joel Fernandes
2018-04-24 17:26                             ` Paul E. McKenney
2018-04-24 18:23                               ` Paul E. McKenney
2018-04-24 18:26                                 ` Paul E. McKenney
2018-04-24 18:59                                   ` Joel Fernandes
2018-04-24 19:01                                     ` Joel Fernandes
2018-04-24 19:09                                     ` Paul E. McKenney
2018-04-24 19:16                                       ` Joel Fernandes
2018-04-24 23:21                                     ` Mathieu Desnoyers
2018-04-24 23:46                                       ` Joel Fernandes
2018-04-25  0:10                                         ` Paul E. McKenney
2018-04-25  4:20                                           ` Paul E. McKenney
2018-04-25 21:27                                             ` Joel Fernandes
2018-04-25 21:35                                               ` Paul E. McKenney
2018-04-25 21:40                                               ` Mathieu Desnoyers
2018-04-25 22:51                                                 ` Steven Rostedt
2018-04-26 15:03                                                   ` Mathieu Desnoyers
2018-04-26 16:08                                                     ` Mathieu Desnoyers
2018-04-25 23:13                                                 ` Joel Fernandes
2018-04-26 15:13                                                   ` Mathieu Desnoyers [this message]
2018-04-26 15:20                                                     ` Joel Fernandes
2018-04-26 15:49                                                     ` Paul E. McKenney
2018-04-23 15:49                 ` Joel Fernandes
2018-04-26  2:18             ` Joel Fernandes
2018-05-01  1:18     ` Joel Fernandes
2018-04-17  4:07 ` [RFC v4 4/4] tracing: Centralize preemptirq tracepoints and unify their usage Joel Fernandes

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