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Date:   Wed, 27 Jul 2022 14:19:22 -0300
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Subject: Re: [RFC] futex2: add NUMA awareness
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To:     Andrey Semashev <andrey.semashev@gmail.com>
Cc:     linux-api@vger.kernel.org, fweimer@redhat.com,
        linux-kernel@vger.kernel.org, Darren Hart <dvhart@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        libc-alpha@sourceware.org, Davidlohr Bueso <dave@stgolabs.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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From:   =?UTF-8?Q?Andr=c3=a9_Almeida?= <andrealmeid@igalia.com>
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Às 13:42 de 22/07/22, Andrey Semashev escreveu:
> On 7/14/22 18:00, André Almeida wrote:
>> Hi Andrey,
>>
>> Thanks for the feedback.
>>
>> Às 08:01 de 14/07/22, Andrey Semashev escreveu:
>>> On 7/14/22 06:18, André Almeida wrote:
>> [...]
>>>>
>>>> Feedback? Who else should I CC?
>>>
>>> Just a few questions:
>>>
>>> Do I understand correctly that notifiers won't be able to wake up
>>> waiters unless they know on which node they are waiting?
>>>
>>
>> If userspace is using NUMA_FLAG, yes. Otherwise all futexes would be
>> located in the default node, and userspace doesn't need to know which
>> one is the default.
>>
>>> Is it possible to wait on a futex on different nodes?
>>
>> Yes, given that you specify `.hint = id` with the proper node id.
> 
> So any given futex_wake(FUTEX_NUMA) operates only within its node, right?
> 
>>> Is it possible to wake waiters on a futex on all nodes? When a single
>>> (or N, where N is not "all") waiter is woken, which node is selected? Is
>>> there a rotation of nodes, so that nodes are not skewed in terms of
>>> notified waiters?
>>
>> Regardless of which node the waiter process is running, what matter is
>> in which node the futex hash table is. So for instance if we have:
>>
>> 	struct futex32_numa f = {.value = 0, hint = 2};
>>
>> And now we add some waiters for this futex:
>>
>> Thread 1, running on node 3:
>>
>> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
>>
>> Thread 2, running on node 0:
>>
>> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
>>
>> Thread 3, running on node 2:
>>
>> 	futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
>>
>> And then, Thread 4, running on node 3:
>>
>> 	futex_wake(&f, 2, FUTEX_NUMA | FUTEX_32);
>>
>> Now, two waiter would wake up (e.g. T1 and T3, node 3 and 2) and they
>> are from different nodes. futex_wake() doesn't provide guarantees of
>> which waiter will be selected, so I can't say which node would be
>> selected.
> 
> In this example, T1, T2 and T3 are all blocking on node 2 (since all of
> them presumably specify hint == 2), right? In this sense, it doesn't
> matter which node they are running on, what matters is what node they
> block on.

yes

> 
> What I'm asking is can I wake all threads blocked on all nodes on the
> same futex? That is, is the following possible?
> 
>   // I'm using hint == -1 to indicate the current node
>   // of the calling thread for waiters and all nodes for notifiers
>   struct futex32_numa f = {.value = 0, .hint = -1};
> 
>   Thread 1, running on node 3, blocks on node 3:
> 
>   futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
>   Thread 2, running on node 0, blocks on node 0:
> 
>   futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
>   Thread 3, running on node 2, blocks on node 2:
> 
>   futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL);
> 
>   And then, Thread 4, running on whatever node:
> 
>   futex_wake(&f, -1, FUTEX_NUMA | FUTEX_32);

this futex_wake will wake all futexes waiting on the node that called
futex_wake(), waking only one futex in this example. They are __not__
the same futex. If they have different nodes, they would have different
information inside the kernel.

if you want to wake them all with the same futex_wake(), they need to be
waiting on the same node.

> 
> Here, futex_wake would wake T1, T2 and T3. Or:
> 
>   futex_wake(&f, 1, FUTEX_NUMA | FUTEX_32);

this would behave exactly as the futex_wake() above.

> 
> Here, futex_wake would wake any one of T1, T2 or T3.
> 
>> There's no policy for fairness/starvation for futex_wake(). Do
>> you think this would be important for the NUMA case?
> 
> I'm not sure yet. If there isn't a cross-node behavior like in my
> example above then, I suppose, it falls to the userspace to ensure fair
> rotation of the wakeups on different nodes. If there is functionality
> like this, I imagine, some sort of fairness would be desired.