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

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Date:   Fri, 6 Sep 2019 08:18:58 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Keith Busch <keith.busch@...el.com>,
        Hannes Reinecke <hare@...e.com>,
        Bart Van Assche <bvanassche@....org>,
        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, Jens Axboe <axboe@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

On Fri, Sep 06, 2019 at 09:48:21AM +0800, Ming Lei wrote:
> When one IRQ flood happens on one CPU:
> 
> 1) softirq handling on this CPU can't make progress
> 
> 2) kernel thread bound to this CPU can't make progress
> 
> For example, network may require softirq to xmit packets, or another irq
> thread for handling keyboards/mice or whatever, or rcu_sched may depend
> on that CPU for making progress, then the irq flood stalls the whole
> system.
> 
> > 
> > AFAIU, there are fast medium where the responses to requests are faster
> > than the time to process them, right?
> 
> Usually medium may not be faster than CPU, now we are talking about
> interrupts, which can be originated from lots of devices concurrently,
> for example, in Long Li'test, there are 8 NVMe drives involved.

Why are all 8 nvmes sharing the same CPU for interrupt handling?
Shouldn't matrix_find_best_cpu_managed() handle selecting the least used
CPU from the cpumask for the effective interrupt handling?