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Date:   Fri, 6 Sep 2019 08:18:58 -0600
From:   Keith Busch <kbusch@kernel.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Keith Busch <keith.busch@intel.com>,
        Hannes Reinecke <hare@suse.com>,
        Bart Van Assche <bvanassche@acm.org>,
        linux-scsi@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Long Li <longli@microsoft.com>,
        John Garry <john.garry@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-nvme@lists.infradead.org, Jens Axboe <axboe@fb.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism
Message-ID: <20190906141858.GA3953@localhost.localdomain>
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 <dd96def4-1121-afbe-2431-9e516a06850c@linaro.org>
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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?