linux-kernel - Re: [PATCH] ARM: smp: Fix missing backtrace IPI statics

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Message-ID: <c55bbffe-9144-f787-eb70-8328be5cfe7c@huawei.com>
Date: Fri, 2 Aug 2024 09:39:33 +0800
From: Jinjie Ruan <ruanjinjie@...wei.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
CC: <dianders@...omium.org>, <mhocko@...e.com>, <akpm@...ux-foundation.org>,
	<maz@...nel.org>, <vschneid@...hat.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: smp: Fix missing backtrace IPI statics



On 2024/8/2 0:36, Russell King (Oracle) wrote:
> On Thu, Aug 01, 2024 at 05:40:22PM +0800, Jinjie Ruan wrote:
>> It is similar to ARM64 commit 916b93f4e865 ("arm64: smp: Fix missing IPI
>> statistics"), commit 56afcd3dbd19 ("ARM: Allow IPIs to be handled as normal
>> interrupts") set CPU_BACKTRACE IPI "IRQ_HIDDEN" flag but not show it in
>> show_ipi_list(), which cause the interrupt kstat_irqs accounting
>> is missing in display.
> 
> I don't see why this needs to be included. If this IPI fires, it means
> that the kernel has suffered a lockup and is probably not very useful.
> So the chances of being able to read out from /proc/interrupts a non-
> zero "CPU backtrace interrupts" figure is highly unlikely.

It is more of a change than a bug.

If we reset the arm32 code to before commit 56afcd3dbd19 ("ARM: Allow
IPIs to be handled as normal interrupts"), the backtrace IPI statics is
shown ok as below, but after that it is missing:

/ # cat /proc/interrupts
           CPU0
 24:          6 GIC-0  34 Level     timer
 25:        469 GIC-0  29 Level     twd
 26:         61 GIC-0  75 Edge      virtio0
 29:          8 GIC-0  44 Level     kmi-pl050
 30:        118 GIC-0  45 Level     kmi-pl050
 31:          0 GIC-0  36 Level     rtc-pl031
 32:          0 GIC-0  41 Level     mmci-pl18x (cmd)
 33:          0 GIC-0  42 Level     mmci-pl18x (pio)
 34:          0 GIC-0  92 Level     arm-pmu
 35:          0 GIC-0  93 Level     arm-pmu
 36:          0 GIC-0  94 Level     arm-pmu
 37:          0 GIC-0  95 Level     arm-pmu
 39:         15 GIC-0  37 Level     uart-pl011
IPI0:          0  CPU wakeup interrupts
IPI1:          0  Timer broadcast interrupts
IPI2:          0  Rescheduling interrupts
IPI3:          0  Function call interrupts
IPI4:          0  CPU stop interrupts
IPI5:          0  IRQ work interrupts
IPI6:          0  completion interrupts
IPI7:          0  CPU backtrace interrupts


> 
> So, I don't see much point to this change.
>