Message-ID: <f4cc60bf-b011-4bdf-96c4-50bc62955a98@efficios.com>
Date: Mon, 7 Oct 2024 15:08:44 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: paulmck@...nel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Boqun Feng <boqun.feng@...il.com>,
 linux-kernel@...r.kernel.org, Linus Torvalds
 <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Nicholas Piggin <npiggin@...il.com>, Michael Ellerman <mpe@...erman.id.au>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 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>,
 Joel Fernandes <joel@...lfernandes.org>,
 Josh Triplett <josh@...htriplett.org>, Uladzislau Rezki <urezki@...il.com>,
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 <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
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 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 2024-10-07 21:06, Paul E. McKenney wrote:
> On Mon, Oct 07, 2024 at 11:18:14AM -0700, Paul E. McKenney wrote:
>> On Mon, Oct 07, 2024 at 10:50:46AM -0400, Mathieu Desnoyers wrote:
>>> On 2024-10-07 12:40, Peter Zijlstra wrote:
>>>> On Sat, Oct 05, 2024 at 02:50:17PM -0400, Mathieu Desnoyers wrote:
>>>>> On 2024-10-05 18:04, Peter Zijlstra wrote:
>>>>
>>>>
>>>>>>> +/*
>>>>>>> + * hp_allocate: Allocate a hazard pointer.
>>>>>>> + *
>>>>>>> + * Allocate a hazard pointer slot for @addr. The object existence should
>>>>>>> + * be guaranteed by the caller. Expects to be called from preempt
>>>>>>> + * disable context.
>>>>>>> + *
>>>>>>> + * Returns a hazard pointer context.
>>>>>>
>>>>>> So you made the WTF'o'meter crack, this here function does not allocate
>>>>>> nothing. Naming is bad. At best this is something like
>>>>>> try-set-hazard-pointer or somesuch.
>>>>>
>>>>> I went with the naming from the 2004 paper from Maged Michael, but I
>>>>> agree it could be clearer.
>>>>>
>>>>> I'm tempted to go for "hp_try_post()" and "hp_remove()", basically
>>>>> "posting" the intent to use a pointer (as in on a metaphorical billboard),
>>>>> and removing it when it's done.
>>>>
>>>> For RCU we've taken to using the word: 'publish', no?
>>>
>>> I'm so glad you suggest this, because it turns out that from all
>>> the possible words you could choose from, 'publish' is probably the
>>> most actively confusing. I'll explain.
>>>
>>> Let me first do a 10'000 feet comparison of RCU vs Hazard Pointers
>>> through a simple example:
>>>
>>> [ Note: I've renamed the HP dereference try_post to HP load try_post
>>>    based on further discussion below. ]
>>>
>>> *** RCU ***
>>>
>>> * Dereference RCU-protected pointer:
>>>      rcu_read_lock();          // [ Begin read transaction ]
>>>      l_p = rcu_dereference(p); // [ Load p: @addr or NULL ]
>>>      if (l_p)
>>>        [ use *l_p ...]
>>>      rcu_read_unlock();        // [ End read transaction ]
>>>
>>> * Publish @addr:    addr = kmalloc();
>>>                      init(addr);
>>>                      rcu_assign_pointer(p, addr);
>>>
>>> * Reclaim @addr:    rcu_assign_pointer(p, NULL); // [ Unpublish @addr ]
>>>                      synchronize_rcu();           // Wait for all pre-existing
>>>                                                   // read transactions to complete.
>>>                      kfree(addr);
>>>
>>>
>>> *** Hazard Pointers ***
>>>
>>> * Load and post a HP-protected pointer:
>>>      l_p = hp_load_try_post(domain, &p, &slot);
>>>      if (l_p) {
>>>        [ use *l_p ...]
>>>        hp_remove(&slot, l_p);
>>>      }
>>>
>>> * Publish @addr:    addr = kmalloc();
>>>                      init(addr);
>>>                      rcu_assign_pointer(p, addr);
>>>
>>> * Reclaim @addr:    rcu_assign_pointer(p, NULL); // [ Unpublish @addr ]
>>>                      hp_scan(domain, addr, NULL);
>>>                      kfree(addr);
>>>
>>> Both HP and RCU have publication guarantees, which can in fact be
>>> implemented in the same way (e.g. rcu_assign_pointer paired with
>>> something that respects address dependencies ordering). A stronger
>>> implementation of this would be pairing a store-release with a
>>> load-acquire: it works, but it would add needless overhead on
>>> weakly-ordered CPUs.
>>>
>>> How the two mechanisms differ is in how they track when it is
>>> safe to reclaim @addr. RCU tracks reader "transactions" begin/end,
>>> and makes sure that all pre-existing transactions are gone before
>>> synchronize_rcu() is allowed to complete. HP does this by tracking
>>> "posted" pointer slots with a HP domain. As long as hp_scan observes
>>> that HP readers are showing interest in @addr, it will wait.
>>>
>>> One notable difference between RCU and HP is that HP knows exactly
>>> which pointer is blocking progress, and from which CPU (at least
>>> with my per-CPU HP domain implementation). Therefore, it is possible
>>> for HP to issue an IPI and make sure the HP user either completes its
>>> use of the pointer quickly, or stops using it right away (e.g. making
>>> the active mm use idle mm instead).
>>>
>>> One strength of RCU is that it can track use of a whole set of RCU
>>> pointers just by tracking reader transaction begin/end, but this is
>>> also one of its weaknesses: a long reader transaction can postpone
>>> completion of grace period for a long time and increase the memory
>>> footprint. In comparison, HP can immediately complete as soon as the
>>> pointer it is scanning for is gone. Even better, it can send an IPI
>>> to the belate CPU and abort use of the pointer using a callback.
>>
>> Plus, in contrast to hazard pointers, rcu_dereference() cannot say "no".
>>
>> This all sounds like arguments *for* use of the term "publish" for
>> hazard pointers rather than against it.  What am I missing here?
> 
> OK, one thing that I was missing is that this was not about the
> counterpart to rcu_assign_pointer(), for which I believe "publish" makes
> a lot of sense, but rather about the setting of a hazard pointer.  Here,
> "protect" is the traditional term of power, which has served users well
> for some years.

After some reading of the C++ specification wording used for hazard
pointers, I am indeed tempted to go for "try_protect()" and
"retire()" to minimize confusion.

Thanks,

Mathieu


> 
> 							Thanx, Paul
> 
>>>>>>> +/*
>>>>>>> + * hp_dereference_allocate: Dereference and allocate a hazard pointer.
>>>>>>> + *
>>>>>>> + * Returns a hazard pointer context. Expects to be called from preempt
>>>>>>> + * disable context.
>>>>>>> + */
>>>>>>
>>>>>> More terrible naming. Same as above, but additionally, I would expect a
>>>>>> 'dereference' to actually dereference the pointer and have a return
>>>>>> value of the dereferenced type.
>>>>>
>>>>> hp_dereference_try_post() ?
>>>>>
>>>>>>
>>>>>> This function seems to double check and update the hp_ctx thing. I'm not
>>>>>> at all sure yet wtf this is doing -- and the total lack of comments
>>>>>> aren't helping.
>>>>>
>>>>> The hp_ctx contains the outputs.
>>>>>
>>>>> The function loads *addr_p to then try_post it into a HP slot. On success,
>>>>> it re-reads the *addr_p (with address dependency) and if it still matches,
>>>>> use that as output address pointer.
>>>>>
>>>>> I'm planning to remove hp_ctx, and just have:
>>>>>
>>>>> /*
>>>>>    * hp_try_post: Try to post a hazard pointer.
>>>>>    *
>>>>>    * Post a hazard pointer slot for @addr. The object existence should
>>>>>    * be guaranteed by the caller. Expects to be called from preempt
>>>>>    * disable context.
>>>>>    *
>>>>>    * Returns true if post succeeds, false otherwise.
>>>>>    */
>>>>> static inline
>>>>> bool hp_try_post(struct hp_domain *hp_domain, void *addr, struct hp_slot **_slot)
>>>>> [...]
>>>>>
>>>>> /*
>>>>>    * hp_dereference_try_post: Dereference and try to post a hazard pointer.
>>>>>    *
>>>>>    * Returns a hazard pointer context. Expects to be called from preempt
>>>>>    * disable context.
>>>>>    */
>>>>> static inline
>>>>> void *__hp_dereference_try_post(struct hp_domain *hp_domain,
>>>>>                                   void * const * addr_p, struct hp_slot **_slot)
>>>>> [...]
>>>>>
>>>>> #define hp_dereference_try_post(domain, p, slot_p)              \
>>>>>           ((__typeof__(*(p))) __hp_dereference_try_post(domain, (void * const *) p, slot_p))
>>>>
>>>> This will compile, but do the wrong thing when p is a regular pointer, no?
>>>
>>> Right, at least in some cases the compiler may not complain, and people used to
>>> rcu_dereference() will expect that "p" is the pointer to load rather than the
>>> address of that pointer. This would be unexpected.
>>>
>>> I must admit that passing the address holding the pointer to load rather than
>>> the pointer to load itself makes it much less troublesome in terms of macro
>>> layers. But perhaps this is another example where we should wander away from the
>>> beaten path and use a word different from "dereference" here. E.g.:
>>>
>>> /*
>>>   * Use a comma expression within typeof: __typeof__((void)**(addr_p), *(addr_p))
>>>   * to generate a compile error if addr_p is not a pointer to a pointer.
>>>   */
>>> #define hp_load_try_post(domain, addr_p, slot_p)                \
>>>          ((__typeof__((void)**(addr_p), *(addr_p))) __hp_load_try_post(domain, (void * const *) (addr_p), slot_p))
>>>
>>>>
>>>>>
>>>>> /* Clear the hazard pointer in @slot. */
>>>>> static inline
>>>>> void hp_remove(struct hp_slot *slot)
>>>>> [...]
>>>>
>>>> Differently weird, but better I suppose :-)
>>>
>>> If you find a better word than "remove" to pair with "post", I'm all in :)
>>>
>>>>
>>>>
>>>>>>> +void hp_scan(struct hp_slot __percpu *percpu_slots, void *addr,
>>>>>>> +	     void (*retire_cb)(int cpu, struct hp_slot *slot, void *addr))
>>>>>>> +{
>>>>>>> +	int cpu;
>>>>>>> +
>>>>>>> +	/*
>>>>>>> +	 * Store A precedes hp_scan(): it unpublishes addr (sets it to
>>>>>>> +	 * NULL or to a different value), and thus hides it from hazard
>>>>>>> +	 * pointer readers.
>>>>>>> +	 */
>>>>>>> +
>>>>>>> +	if (!addr)
>>>>>>> +		return;
>>>>>>> +	/* Memory ordering: Store A before Load B. */
>>>>>>> +	smp_mb();
>>>>>>> +	/* Scan all CPUs slots. */
>>>>>>> +	for_each_possible_cpu(cpu) {
>>>>>>> +		struct hp_slot *slot = per_cpu_ptr(percpu_slots, cpu);
>>>>>>> +
>>>>>>> +		if (retire_cb && smp_load_acquire(&slot->addr) == addr)	/* Load B */
>>>>>>> +			retire_cb(cpu, slot, addr);
>>>>>>
>>>>>> Is retirce_cb allowed to cmpxchg the thing?
>>>>>
>>>>> It could, but we'd need to make sure the slot is not re-used by another
>>>>> hp_try_post() before the current user removes its own post. It would
>>>>> need to synchronize with the current HP user (e.g. with IPI).
>>>>>
>>>>> I've actually renamed retire_cb to "on_match_cb".
>>>>
>>>> Hmm, I think I see. Would it make sense to pass the expected addr to
>>>> hp_remove() and double check we don't NULL out something unexpected? --
>>>> maybe just for a DEBUG option.
>>>>
>>>> I'm always seeing the NOHZ_FULL guys hating on this :-)
>>>
>>> That's a fair point. Sure, we can do this as an extra safety net. For now I
>>> will just make the check always present, we can always move it to a debug
>>> option later.
>>>
>>> And now I notice that hp_remove is also used for CPU hotplug (grep
>>> matches for cpuhp_remove_state()). I wonder if we should go for something
>>> more grep-friendly than "hp_", e.g. "hazptr_" and rename hp.h to hazptr.h ?
>>>
>>> Thanks,
>>>
>>> Mathieu
>>>
>>>
>>> -- 
>>> Mathieu Desnoyers
>>> EfficiOS Inc.
>>> https://www.efficios.com
>>>

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

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