Date: Tue, 24 Feb 2015 15:46:27 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Harrison Grundy <harrison.grundy@astrodoggroup.com> Cc: freebsd-arch@freebsd.org Subject: Re: locks and kernel randomness... Message-ID: <20150224134627.GX74514@kib.kiev.ua> In-Reply-To: <54EBFFDC.4090905@astrodoggroup.com> References: <20150224012026.GY46794@funkthat.com> <20150224015721.GT74514@kib.kiev.ua> <54EBDC1C.3060007@astrodoggroup.com> <20150224024250.GV74514@kib.kiev.ua> <54EBFFDC.4090905@astrodoggroup.com>
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On Mon, Feb 23, 2015 at 08:36:44PM -0800, Harrison Grundy wrote: > > > On 02/23/15 18:42, Konstantin Belousov wrote: > > 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. > > > > Talking to some of the arm and MIPS developers, it appears > get_cyclecount() may be slow on some older ARM hardware... (In > particular, hardware that doesn't support SMP anyway.) > > However, after a quick test on some machines here, I don't think this > function actually needs randomness, due to the large number of other > pathways ULE uses to balance load. Well, system does benefit from some irregularity in a scheduler. It is in fact required for locks to work. > > New patch attached to the PR that simply removes the randomness entirely. Since my groundless worries about get_cyclecount() were defeated, I think your first patch is fine and is the way to go.
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