linux-kernel - RE: [PATCH v2 05/19] crypto: x86/crc

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Date:   Fri, 14 Oct 2022 04:02:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Robert Elliott' <elliott@....com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "ap420073@...il.com" <ap420073@...il.com>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 05/19] crypto: x86/crc - limit FPU preemption

From: Robert Elliott
> Sent: 12 October 2022 22:59
> 
> As done by the ECB and CBC helpers in arch/x86/crypt/ecb_cbc_helpers.h,
> limit the number of bytes processed between kernel_fpu_begin() and
> kernel_fpu_end() calls.
> 
> Those functions call preempt_disable() and preempt_enable(), so
> the CPU core is unavailable for scheduling while running, leading to:
>     rcu: INFO: rcu_preempt detected expedited stalls on CPUs/tasks: ...

How long were the buffers being processed when the rcu stall was reported?
It looks like you are adding kernel_fpu_end(); kernel_fpu_begin()
pairs every 4096 bytes.
I'd guess the crc instruction runs at 4 bytes/clock
(or at least gets somewhere near that).
So you are talking of few thousand clocks at most.
A pci read from a device can easily take much longer than that.
So I'm surprised you need to do such small buffers to avoid
rcu stalls.

The kernel_fpu_end(); kernel_fpu_begin() pair pair will also cost.
(Maybe not as much as the first kernel_fpu_begin() ?)

Some performance figures might be enlightening.

	David

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