linux-kernel - Re: [PATCH v2] random: set fast pool count to zero in cpuhp teardown

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Date:   Mon, 14 Feb 2022 15:17:30 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v2] random: set fast pool count to zero in cpuhp teardown

On 2022-02-14 14:37:35 [+0100], Jason A. Donenfeld wrote:
> Rather than having to use expensive atomics, which were visibly the most
> expensive thing in the entire irq handler, simply take care of the
> extreme edge case of resetting count to 0 in the cpuhp teardown handler,
> after no more interrupts will arrive on that CPU. This simplifies the
> code a bit and lets us use vanilla variables rather than atomics, and
> performance should be improved.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Jonathan Neuschäfer <j.neuschaefer@....net>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Sultan Alsawaf <sultan@...neltoast.com>
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Sebastian -
> 
> v2 moves the teardown to CPUHP_OFFLINE…CPUHP_BRINGUP_CPU, per our
> discussion.

My suggestion was to move it to the startup handler with the code
snippet I had.
As I tried to explain, this may have two problems:
- worker scheduled during CPU-UP before CPUHP_AP_WORKQUEUE_ONLINE are
  probably unbound.

- worker scheduled during CPU-DOWN after CPUHP_AP_WORKQUEUE_ONLINE are
  probably unbound.

The unbound worker may run on any CPU and thus do nothing.
In the CPU-DOWN case before: should we rollback before
CPUHP_RANDOM_PREPARE but after CPUHP_AP_WORKQUEUE_ONLINE then the needed
reset (in case the worker did nothing because it was on the wrong CPU)
will not happen.
Therefore I think, moving it to startup, online, (as suggested in
https://lore.kernel.org/all/Ygo3%2FpuhZFpuX91x@linutronix.de/).

will not have any of this downsides/ corner cases.

Sebastian