Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

From: Gao Xiang <hsiangkao@linux.alibaba.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: paulmck@kernel.org, Sandeep Dhavale <dhavale@google.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	AngeloGioacchino Del Regno
	<angelogioacchino.delregno@collabora.com>,
	linux-erofs@lists.ozlabs.org, xiang@kernel.org,
	Will Shiu <Will.Shiu@mediatek.com>,
	kernel-team@android.com, rcu@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org
Subject: Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC
Date: Fri, 14 Jul 2023 21:51:16 +0800	[thread overview]
Message-ID: <fccca41d-8a72-27cf-e589-409f54cd5811@linux.alibaba.com> (raw)
In-Reply-To: <CAEXW_YQCpUsPz24H4Mux6wOH1=RFRR-gsXLFTbJ37MgUJo3kCw@mail.gmail.com>

On 2023/7/14 21:42, Joel Fernandes wrote:
> On Thu, Jul 13, 2023 at 11:17 PM Gao Xiang <hsiangkao@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2023/7/14 10:16, Paul E. McKenney wrote:
>>> On Thu, Jul 13, 2023 at 09:33:35AM -0700, Paul E. McKenney wrote:
>>>> On Thu, Jul 13, 2023 at 11:33:24AM -0400, Joel Fernandes wrote:
>>
>> ...
>>
>>>>>
>>>>> >From what Sandeep described, the code path is in an RCU reader. My
>>>>> question is more, why doesn't it use SRCU instead since it clearly
>>>>> does so if BLK_MQ_F_BLOCKING. What are the tradeoffs? IMHO, a deeper
>>>>> dive needs to be made into that before concluding that the fix is to
>>>>> use rcu_read_lock_any_held().
>>>>
>>>> How can this be solved?
>>>>
>>>> 1.   Always use a workqueue.  Simple, but is said to have performance
>>>>       issues.
>>>>
>>>> 2.   Pass a flag in that indicates whether or not the caller is in an
>>>>       RCU read-side critical section.  Conceptually simple, but might
>>>>       or might not be reasonable to actually implement in the code as
>>>>       it exists now.  (You tell me!)
>>>>
>>>> 3.   Create a function in z_erofs that gives you a decent
>>>>       approximation, maybe something like the following.
>>>>
>>>> 4.   Other ideas here.
>>>
>>> 5.    #3 plus make the corresponding Kconfig option select
>>>        PREEMPT_COUNT, assuming that any users needing compression in
>>>        non-preemptible kernels are OK with PREEMPT_COUNT being set.
>>>        (Some users of non-preemptible kernels object strenuously
>>>        to the added overhead from CONFIG_PREEMPT_COUNT=y.)
>>
>> I'm not sure if it's a good idea
> 
> I think it is a fine idea.
> 
>> we need to work on
>> CONFIG_PREEMPT_COUNT=n (why not?), we could just always trigger a
>> workqueue for this.
>>
> 
> So CONFIG_PREEMPT_COUNT=n users don't deserve good performance? ;-)

I'm not sure if non-preemptible kernel users really care about
such sensitive latencies, I don't know, my 2 cents.

Thanks,
Gao Xiang

> 
> thanks,
> 
>   - Joel

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