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From: "André Almeida" <andrealmeid@collabora.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
	acme@kernel.org, Andrey Semashev <andrey.semashev@gmail.com>,
	corbet@lwn.net, Davidlohr Bueso <dave@stgolabs.net>,
	Darren Hart <dvhart@infradead.org>,
	fweimer@redhat.com, joel@joelfernandes.org, kernel@collabora.com,
	krisman@collabora.com, libc-alpha@sourceware.org,
	linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, malteskarupke@fastmail.fm,
	Ingo Molnar <mingo@redhat.com>,
	pgriffais@valvesoftware.com, Peter Oskolkov <posk@posk.io>,
	Steven Rostedt <rostedt@goodmis.org>,
	shuah@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	z.figura12@gmail.com,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v4 00/15] Add futex2 syscalls
Date: Tue, 8 Jun 2021 12:04:18 -0300	[thread overview]
Message-ID: <7ab1a38e-5ba6-843d-9fa8-7480914c3d15@collabora.com> (raw)
In-Reply-To: <YL99cR0H+7xgU8L1@hirez.programming.kicks-ass.net>

Às 11:23 de 08/06/21, Peter Zijlstra escreveu:
> On Tue, Jun 08, 2021 at 02:26:22PM +0200, Sebastian Andrzej Siewior wrote:
>> On 2021-06-07 12:40:54 [-0300], André Almeida wrote:
>>>
>>> When I first read Thomas proposal for per table process, I thought that
>>> the main goal there was to solve NUMA locality issues, not RT latency,
>>> but I think you are right. However, re-reading the thread at [0], it
>>> seems that the RT problems where not completely solved in that
>>> interface, maybe the people involved with that patchset can help to shed
>>> some light on it.
>>>
>>> Otherwise, this same proposal could be integrated in futex2, given that
>>> we would only need to provide to userland some extra flags and add some
>>> `if`s around the hash table code (in a very similar way the NUMA code
>>> will be implemented in futex2).
>>
>> There are slides at [0] describing some attempts and the kernel tree [1]
>> from that time.
>>
>> The process-table solves the problem to some degree that two random
>> process don't collide on the same hash bucket. But as Peter Zijlstra
>> pointed out back then two threads from the same task could collide on
>> the same hash bucket (and with ASLR not always). So the collision is
>> there but limited and this was not perfect.
>>
>> All the attempts with API extensions didn't go well because glibc did
>> not want to change a bit. This starts with a mutex that has a static
>> initializer which has to work (I don't remember why the first
>> pthread_mutex_lock() could not fail with -ENOMEM but there was
>> something) and ends with glibc's struct mutex which is full and has no
>> room for additional data storage.
>>
>> The additional data in user's struct mutex + init would have the benefit
>> that instead uaddr (which is hashed for the in-kernel lookup) a cookie
>> could be used for the hash-less lookup (and NUMA pointer where memory
>> should be stored).
>>
>> So. We couldn't change a thing back then so nothing did happen. We
>> didn't want to create a new interface and a library implementing it plus
>> all the functionality around it (like pthread_cond, phtread_barrier, …).
>> Not to mention that if glibc continues to use the "old" locking
>> internally then the application is still affected by the hash-collision
>> locking (or the NUMA problem) should it block on the lock.
> 
> There's more futex users than glibc, and some of them are really hurting
> because of the NUMA issue. Oracle used to (I've no idea what they do or
> do not do these days) use sysvsem because the futex hash table was a
> massive bottleneck for them.
> 
> And as Nick said, other vendors are having the same problems.

Since we're talking about NUMA, which userspace communities would be
able to provide feedback about the futex2() NUMA-aware feature, to check
if this interface would help solving those issues?

> 
> And if you don't extend the futex to store the nid you put the waiter in
> (see all the problems above) you will have to do wakeups on all nodes,
> which is both slower than it is today, and scales possibly even worse.
> 
> The whole numa-aware qspinlock saga is in part because of futex.
> 
> 
> That said; if we're going to do the whole futex-vector thing, we really
> do need a new interface, because the futex multiplex monster is about to
> crumble (see the fun wrt timeouts for example).
> 
> And if we're going to do a new interface, we ought to make one that can
> solve all these problems. Now, ideally glibc will bring forth some
> opinions, but if they don't want to play, we'll go back to the good old
> days of non-standard locking libraries.. we're halfway there already due
> to glibc not wanting to break with POSIX were we know POSIX was just
> dead wrong broken.
> 
> See: https://github.com/dvhart/librtpi
> 
> 

  parent reply	other threads:[~2021-06-08 15:04 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-03 19:59 André Almeida
2021-06-03 19:59 ` [PATCH v4 01/15] futex2: Implement wait and wake functions André Almeida
2021-06-03 19:59 ` [PATCH v4 02/15] futex2: Add support for shared futexes André Almeida
2021-06-03 19:59 ` [PATCH v4 03/15] futex2: Implement vectorized wait André Almeida
2021-06-03 19:59 ` [PATCH v4 04/15] futex2: Implement requeue operation André Almeida
2021-06-03 19:59 ` [PATCH v4 05/15] futex2: Implement support for different futex sizes André Almeida
2021-06-04  0:23   ` kernel test robot
2021-06-06 19:12   ` Davidlohr Bueso
2021-06-06 23:01     ` Andrey Semashev
2021-06-03 19:59 ` [PATCH v4 06/15] futex2: Add compatibility entry point for x86_x32 ABI André Almeida
2021-06-03 19:59 ` [PATCH v4 07/15] docs: locking: futex2: Add documentation André Almeida
2021-06-06 19:23   ` Davidlohr Bueso
2021-06-03 19:59 ` [PATCH v4 08/15] selftests: futex2: Add wake/wait test André Almeida
2021-06-03 19:59 ` [PATCH v4 09/15] selftests: futex2: Add timeout test André Almeida
2021-06-03 19:59 ` [PATCH v4 10/15] selftests: futex2: Add wouldblock test André Almeida
2021-06-03 19:59 ` [PATCH v4 11/15] selftests: futex2: Add waitv test André Almeida
2021-06-03 19:59 ` [PATCH v4 12/15] selftests: futex2: Add requeue test André Almeida
2021-06-03 19:59 ` [PATCH v4 13/15] selftests: futex2: Add futex sizes test André Almeida
2021-06-03 19:59 ` [PATCH v4 14/15] perf bench: Add futex2 benchmark tests André Almeida
2021-06-03 19:59 ` [PATCH v4 15/15] kernel: Enable waitpid() for futex2 André Almeida
2021-06-04  4:51 ` [PATCH v4 00/15] Add futex2 syscalls Zebediah Figura
2021-06-04 17:04   ` André Almeida
2021-06-04 11:36 ` Nicholas Piggin
2021-06-04 20:01   ` André Almeida
2021-06-05  1:09     ` Nicholas Piggin
2021-06-05  8:56       ` Andrey Semashev
2021-06-06 11:57         ` Nicholas Piggin
2021-06-06 13:15           ` Andrey Semashev
2021-06-08  1:25             ` Nicholas Piggin
2021-06-08 11:03               ` Andrey Semashev
2021-06-08 11:13                 ` Greg KH
2021-06-08 11:44                   ` Peter Zijlstra
2021-06-08 14:31                     ` Davidlohr Bueso
2021-06-08 12:06                   ` Andrey Semashev
2021-06-08 12:33                     ` Greg KH
2021-06-08 12:35                     ` Greg KH
2021-06-08 13:18                       ` Andrey Semashev
2021-06-08 13:27                         ` Greg KH
2021-06-08 13:41                           ` Andrey Semashev
2021-06-08 17:06                         ` Zebediah Figura
2021-06-08 14:14                   ` André Almeida
2021-06-07 15:40       ` André Almeida
2021-06-08  1:31         ` Nicholas Piggin
2021-06-08  2:33         ` Davidlohr Bueso
2021-06-08  4:45           ` Nicholas Piggin
2021-06-08 12:26         ` Sebastian Andrzej Siewior
2021-06-08 14:23           ` Peter Zijlstra
2021-06-08 14:57             ` Sebastian Andrzej Siewior
2021-06-08 15:04             ` André Almeida [this message]
2021-06-08 18:08             ` Adhemerval Zanella
2021-06-08 18:19               ` Florian Weimer
2021-06-08 18:22                 ` Adhemerval Zanella
2021-06-09 16:26             ` David Laight

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