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Date:      Mon, 23 Feb 2015 18:04:12 -0800
From:      Harrison Grundy <harrison.grundy@astrodoggroup.com>
To:        freebsd-arch@freebsd.org
Subject:   Re: locks and kernel randomness...
Message-ID:  <54EBDC1C.3060007@astrodoggroup.com>
In-Reply-To: <20150224015721.GT74514@kib.kiev.ua>
References:  <20150224012026.GY46794@funkthat.com> <20150224015721.GT74514@kib.kiev.ua>

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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.

--- Harrison



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