From owner-freebsd-hackers@freebsd.org Wed Aug 22 06:42:56 2018 Return-Path: Delivered-To: freebsd-hackers@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id C92FC108E9B2 for ; Wed, 22 Aug 2018 06:42:55 +0000 (UTC) (envelope-from sebastian.huber@embedded-brains.de) Received: from dedi548.your-server.de (dedi548.your-server.de [85.10.215.148]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 5FF1985055; Wed, 22 Aug 2018 06:42:55 +0000 (UTC) (envelope-from sebastian.huber@embedded-brains.de) Received: from [78.46.172.2] (helo=sslproxy05.your-server.de) by dedi548.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.85_2) (envelope-from ) id 1fsMqz-0005Hs-FL; Wed, 22 Aug 2018 08:42:53 +0200 Received: from [82.135.62.35] (helo=mail.embedded-brains.de) by sslproxy05.your-server.de with esmtpsa (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256) (Exim 4.89) (envelope-from ) id 1fsMqz-000PPQ-8d; Wed, 22 Aug 2018 08:42:53 +0200 Received: from localhost (localhost.localhost [127.0.0.1]) by mail.embedded-brains.de (Postfix) with ESMTP id C73982A165C; Wed, 22 Aug 2018 08:42:54 +0200 (CEST) Received: from mail.embedded-brains.de ([127.0.0.1]) by localhost (zimbra.eb.localhost [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id OSXoiEVZger6; Wed, 22 Aug 2018 08:42:52 +0200 (CEST) Received: from localhost (localhost.localhost [127.0.0.1]) by mail.embedded-brains.de (Postfix) with ESMTP id 4AE9C2A167E; Wed, 22 Aug 2018 08:42:52 +0200 (CEST) X-Virus-Scanned: amavisd-new at zimbra.eb.localhost Received: from mail.embedded-brains.de ([127.0.0.1]) by localhost (zimbra.eb.localhost [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id t92RFRz0cIzN; Wed, 22 Aug 2018 08:42:52 +0200 (CEST) Received: from [192.168.96.149] (unknown [192.168.96.149]) by mail.embedded-brains.de (Postfix) with ESMTPSA id 176D12A165C; Wed, 22 Aug 2018 08:42:52 +0200 (CEST) Subject: Re: epoch(9) background information? From: Sebastian Huber To: FreeBSD Cc: Matthew Macy References: Message-ID: <15e3f080-2f82-a243-80e9-f0a916445828@embedded-brains.de> Date: Wed, 22 Aug 2018 08:42:49 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: quoted-printable X-Authenticated-Sender: smtp-embedded@poldinet.de X-Virus-Scanned: Clear (ClamAV 0.100.1/24863/Tue Aug 21 18:48:32 2018) X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.27 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 22 Aug 2018 06:42:56 -0000 On 22/08/18 08:34, Sebastian Huber wrote: > On 21/08/18 15:38, Jacques Fourie wrote: >> >> >> On Tue, Aug 21, 2018 at 8:33 AM, Sebastian Huber=20 >> > > wrote: >> >> =C2=A0=C2=A0=C2=A0 Hello, >> >> =C2=A0=C2=A0=C2=A0 I update currently a port of the FreeBSD network st= ack, etc. to >> =C2=A0=C2=A0=C2=A0 the real-time operating system RTEMS from the head = version at >> =C2=A0=C2=A0=C2=A0 2017-04-04 to the head version of today. I noticed = that some >> =C2=A0=C2=A0=C2=A0 read-write locks are replaced by a relatively new s= tuff called >> =C2=A0=C2=A0=C2=A0 EPOCH(9). Is there some background information avai= lable for this? >> =C2=A0=C2=A0=C2=A0 The man page is a bit vague and searching for somet= hing named >> =C2=A0=C2=A0=C2=A0 epoch on the internet is not really great. For exam= ple, what is >> =C2=A0=C2=A0=C2=A0 the motivation for this change? How is this related= to >> =C2=A0=C2=A0=C2=A0 read-copy-update (RCU)? >> >> =C2=A0=C2=A0=C2=A0 -- =C2=A0=C2=A0=C2=A0 Sebastian Huber, embedded bra= ins GmbH >> >> =C2=A0=C2=A0=C2=A0 Address : Dornierstr. 4, D-82178 Puchheim, Germany >> >> =C2=A0=C2=A0=C2=A0 Phone=C2=A0 =C2=A0: +49 89 189 47 41-16 >> =C2=A0=C2=A0=C2=A0 Fax=C2=A0 =C2=A0 =C2=A0: +49 89 189 47 41-09 >> =C2=A0=C2=A0=C2=A0 E-Mail=C2=A0 : sebastian.huber@embedded-brains.de >> =C2=A0=C2=A0=C2=A0 >> =C2=A0=C2=A0=C2=A0 PGP=C2=A0 =C2=A0 =C2=A0: Public key available on re= quest. >> >> =C2=A0=C2=A0=C2=A0 Diese Nachricht ist keine gesch=C3=A4ftliche Mittei= lung im Sinne des=20 >> EHUG. >> >> =C2=A0=C2=A0=C2=A0 _______________________________________________ >> =C2=A0=C2=A0=C2=A0 freebsd-hackers@freebsd.org >> =C2=A0=C2=A0=C2=A0 mailing list >> =C2=A0=C2=A0=C2=A0 https://lists.freebsd.org/mailman/listinfo/freebsd-= hackers >> >> =C2=A0=C2=A0=C2=A0 To unsubscribe, send any mail to >> =C2=A0=C2=A0=C2=A0 "freebsd-hackers-unsubscribe@freebsd.org >> =C2=A0=C2=A0=C2=A0 " >> >> >> Additional information is available here :=20 >> http://concurrencykit.org/presentations/ebr.pdf=20 >> . The way I=20 >> understand it is that it is mostly used in place of read locks to=20 >> provide liveness guarantees without using atomics. Additional detail=20 >> is available in the commit messages. As an example see r333813 for=20 >> some performance data. >> > > Thanks, for the reference. The "epoch reclamation" are good keywords=20 > to find more information. > > What is the right mailing list to ask questions about the epoch=20 > implementation of the FreeBSD kernel? > > To support this machinery in RTEMS is a bit difficult (in particular=20 > EPOCH_LOCKED). Since RTEMS is supposed to be a real-time operating=20 > system it supports only fixed-priority and job-level fixed priority=20 > (EDF) schedulers. To allow some scaling to larger SMP systems it=20 > supports clustered scheduling together with the mutual exclusion=20 > locking protocols MrsP=20 > (http://www-users.cs.york.ac.uk/~burns/MRSPpaper.pdf) and OMIP=20 > (http://www.mpi-sws.org/~bbb/papers/pdf/ecrts13b.pdf). This makes the=20 > thread pinning hard to implement (which is very easy to support in=20 > FreeBSD). The locking protocols may temporarily move a thread which=20 > owns a mutex to a foreign scheduler instance, e.g. a thread which=20 > wants to obtain the mutex helps the owner to make progress if it was=20 > pre-empted in its home scheduler instance. Due to a timeout of the=20 > helper the owner may loose the right to execute in the foreign=20 > scheduler instance. This would make it impossible to fulfil the=20 > processor pinning constraint (e.g. the thread priority in the foreign=20 > scheduler instance is undefined). > > It would save me a lot of trouble if I could assume that EPOCH_LOCKED=20 > is an exotic feature which is unlikely to get used in FreeBSD. > Another question, is it a common use case to call epoch_enter_preempt()=20 and epoch_exit_preempt() while owning a mutex? --=20 Sebastian Huber, embedded brains GmbH Address : Dornierstr. 4, D-82178 Puchheim, Germany Phone : +49 89 189 47 41-16 Fax : +49 89 189 47 41-09 E-Mail : sebastian.huber@embedded-brains.de PGP : Public key available on request. Diese Nachricht ist keine gesch=C3=A4ftliche Mitteilung im Sinne des EHUG= .