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From: David Laight <David.Laight@ACULAB.COM>
To: 'Peter Zijlstra' <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "André Almeida" <andrealmeid@collabora.com>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"acme@kernel.org" <acme@kernel.org>,
	"Andrey Semashev" <andrey.semashev@gmail.com>,
	"corbet@lwn.net" <corbet@lwn.net>,
	"Davidlohr Bueso" <dave@stgolabs.net>,
	"Darren Hart" <dvhart@infradead.org>,
	"fweimer@redhat.com" <fweimer@redhat.com>,
	"joel@joelfernandes.org" <joel@joelfernandes.org>,
	"kernel@collabora.com" <kernel@collabora.com>,
	"krisman@collabora.com" <krisman@collabora.com>,
	"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>,
	"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>,
	"malteskarupke@fastmail.fm" <malteskarupke@fastmail.fm>,
	"Ingo Molnar" <mingo@redhat.com>,
	"pgriffais@valvesoftware.com" <pgriffais@valvesoftware.com>,
	"Peter Oskolkov" <posk@posk.io>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"shuah@kernel.org" <shuah@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"z.figura12@gmail.com" <z.figura12@gmail.com>
Subject: RE: [PATCH v4 00/15] Add futex2 syscalls
Date: Wed, 9 Jun 2021 16:26:09 +0000	[thread overview]
Message-ID: <ae5b077863fb4853b4f26c3f0b176ac0@AcuMS.aculab.com> (raw)
In-Reply-To: <YL99cR0H+7xgU8L1@hirez.programming.kicks-ass.net>

From: Peter Zijlstra
> Sent: 08 June 2021 15:24
...
> 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.

I had a problem with pthread_broadcast().
I've got multiple threads all bound to different cpu and
I really do want to wake them all up at once.
Now, I know they'll spin on the mutex for a bit - but that
is soon released (the 'adaptive' spin is probably long enough).

But what actually happens (probably because of the way glibc
is constrained by the futext system call) is:
1) The first thread is woken.
2) It's cpu comes out of sleep.
3) Once running it wakes the second thread.
4) It's cpu comes out of sleep.
...
So you get a very slow ripple of the threads starting.

Worse still, if the thread can't be scheduled (eg because
its cpu is running all the network stack softint code)
then none of the later threads start running.

I've mitigated it by using a separate cv for each thread
and looping to wake them all - but it shouldn't really
be necessary.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

      parent reply	other threads:[~2021-06-09 16:26 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
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 [this message]

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