linux-kernel - Re: [PATCH v6] random: defer fast pool mixing to worker

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Message-ID: <CAHmME9okFEVqTM9c9fQY02UiiNBkpaZMLSYMhzHs8FqtpV1F1g@mail.gmail.com>
Date:   Sun, 13 Feb 2022 22:04:17 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v6] random: defer fast pool mixing to worker

Hey Sebastian,

On Fri, Feb 11, 2022 at 6:26 PM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
> Sure. I can do the CPU-HP notifier later one once we are done with
> everything. I acked the v7, don't see a road block.

I've been running this over the weekend and performance is generally okay.

# perf top --symbols $(objdump -t
/usr/src/linux/drivers/char/random.o|fgrep 'F .text'|awk '{print
$6}'|tr '\n' ,) -F max -g

One thing I noticed though was that add_interrupt_randomness spends
most of its time on:

    lock   xadd   %eax,0x38(%rbp)

So, as expected, those atomics really need to go. Indeed we might be
best off with the CPU hotplug notifier for setting the count back to
zero.

Do you want to prepare a patch for this? Or should I take a stab at it?

Jason