Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)

From: Waiman Long <longman@redhat.com>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	rostedt <rostedt@goodmis.org>,
	Byungchul Park <byungchul.park@lge.com>,
	Radoslaw Burny <rburny@google.com>, Tejun Heo <tj@kernel.org>,
	rcu <rcu@vger.kernel.org>, cgroups <cgroups@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	paulmck <paulmck@kernel.org>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)
Date: Wed, 9 Feb 2022 21:12:20 -0500	[thread overview]
Message-ID: <8c85f624-690e-1b96-2e7e-2c1695373437@redhat.com> (raw)
In-Reply-To: <CAM9d7chH0Pvxx_FURL0sZvawwenRmjPyfac_9oinOaRwv8isng@mail.gmail.com>

On 2/9/22 19:27, Namhyung Kim wrote:
> On Wed, Feb 9, 2022 at 12:17 PM Waiman Long <longman@redhat.com> wrote:
>>
>> On 2/9/22 14:45, Namhyung Kim wrote:
>>> On Wed, Feb 9, 2022 at 11:28 AM Mathieu Desnoyers
>>> <mathieu.desnoyers@efficios.com> wrote:
>>>> ----- On Feb 9, 2022, at 2:22 PM, Namhyung Kim namhyung@kernel.org wrote:
>>>>> I'm also concerning dynamic allocated locks in a data structure.
>>>>> If we keep the info in a hash table, we should delete it when the
>>>>> lock is gone.  I'm not sure we have a good place to hook it up all.
>>>> I was wondering about this use case as well. Can we make it mandatory to
>>>> declare the lock "class" (including the name) statically, even though the
>>>> lock per-se is allocated dynamically ? Then the initialization of the lock
>>>> embedded within the data structure would simply refer to the lock class
>>>> definition.
>>> Isn't it still the same if we have static lock classes that the entry needs
>>> to be deleted from the hash table when it frees the data structure?
>>> I'm more concerned about free than alloc as there seems to be no
>>> API to track that in a place.
>> We may have to invent some new APIs to do that. For example,
>> spin_lock_exit() can be the counterpart of spin_lock_init() and so on.
>> Of course, existing kernel code have to be modified to designate the
>> point after which a lock is no longer being used or is freed.
> Yeah, but I'm afraid that it could be easy to miss something.
> Also it would add some runtime overhead due to maintaining
> the hash table even if the tracepoints are not used.

Yes, there is some overhead at lock init and exit time, but it is 
generally negligible if the lock is used multiple times. The hash table 
will consume some additional memory if configured, but it shouldn't be much.

Cheers,
Longman