linux-kernel - Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

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Date:   Wed, 4 Sep 2019 10:07:48 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, Hannes Reinecke <hare@...e.com>,
        Sagi Grimberg <sagi@...mberg.me>, linux-scsi@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Long Li <longli@...rosoft.com>,
        John Garry <john.garry@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org,
        Keith Busch <keith.busch@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

On 9/3/19 12:50 AM, Daniel Lezcano wrote:
> On 03/09/2019 09:28, Ming Lei wrote:
>> On Tue, Sep 03, 2019 at 08:40:35AM +0200, Daniel Lezcano wrote:
>>> It is a scheduler problem then ?
>>
>> Scheduler can do nothing if the CPU is taken completely by handling
>> interrupt & softirq, so seems not a scheduler problem, IMO.
> 
> Why? If there is a irq pressure on one CPU reducing its capacity, the
> scheduler will balance the tasks on another CPU, no?

Only if CONFIG_IRQ_TIME_ACCOUNTING has been enabled. However, I don't 
know any Linux distro that enables that option. That's probably because 
that option introduces two rdtsc() calls in each interrupt. Given the 
overhead introduced by this option, I don't think this is the solution 
Ming is looking for.

See also irqtime_account_irq() in kernel/sched/cputime.c.

Bart.