linux-kernel - Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

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Message-ID: <69c7e71a-2076-4fa7-90f3-534b52d74345@efficios.com>
Date: Sat, 5 Oct 2024 08:05:05 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Nicholas Piggin <npiggin@...il.com>,
 Michael Ellerman <mpe@...erman.id.au>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Paul E. McKenney" <paulmck@...nel.org>, Will Deacon <will@...nel.org>,
 Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>,
 Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Frederic Weisbecker <frederic@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan
 <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
 Mateusz Guzik <mjguzik@...il.com>,
 Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, rcu@...r.kernel.org,
 linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

On 2024-10-04 23:25, Joel Fernandes wrote:
> On Fri, Oct 4, 2024 at 2:29 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.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