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Date:   Sat, 7 Dec 2019 16:03:35 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     John Garry <john.garry@huawei.com>
Cc:     tglx@linutronix.de, chenxiang66@hisilicon.com,
        bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        maz@kernel.org, hare@suse.com, hch@lst.de, axboe@kernel.dk,
        bvanassche@acm.org, peterz@infradead.org, mingo@redhat.com
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity
 for managed interrupt
Message-ID: <20191207080335.GA6077@ming.t460p>
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On Fri, Dec 06, 2019 at 10:35:04PM +0800, John Garry wrote:
> Currently the cpu allowed mask for the threaded part of a threaded irq
> handler will be set to the effective affinity of the hard irq.
>=20
> Typically the effective affinity of the hard irq will be for a single cpu=
. As such,
> the threaded handler would always run on the same cpu as the hard irq.
>=20
> We have seen scenarios in high data-rate throughput testing that the cpu
> handling the interrupt can be totally saturated handling both the hard
> interrupt and threaded handler parts, limiting throughput.

Frankly speaking, I never observed that single CPU is saturated by one stor=
age
completion queue's interrupt load. Because CPU is still much quicker than
current storage device.=20

If there are more drives, one CPU won't handle more than one queue(drive)'s
interrupt if (nr_drive * nr_hw_queues) < nr_cpu_cores.

So could you describe your case in a bit detail? Then we can confirm
if this change is really needed.

>=20
> For when the interrupt is managed, allow the threaded part to run on all
> cpus in the irq affinity mask.

I remembered that performance drop is observed by this approach in some
test.


Thanks,=20
Ming