From: Peter Zijlstra <peterz@infradead.org>
To: Long Li <longli@microsoft.com>
Cc: "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
Ingo Molnar <mingo@redhat.com>,
Keith Busch <keith.busch@intel.com>, Jens Axboe <axboe@fb.com>,
Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] nvme: complete request in work queue on CPU with flooded interrupts
Date: Wed, 21 Aug 2019 12:35:59 +0200 [thread overview]
Message-ID: <20190821103559.GZ2349@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CY4PR21MB0741A817BEB880C8DC526ECFCEAA0@CY4PR21MB0741.namprd21.prod.outlook.com>
On Wed, Aug 21, 2019 at 08:37:55AM +0000, Long Li wrote:
> >>>Subject: Re: [PATCH 3/3] nvme: complete request in work queue on CPU
> >>>with flooded interrupts
> >>>
> >>>On Mon, Aug 19, 2019 at 11:14:29PM -0700, longli@linuxonhyperv.com
> >>>wrote:
> >>>> From: Long Li <longli@microsoft.com>
> >>>>
> >>>> When a NVMe hardware queue is mapped to several CPU queues, it is
> >>>> possible that the CPU this hardware queue is bound to is flooded by
> >>>> returning I/O for other CPUs.
> >>>>
> >>>> For example, consider the following scenario:
> >>>> 1. CPU 0, 1, 2 and 3 share the same hardware queue 2. the hardware
> >>>> queue interrupts CPU 0 for I/O response 3. processes from CPU 1, 2 and
> >>>> 3 keep sending I/Os
> >>>>
> >>>> CPU 0 may be flooded with interrupts from NVMe device that are I/O
> >>>> responses for CPU 1, 2 and 3. Under heavy I/O load, it is possible
> >>>> that CPU 0 spends all the time serving NVMe and other system
> >>>> interrupts, but doesn't have a chance to run in process context.
> >>>
> >>>Ideally -- and there is some code to affect this, the load-balancer will move
> >>>tasks away from this CPU.
> >>>
> >>>> To fix this, CPU 0 can schedule a work to complete the I/O request
> >>>> when it detects the scheduler is not making progress. This serves multiple
> >>>purposes:
> >>>
> >>>Suppose the task waiting for the IO completion is a RT task, and you've just
> >>>queued it to a regular work. This is an instant priority inversion.
>
> This is a choice. We can either not "lock up" the CPU, or finish the I/O on time from IRQ handler. I think throttling only happens in extreme conditions, which is rare. The purpose is to make the whole system responsive and happy.
Can you please use a sane MUA.. this is unreadable garbage.
next prev parent reply other threads:[~2019-08-21 10:36 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-20 6:14 [PATCH 0/3] fix interrupt swamp in NVMe longli
2019-08-20 6:14 ` [PATCH 1/3] sched: define a function to report the number of context switches on a CPU longli
2019-08-20 9:38 ` Peter Zijlstra
2019-08-21 8:20 ` Long Li
2019-08-21 10:34 ` Peter Zijlstra
2019-08-20 9:39 ` Peter Zijlstra
2019-08-20 6:14 ` [PATCH 2/3] sched: export idle_cpu() longli
2019-08-20 6:14 ` [PATCH 3/3] nvme: complete request in work queue on CPU with flooded interrupts longli
2019-08-20 9:52 ` Peter Zijlstra
2019-08-21 8:37 ` Long Li
2019-08-21 10:35 ` Peter Zijlstra [this message]
2019-08-20 17:33 ` Sagi Grimberg
2019-08-21 8:39 ` Long Li
2019-08-21 17:36 ` Long Li
2019-08-21 21:54 ` Sagi Grimberg
2019-08-24 0:13 ` Long Li
2019-08-23 3:21 ` Ming Lei
2019-08-24 0:27 ` Long Li
2019-08-24 12:55 ` Ming Lei
2019-08-20 8:25 ` [PATCH 0/3] fix interrupt swamp in NVMe Ming Lei
2019-08-20 8:59 ` John Garry
2019-08-20 15:05 ` Keith Busch
2019-08-21 7:47 ` Long Li
2019-08-21 9:44 ` Ming Lei
2019-08-21 10:03 ` John Garry
2019-08-21 16:27 ` Long Li
2019-08-22 1:33 ` Ming Lei
2019-08-22 2:00 ` Keith Busch
2019-08-22 2:23 ` Ming Lei
2019-08-22 9:48 ` Thomas Gleixner
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