linux-kernel - Re: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop

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Message-ID: <87imc1v5xq.fsf@nanos.tec.linutronix.de>
Date:   Sat, 26 Sep 2020 00:37:37 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     qianjun.kernel@...il.com, peterz@...radead.org, will@...nel.org,
        luto@...nel.org, linux-kernel@...r.kernel.org,
        laoar.shao@...il.com, qais.yousef@....com, urezki@...il.com
Subject: Re: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop

On Fri, Sep 25 2020 at 01:08, Frederic Weisbecker wrote:

> On Thu, Sep 24, 2020 at 05:37:42PM +0200, Thomas Gleixner wrote:
>> Subject: softirq; Prevent starvation of higher softirq vectors
>> From: Thomas Gleixner <tglx@...utronix.de>
>> Date: Thu, 24 Sep 2020 10:40:24 +0200
>> 
>> From: Thomas Gleixner <tglx@...utronix.de>
>> 
>> The early termination of the softirq processing loop can lead to starvation
>> of the higher numbered soft interrupt vectors because each run starts at
>> the lowest bit. If the loop terminates then the already processed bits can
>> be raised again before the next loop starts. If these lower bits run into
>> the termination again, then a re-raise might starve the higher bits forever.
>> 
>> To prevent this, store the leftovers of the previous run in the upper 16
>> bit of the local softirq_pending storage and ensure that these are
>> processed before any newly raised bits are handled.
>> 
>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>> ---
>>  kernel/softirq.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++--------
>>  1 file changed, 50 insertions(+), 8 deletions(-)
>> 
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -259,11 +259,23 @@ static inline bool __softirq_needs_break
>>  	return need_resched() || __softirq_timeout(tbreak);
>>  }
>>  
>> +/*
>> + * local_softirq_pending() is split into two 16 bit words. The low word
>> + * contains the bits set by raise_softirq(), the high word contains pending
>> + * bits which have not been processed in an early terminated run. This is
>> + * required to prevent starvation of the higher numbered softirqs.
>> + */
>> +#define SIRQ_PREV_SHIFT		16
>
> Note that in the case of x86, irq_start.__softirq_pending is a u16.
>
> The origin is there: 9aee5f8a7e30330d0a8f4c626dc924ca5590aba5
> "x86/irq: Demote irq_cpustat_t::__softirq_pending to u16"

Bah, crap. I knew I that and wanted to fix it up but then forgot.

Thanks for reminding me of my slowly upcoming alzheimer!