From: Mike Galbraith <efault@gmx.de>
To: mfranklin@comfiletech.com, linux-rt-users@vger.kernel.org
Subject: Re: i2c jitter is worse in PREEMPT_RT kernel than stock Raspberry Pi kernel
Date: Wed, 13 Dec 2023 09:15:14 +0100 [thread overview]
Message-ID: <3457c4604d0b0318e26336b24a0b36808080a25e.camel@gmx.de> (raw)
In-Reply-To: <3feecb7c-72a9-4778-930f-764495a91448@comfiletech.com>
On Wed, 2023-12-13 at 12:33 +0900, Michael Franklin wrote:
>
> I tested the stock kernel with `threadirqs` and indeed the jitter was worse, but the RT kernel still
> seemed to be more jittery.
>
> However, based on what you mentioned, I decided to look further interrupts/IRQ behavior, and found
> this in /proc/interrupts:
>
> PREEMPT_RT kernel:
> CPU0 CPU1 CPU2 CPU3
> 109: 22625815 0 0 0 rp1_irq_chip 8 Level 1f00074000.i2c
> IPI0: 19640 12018430 6870398 5548908 Rescheduling interrupts
> IPI1: 713 26295 18104 367 Function call interrupts
>
> Stock Kernel:
> CPU0 CPU1 CPU2 CPU3
> 109: 11129247 0 0 0 rp1_irq_chip 8 Level 1f00074000.i2c
> IPI0: 582 620 572 694 Rescheduling interrupts
> IPI1: 40061 12360 108946 5475437 Function call interrupts
>
> Stock Kernel with `threadirqs`:
> CPU0 CPU1 CPU2 CPU3
> 109: 21774128 0 0 0 rp1_irq_chip 8 Level 1f00074000.i2c
> IPI0: 674 657 687 617 Rescheduling interrupts
> IPI1: 58655 127527 21331238 5780380 Function call interrupts
>
> There you can see that, in the PREEMPT_RT kernel, there are a very large number of 'Rescheduling
> interrupts' and only a few 'Function call interrupts'. However, in the stock kernel it is the
> opposite -- a large number of 'Function call interrupts' and a few 'Rescheduling interrupts'.
Part of that is mitigation ala 539fbb5be0, and part is RT being its
naturally twitchy self.
-Mike
prev parent reply other threads:[~2023-12-13 8:15 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-12 2:53 i2c jitter is worse in PREEMPT_RT kernel than stock Raspberry Pi kernel Michael Franklin
2023-12-12 12:42 ` Mike Galbraith
2023-12-13 3:33 ` Michael Franklin
2023-12-13 4:37 ` Michael Franklin
2023-12-13 8:15 ` Mike Galbraith [this message]
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