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[85.223.209.22]) by smtp.gmail.com with ESMTPSA id w3sm559066lfe.18.2019.02.22.04.38.08 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 22 Feb 2019 04:38:08 -0800 (PST) Subject: Re: [Xen-devel] xen/evtchn and forced threaded irq To: Julien Grall , Boris Ostrovsky Cc: Juergen Gross , Stefano Stabellini , Andrew Cooper , "linux-kernel@vger.kernel.org" , Jan Beulich , xen-devel , Dave P Martin References: <5e256d9a-572c-e01e-7706-407f99245b00@arm.com> <20190220000209.GA4091@localhost.localdomain> <21214d47-9c68-e6bf-007a-4047cc2b02f9@oracle.com> <8f7445d7-fa50-f3e9-44f5-cc2aebd020f4@arm.com> <15bc52cb-82d8-4d2c-e5a8-c6ae8de90276@oracle.com> <5df8888a-4a29-fccd-bac2-a9c170244029@arm.com> From: Oleksandr Andrushchenko Message-ID: Date: Fri, 22 Feb 2019 14:38:07 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <5df8888a-4a29-fccd-bac2-a9c170244029@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/20/19 10:46 PM, Julien Grall wrote: > (+ Andrew and Jan for feedback on the event channel interrupt) > > Hi Boris, > > Thank you for the your feedback. > > On 2/20/19 8:04 PM, Boris Ostrovsky wrote: >> On 2/20/19 1:05 PM, Julien Grall wrote: >>> Hi, >>> >>> On 20/02/2019 17:07, Boris Ostrovsky wrote: >>>> On 2/20/19 9:15 AM, Julien Grall wrote: >>>>> Hi Boris, >>>>> >>>>> Thank you for your answer. >>>>> >>>>> On 20/02/2019 00:02, Boris Ostrovsky wrote: >>>>>> On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote: >>>>>>> Hi all, >>>>>>> >>>>>>> I have been looking at using Linux RT in Dom0. Once the guest is >>>>>>> started, >>>>>>> the console is ending to have a lot of warning (see trace below). >>>>>>> >>>>>>> After some investigation, this is because the irq handler will now >>>>>>> be threaded. >>>>>>> I can reproduce the same error with the vanilla Linux when passing >>>>>>> the option >>>>>>> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 >>>>>>> that has >>>>>>> not RT support). >>>>>>> >>>>>>> FWIW, the interrupt for port 6 is used to for the guest to >>>>>>> communicate with >>>>>>> xenstore. >>>>>>> >>>>>>>    From my understanding, this is happening because the interrupt >>>>>>> handler is now >>>>>>> run in a thread. So we can have the following happening. >>>>>>> >>>>>>>       Interrupt context            |     Interrupt thread >>>>>>>                                    | >>>>>>>       receive interrupt port 6     | >>>>>>>       clear the evtchn port        | >>>>>>>       set IRQF_RUNTHREAD            | >>>>>>>       kick interrupt thread        | >>>>>>>                                    |    clear IRQF_RUNTHREAD >>>>>>>                                    |    call evtchn_interrupt >>>>>>>       receive interrupt port 6     | >>>>>>>       clear the evtchn port        | >>>>>>>       set IRQF_RUNTHREAD           | >>>>>>>       kick interrupt thread        | >>>>>>>                                    |    disable interrupt port 6 >>>>>>>                                    | evtchn->enabled = false >>>>>>>                                    |    [....] >>>>>>>                                    | >>>>>>>                                    |    *** Handling the second >>>>>>> interrupt *** >>>>>>>                                    |    clear IRQF_RUNTHREAD >>>>>>>                                    |    call evtchn_interrupt >>>>>>>                                    |    WARN(...) >>>>>>> >>>>>>> I am not entirely sure how to fix this. I have two solutions in >>>>>>> mind: >>>>>>> >>>>>>> 1) Prevent the interrupt handler to be threaded. We would also >>>>>>> need to >>>>>>> switch from spin_lock to raw_spin_lock as the former may sleep on >>>>>>> RT-Linux. >>>>>>> >>>>>>> 2) Remove the warning >>>>>> >>>>>> I think access to evtchn->enabled is racy so (with or without the >>>>>> warning) we can't use it reliably. >>>>> >>>>> Thinking about it, it would not be the only issue. The ring is sized >>>>> to contain only one instance of the same event. So if you receive >>>>> twice the event, you may overflow the ring. >>>> >>>> Hm... That's another argument in favor of "unthreading" the handler. >>> >>> I first thought it would be possible to unthread it. However, >>> wake_up_interruptible is using a spin_lock. On RT spin_lock can sleep, >>> so this cannot be used in an interrupt context. >>> >>> So I think "unthreading" the handler is not an option here. >> >> That sounds like a different problem. I.e. there are two issues: >> * threaded interrupts don't work properly (races, ring overflow) >> * evtchn_interrupt() (threaded or not) has spin_lock(), which is not >> going to work for RT > > I am afraid that's not correct, you can use spin_lock() in threaded > interrupt handler. > >> The first can be fixed by using non-threaded handlers. > > The two are somewhat related, if you use a non-threaded handler then > you are not going to help the RT case. > > In general, the unthreaded solution should be used in the last resort. > >>> >>>> >>>>> >>>>>> >>>>>> Another alternative could be to queue the irq if !evtchn->enabled >>>>>> and >>>>>> handle it in evtchn_write() (which is where irq is supposed to be >>>>>> re-enabled). >>>>> What do you mean by queue? Is it queueing in the ring? >>>> >>>> >>>> No, I was thinking about having a new structure for deferred >>>> interrupts. >>> >>> Hmmm, I am not entirely sure what would be the structure here. Could >>> you expand your thinking? >> >> Some sort of a FIFO that stores {irq, data} tuple. It could obviously be >> implemented as a ring but not necessarily as Xen shared ring (if that's >> what you were referring to). > > The underlying question is what happen if you miss an interrupt. Is it > going to be ok? If no, then we have to record everything and can't > kill/send an error to the user app because it is not its fault. > > This means a FIFO would not be a viable. How do you size it? Static > (i.e pre-defined) size is not going to be possible because you don't > know how many interrupt you are going to receive before the thread > handler runs. You can't make it grow dynamically as it make become > quite big for the same reason. > > Discussing with my team, a solution that came up would be to introduce > one atomic field per event to record the number of event received. I > will explore that solution tomorrow. How will this help if events have some payload? > > Cheers, >