linux-kernel - Re: [PATCH] random: vary jitter iterations based on cycle counter speed

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Message-ID: <Ys7ZURKkoHw8Bp+q@zx2c4.com>
Date:   Wed, 13 Jul 2022 16:40:17 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>,
        Theodore Ts'o <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] random: vary jitter iterations based on cycle counter
 speed

Hi Vladimir,

On Wed, Jul 13, 2022 at 03:31:05PM +0100, Vladimir Murzin wrote:
> I've just seen on the platform with slow(ish) timer that it is now considered
> as a source of entropy with samples_per_bit set to 27 (5.19-rc6 has MAX_SAMPLES_PER_BIT
> set to 32). Because of that I see significant delays and I'm trying to understand what
> could be wrong with my setup.
> 
> I observe one credit_init_bits(1) call (via entropy_timer()) per ~970 schedule() calls.
> Is that somewhat expected? Does it make sense at all?

How slow are we talking? Seconds? Minutes? Is it too slow? It's possible
that MAX_SAMPLES_PER_BIT=32 is a bit high as a threshold and I should
reduce that a bit.

Also, out of curiosity, why is your timer so slow?

Jason