From owner-freebsd-arch@FreeBSD.ORG Tue Feb 24 04:23:51 2015 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id BA729CFF for ; Tue, 24 Feb 2015 04:23:51 +0000 (UTC) Received: from gold.funkthat.com (gate2.funkthat.com [208.87.223.18]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "gold.funkthat.com", Issuer "gold.funkthat.com" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 97C0B657 for ; Tue, 24 Feb 2015 04:23:51 +0000 (UTC) Received: from gold.funkthat.com (localhost [127.0.0.1]) by gold.funkthat.com (8.14.5/8.14.5) with ESMTP id t1O4NnBL097868 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 23 Feb 2015 20:23:49 -0800 (PST) (envelope-from jmg@gold.funkthat.com) Received: (from jmg@localhost) by gold.funkthat.com (8.14.5/8.14.5/Submit) id t1O4NmJW097867; Mon, 23 Feb 2015 20:23:48 -0800 (PST) (envelope-from jmg) Date: Mon, 23 Feb 2015 20:23:48 -0800 From: John-Mark Gurney To: Konstantin Belousov Subject: Re: locks and kernel randomness... Message-ID: <20150224042348.GA46794@funkthat.com> References: <20150224012026.GY46794@funkthat.com> <20150224015721.GT74514@kib.kiev.ua> <54EBDC1C.3060007@astrodoggroup.com> <20150224024250.GV74514@kib.kiev.ua> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150224024250.GV74514@kib.kiev.ua> X-Operating-System: FreeBSD 9.1-PRERELEASE amd64 X-PGP-Fingerprint: 54BA 873B 6515 3F10 9E88 9322 9CB1 8F74 6D3F A396 X-Files: The truth is out there X-URL: http://resnet.uoregon.edu/~gurney_j/ X-Resume: http://resnet.uoregon.edu/~gurney_j/resume.html X-TipJar: bitcoin:13Qmb6AeTgQecazTWph4XasEsP7nGRbAPE X-to-the-FBI-CIA-and-NSA: HI! HOW YA DOIN? can i haz chizburger? User-Agent: Mutt/1.5.21 (2010-09-15) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.7 (gold.funkthat.com [127.0.0.1]); Mon, 23 Feb 2015 20:23:49 -0800 (PST) Cc: Harrison Grundy , freebsd-arch@freebsd.org X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 24 Feb 2015 04:23:51 -0000 Konstantin Belousov wrote this message on Tue, Feb 24, 2015 at 04:42 +0200: > On Mon, Feb 23, 2015 at 06:04:12PM -0800, Harrison Grundy wrote: > > > > > > On 02/23/15 17:57, Konstantin Belousov wrote: > > > On Mon, Feb 23, 2015 at 05:20:26PM -0800, John-Mark Gurney wrote: > > >> I'm working on simplifying kernel randomness interfaces. I would > > >> like to get read of all weak random generators, and this means > > >> replacing read_random and random(9) w/ effectively arc4rand(9) > > >> (to be replaced by ChaCha or Keccak in the future). > > >> > > >> The issue is that random(9) is called from any number of > > >> contexts, such as the scheduler. This makes locking a bit more > > >> interesting. Currently, both arc4rand(9) and yarrow/fortuna use > > >> a default mtx lock to protect their state. This obviously isn't > > >> compatible w/ the scheduler, and possibly other calling > > >> contexts. > > >> > > >> I have a patch[1] that unifies the random interface. It converts > > >> a few of the locks from mtx default to mtx spin to deal w/ this. > > > This is definitely an overkill. The rebalancing minor use of > > > randomness absolutely does not require cryptographical-strenght > > > randomness to select a moment to rebalance thread queue. Imposing > > > the spin lock on the whole random machinery just to allow the same > > > random gathering code to be used for balance_ticks is detriment to > > > the system responsivness. Scheduler is fine even with congruential > > > generators, as you could see in the cpu_search(), look for the > > > '69069'. > > > > > > Please do not enforce yet another spinlock for the system. > > > _______________________________________________ > > > > The patch attached to > > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197922 switches > > sched_balance to use get_cyclecount, which is also a suitable source > > of entropy for this purpose. > > > > It would also be possible to make the scheduler deterministic here, > > using cpuid or some such thing to make sure all CPUs don't fire the > > balancer at the same time. > > > > The patch in the PR is probably in the right direction, but might be too > simple, unless somebody dispel my fallacy. I remember seeing claims that > on the very low-end embedded devices the get_cyclecount() method may > be non-functional, i.e. returning some constant, probably 0. I somehow > associate MIPS arch with this bias. Well, the docs say: The speed and the maximum value of each counter is CPU-dependent. Some CPUs (such as the Intel 80486) do not have such a register, so get_cyclecount() on these platforms returns a (monotonic) combination of numbers represented by the structure returned by binuptime(9). The clang builtin of cycle counter is documented as returning 0 when it isn't available. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."