From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: linux-kernel@vger.kernel.org,
"Thomas Gleixner" <tglx@linutronix.de>,
"Peter Zijlstra" <peterz@infradead.org>,
"Theodore Ts'o" <tytso@mit.edu>,
"Jonathan Neuschäfer" <j.neuschaefer@gmx.net>,
"Sultan Alsawaf" <sultan@kerneltoast.com>,
"Dominik Brodowski" <linux@dominikbrodowski.net>
Subject: Re: [PATCH v2] random: set fast pool count to zero in cpuhp teardown
Date: Mon, 14 Feb 2022 15:17:30 +0100 [thread overview]
Message-ID: <Ygpkej/vnMvUDIlP@linutronix.de> (raw)
In-Reply-To: <20220214133735.966528-1-Jason@zx2c4.com>
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@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Sultan Alsawaf <sultan@kerneltoast.com>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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
next prev parent reply other threads:[~2022-02-14 14:17 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-13 21:53 [PATCH] random: set fast pool count to zero in cpuhp teardown Jason A. Donenfeld
2022-02-14 11:07 ` Sebastian Andrzej Siewior
2022-02-14 13:28 ` Jason A. Donenfeld
2022-02-14 13:37 ` [PATCH v2] " Jason A. Donenfeld
2022-02-14 14:17 ` Sebastian Andrzej Siewior [this message]
2022-02-14 14:42 ` Jason A. Donenfeld
2022-02-14 14:49 ` Sebastian Andrzej Siewior
2022-02-14 14:52 ` Jason A. Donenfeld
2022-02-14 15:06 ` Sebastian Andrzej Siewior
2022-02-14 15:10 ` Jason A. Donenfeld
2022-02-14 15:17 ` Sebastian Andrzej Siewior
2022-02-14 16:15 ` Jason A. Donenfeld
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