Date: Tue, 24 Feb 2015 07:58:19 -0700 From: Warner Losh <imp@bsdimp.com> To: Harrison Grundy <harrison.grundy@astrodoggroup.com> Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-arch@freebsd.org Subject: Re: locks and kernel randomness... Message-ID: <E745148E-12AF-4205-B340-B1795B44B257@bsdimp.com> 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 Feb 23, 2015, at 9:36 PM, Harrison Grundy <harrison.grundy@astrodoggroup.com> 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.) It simply doesn’t exist on older ARM hardware. Some SoCs have something similar to a real-time clock that you can read, but that’s not reliable for this use. > 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. > > New patch attached to the PR that simply removes the randomness entirely. Are you sure about that? Warnerhome | help
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