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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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