Date: Tue, 24 Feb 2015 04:42:50 +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: <20150224024250.GV74514@kib.kiev.ua> In-Reply-To: <54EBDC1C.3060007@astrodoggroup.com> References: <20150224012026.GY46794@funkthat.com> <20150224015721.GT74514@kib.kiev.ua> <54EBDC1C.3060007@astrodoggroup.com>
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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.
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