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

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Message-ID: <20190906185210.GA4260@localhost.localdomain>
Date:   Fri, 6 Sep 2019 12:52:11 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...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,
        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 Fri, Sep 06, 2019 at 11:30:57AM -0700, Sagi Grimberg wrote:
> 
> > 
> > Ok, so the real problem is per-cpu bounded tasks.
> > 
> > I share Thomas opinion about a NAPI like approach.
> 
> We already have that, its irq_poll, but it seems that for this
> use-case, we get lower performance for some reason. I'm not
> entirely sure why that is, maybe its because we need to mask interrupts
> because we don't have an "arm" register in nvme like network devices
> have?

For MSI, that's the INTMS/INTMC NVMe registers. MSI-x, though, has to
disarm it in its table entry, and the Linux implementation will do a
posted read in that path, which is a bit too expensive.