From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Nicholas Piggin <npiggin@gmail.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Paul E. McKenney" <paulmck@kernel.org>,
Will Deacon <will@kernel.org>,
Alan Stern <stern@rowland.harvard.edu>,
John Stultz <jstultz@google.com>,
Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>,
Frederic Weisbecker <frederic@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Uladzislau Rezki <urezki@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Zqiang <qiang.zhang1211@gmail.com>,
Ingo Molnar <mingo@redhat.com>, Waiman Long <longman@redhat.com>,
Mark Rutland <mark.rutland@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Vlastimil Babka <vbabka@suse.cz>,
maged.michael@gmail.com, Mateusz Guzik <mjguzik@gmail.com>,
Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers
Date: Sat, 5 Oct 2024 08:05:05 -0400 [thread overview]
Message-ID: <69c7e71a-2076-4fa7-90f3-534b52d74345@efficios.com> (raw)
In-Reply-To: <CAEXW_YQfyPapBwSuqvFfs+XzSPaSCC3FDKsvb_Up+h-dnqgXeA@mail.gmail.com>
On 2024-10-04 23:25, Joel Fernandes wrote:
> On Fri, Oct 4, 2024 at 2:29 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> This API provides existence guarantees of objects through Hazard
>> Pointers (HP). This minimalist implementation is specific to use
>> with preemption disabled, but can be extended further as needed.
>>
>> Each HP domain defines a fixed number of hazard pointer slots (nr_cpus)
>> across the entire system.
>>
>> Its main benefit over RCU is that it allows fast reclaim of
>> HP-protected pointers without needing to wait for a grace period.
>>
>> It also allows the hazard pointer scan to call a user-defined callback
>> to retire a hazard pointer slot immediately if needed. This callback
>> may, for instance, issue an IPI to the relevant CPU.
>>
>> There are a few possible use-cases for this in the Linux kernel:
>>
>> - Improve performance of mm_count by replacing lazy active mm by HP.
>> - Guarantee object existence on pointer dereference to use refcount:
>> - replace locking used for that purpose in some drivers,
>> - replace RCU + inc_not_zero pattern,
>> - rtmutex: Improve situations where locks need to be taken in
>> reverse dependency chain order by guaranteeing existence of
>> first and second locks in traversal order, allowing them to be
>> locked in the correct order (which is reverse from traversal
>> order) rather than try-lock+retry on nested lock.
>>
>> References:
>>
>> [1]: M. M. Michael, "Hazard pointers: safe memory reclamation for
>> lock-free objects," in IEEE Transactions on Parallel and
>> Distributed Systems, vol. 15, no. 6, pp. 491-504, June 2004
> [ ... ]
>> ---
>> Changes since v0:
>> - Remove slot variable from hp_dereference_allocate().
>> ---
>> include/linux/hp.h | 158 +++++++++++++++++++++++++++++++++++++++++++++
>> kernel/Makefile | 2 +-
>> kernel/hp.c | 46 +++++++++++++
>
> Just a housekeeping comment, ISTR Linus looking down on adding bodies
> of C code to header files (like hp_dereference_allocate). I understand
> maybe the rationale is that the functions included are inlined. But do
> all of them have to be inlined? Such headers also hurt code browsing
> capabilities in code browsers like clangd. clangd doesn't understand
> header files because it can't independently compile them -- it uses
> the compiler to generate and extract the AST for superior code
> browsing/completion.
>
> Also have you looked at the benefits of inlining for hp.h?
> hp_dereference_allocate() seems large enough that inlining may not
> matter much, but I haven't compiled it and looked at the asm myself.
Here is a comparison in userspace:
* With "hp dereference allocate" inlined:
test_hpref_benchmark (smp_mb) nr_reads 1994298193 nr_writes 22293162 nr_ops 2016591355
test_hpref_benchmark (barrier/membarrier) nr_reads 15208690879 nr_writes 1893785 nr_ops 15210584664
* With "hp dereference allocate" implemented as a function call:
test_hpref_benchmark (smp_mb) nr_reads 1558924716 nr_writes 14261028 nr_ops 1573185744
test_hpref_benchmark (barrier/membarrier) nr_reads 5881131707 nr_writes 2005140 nr_ops 5883136847
So the overhead of the function call when using symmetric memory barriers
between hp allocate/hp scan is a 20% slowdown.
It's worse in the asymmetric barrier/membarrier case, introducing a 61%
slowdown.
Given that the overhead is noticeable, I am tempted to leave the hazard
pointer allocate/retire as inline functions.
About code browsers like clangd, I would recommend improving the tooling
rather than alter the design of the code based on current tooling
limitations.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
next prev parent reply other threads:[~2024-10-05 12:07 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-10-04 18:27 [RFC PATCH v2 0/4] sched+mm: Track lazy active mm existence with hazard pointers Mathieu Desnoyers
2024-10-04 18:27 ` [RFC PATCH v2 1/4] compiler.h: Introduce ptr_eq() to preserve address dependency Mathieu Desnoyers
2024-10-04 18:27 ` [RFC PATCH v2 2/4] Documentation: RCU: Refer to ptr_eq() Mathieu Desnoyers
2024-10-04 21:15 ` Joel Fernandes
2024-10-06 19:52 ` David Laight
2024-10-06 20:39 ` Paul E. McKenney
2024-10-07 11:01 ` Sebastian Andrzej Siewior
2024-10-04 18:27 ` [RFC PATCH v2 3/4] hp: Implement Hazard Pointers Mathieu Desnoyers
2024-10-04 21:25 ` Joel Fernandes
2024-10-05 12:05 ` Mathieu Desnoyers [this message]
2024-10-05 11:19 ` Frederic Weisbecker
2024-10-05 11:42 ` Mathieu Desnoyers
2024-10-05 16:04 ` Peter Zijlstra
2024-10-05 16:07 ` Peter Zijlstra
2024-10-05 18:56 ` Mathieu Desnoyers
2024-10-07 10:42 ` Peter Zijlstra
2024-10-07 13:22 ` Mathieu Desnoyers
2024-10-05 18:50 ` Mathieu Desnoyers
2024-10-07 10:40 ` Peter Zijlstra
2024-10-07 14:50 ` Mathieu Desnoyers
2024-10-07 18:18 ` Paul E. McKenney
2024-10-07 19:06 ` Paul E. McKenney
2024-10-07 19:08 ` Mathieu Desnoyers
2024-10-04 18:27 ` [RFC PATCH v2 4/4] sched+mm: Use hazard pointers to track lazy active mm existence Mathieu Desnoyers
2024-10-07 14:50 ` kernel test robot
2024-10-07 15:05 ` Mathieu Desnoyers
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