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Subject: Re: [RFC PATCH] irqchip/gic-v3: Make gic_handle_irq() notrace
To:     Marc Zyngier <marc.zyngier@arm.com>, <julien.thierry@arm.com>,
        <rostedt@goodmis.org>
CC:     <mingo@redhat.com>, <catalin.marinas@arm.com>,
        <will.deacon@arm.com>, <mark.rutland@arm.com>,
        <linux@armlinux.org.uk>, <james.morse@arm.com>,
        <suzuki.poulose@arm.com>, <wanghaibin.wang@huawei.com>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>
References: <1553865808-30604-1-git-send-email-yuzenghui@huawei.com>
 <237b00d0-8a8d-ede6-91b1-c3e23e61925c@arm.com>
From:   Zenghui Yu <yuzenghui@huawei.com>
Message-ID: <bf434b0c-2b17-b92d-3171-a57203486c84@huawei.com>
Date:   Fri, 29 Mar 2019 23:02:05 +0800
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Hi Marc,

On 2019/3/29 21:58, Marc Zyngier wrote:
> Hi Zenghui,
> 
> On 29/03/2019 13:23, Zenghui Yu wrote:
>> Enable pseudo NMI together with function_graph tracer, will lead
>> the system to a hang. This is easy to reproduce,
>>
>>    1) Set "irqchip.gicv3_pseudo_nmi=1" on the kernel command line
>>    2) echo function_graph > /sys/kernel/debug/tracing/current_tracer
>>
>> This patch (RFC) set gic_handle_irq() as notrace and it seems works
>> fine now. But I have no idea about what the issue is exactly, and
>> you can regard this patch as a report then :)
>>
>> Can someone give a look at it and provide some explanations ?
>>
>> Thanks!
>>
>> Cc: Julien Thierry <julien.thierry@arm.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>> ---
>>   drivers/irqchip/irq-gic-v3.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
>> index 15e55d3..8d0c25f 100644
>> --- a/drivers/irqchip/irq-gic-v3.c
>> +++ b/drivers/irqchip/irq-gic-v3.c
>> @@ -487,7 +487,7 @@ static inline void gic_handle_nmi(u32 irqnr, struct pt_regs *regs)
>>   		gic_deactivate_unhandled(irqnr);
>>   }
>>   
>> -static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
>> +static asmlinkage notrace void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
>>   {
>>   	u32 irqnr;
>>   
>>
> 
> 
> That's interesting. Do you have any out of tree patch that actually
> makes use of the pseudo-NMI feature? Without those patches, the
> behaviour should stay unchanged.

I am at commit 1a9df9e29c2afecf6e3089442d429b377279ca3c. No more
patches, and this is the most confusing. Just out of curiosity, I
wanted to run Julien's "Use NMI for perf interrupt" patch (posted
on the mailing list), so I have to enable NMI first.

That said, with
   1) Select Kernel Feature -> Support for NMI-like interrupts
   2) Set "irqchip.gicv3_pseudo_nmi=1" on the kernel command line
   3) No pseudo-NMIs have been generated at all
and this issue was hit.


thanks,

zenghui

> 
> On the other hand, if you can generate pseudo-NMIs, you could end-up
> tracing gic_handle_irq whilst being inside the tracing code with
> interrupts being notionally disabled (and that could be pretty bad).
> 
> So, patches or no patches?
> 
> Thanks,
> 
> 	M.
>