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Tsirkin" , peterz@infradead.org, maz@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Halil Pasic , eperezma@redhat.com, tglx@linutronix.de X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" T24gV2VkLCAyNyBBcHIgMjAyMiAxMToyNzowMyArMDIwMApDb3JuZWxpYSBIdWNrIDxjb2h1Y2tA cmVkaGF0LmNvbT4gd3JvdGU6Cgo+IE9uIFR1ZSwgQXByIDI2IDIwMjIsICJNaWNoYWVsIFMuIFRz aXJraW4iIDxtc3RAcmVkaGF0LmNvbT4gd3JvdGU6Cj4gCj4gPiBPbiBUdWUsIEFwciAyNiwgMjAy MiBhdCAwNTo0NzoxN1BNICswMjAwLCBDb3JuZWxpYSBIdWNrIHdyb3RlOiAgCj4gPj4gT24gTW9u LCBBcHIgMjUgMjAyMiwgIk1pY2hhZWwgUy4gVHNpcmtpbiIgPG1zdEByZWRoYXQuY29tPiB3cm90 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02:43:20 +0000 Received: from d06av23.portsmouth.uk.ibm.com (d06av23.portsmouth.uk.ibm.com [9.149.105.59]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 23S2hIEF55771408 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 28 Apr 2022 02:43:18 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6F863A404D; Thu, 28 Apr 2022 02:43:18 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C402EA4040; Thu, 28 Apr 2022 02:43:17 +0000 (GMT) Received: from li-e979b1cc-23ba-11b2-a85c-dfd230f6cf82 (unknown [9.171.21.163]) by d06av23.portsmouth.uk.ibm.com (Postfix) with SMTP; Thu, 28 Apr 2022 02:43:17 +0000 (GMT) Date: Thu, 28 Apr 2022 04:43:15 +0200 From: Halil Pasic To: Cornelia Huck Cc: "Michael S. Tsirkin" , Jason Wang , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, sgarzare@redhat.com, eperezma@redhat.com, lulu@redhat.com, tglx@linutronix.de, peterz@infradead.org, paulmck@kernel.org, maz@kernel.org, Halil Pasic Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs() Message-ID: <20220428044315.3945e660.pasic@linux.ibm.com> In-Reply-To: <87ilqu7u6w.fsf@redhat.com> References: <20220425040512-mutt-send-email-mst@kernel.org> <87a6c98rwf.fsf@redhat.com> <20220425095742-mutt-send-email-mst@kernel.org> <20220426042911.544477f9.pasic@linux.ibm.com> <20220425233434-mutt-send-email-mst@kernel.org> <20220425233604-mutt-send-email-mst@kernel.org> <20220425235134-mutt-send-email-mst@kernel.org> <20220425235415-mutt-send-email-mst@kernel.org> <87o80n7soq.fsf@redhat.com> <20220426124243-mutt-send-email-mst@kernel.org> <87ilqu7u6w.fsf@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: nVGGsjJyI-Dp1hqx04GKo6n8n4njrryo X-Proofpoint-GUID: -3sWKC2dVUz214jceQbUHmppgsr5vVfy X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.858,Hydra:6.0.486,FMLib:17.11.64.514 definitions=2022-04-27_04,2022-04-27_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 mlxlogscore=999 bulkscore=0 spamscore=0 priorityscore=1501 malwarescore=0 adultscore=0 suspectscore=0 lowpriorityscore=0 mlxscore=0 clxscore=1015 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2204280012 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 27 Apr 2022 11:27:03 +0200 Cornelia Huck wrote: > On Tue, Apr 26 2022, "Michael S. Tsirkin" wrote: > > > On Tue, Apr 26, 2022 at 05:47:17PM +0200, Cornelia Huck wrote: > >> On Mon, Apr 25 2022, "Michael S. Tsirkin" wrote: > >> > >> > On Mon, Apr 25, 2022 at 11:53:24PM -0400, Michael S. Tsirkin wrote: > >> >> On Tue, Apr 26, 2022 at 11:42:45AM +0800, Jason Wang wrote: > >> >> > > >> >> > 在 2022/4/26 11:38, Michael S. Tsirkin 写道: > >> >> > > On Mon, Apr 25, 2022 at 11:35:41PM -0400, Michael S. Tsirkin wrote: > >> >> > > > On Tue, Apr 26, 2022 at 04:29:11AM +0200, Halil Pasic wrote: > >> >> > > > > On Mon, 25 Apr 2022 09:59:55 -0400 > >> >> > > > > "Michael S. Tsirkin" wrote: > >> >> > > > > > >> >> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote: > >> >> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" wrote: > >> >> > > > > > > > On Mon, Apr 25, 2022 at 10:44:15AM +0800, Jason Wang wrote: > >> >> > > > > > > > > This patch tries to implement the synchronize_cbs() for ccw. For the > >> >> > > > > > > > > vring_interrupt() that is called via virtio_airq_handler(), the > >> >> > > > > > > > > synchronization is simply done via the airq_info's lock. For the > >> >> > > > > > > > > vring_interrupt() that is called via virtio_ccw_int_handler(), a per > >> >> > > > > > > > > device spinlock for irq is introduced ans used in the synchronization > >> >> > > > > > > > > method. > >> >> > > > > > > > > > >> >> > > > > > > > > Cc: Thomas Gleixner > >> >> > > > > > > > > Cc: Peter Zijlstra > >> >> > > > > > > > > Cc: "Paul E. McKenney" > >> >> > > > > > > > > Cc: Marc Zyngier > >> >> > > > > > > > > Cc: Halil Pasic > >> >> > > > > > > > > Cc: Cornelia Huck > >> >> > > > > > > > > Signed-off-by: Jason Wang > >> >> > > > > > > > > >> >> > > > > > > > This is the only one that is giving me pause. Halil, Cornelia, > >> >> > > > > > > > should we be concerned about the performance impact here? > >> >> > > > > > > > Any chance it can be tested? > >> >> > > > > > > We can have a bunch of devices using the same airq structure, and the > >> >> > > > > > > sync cb creates a choke point, same as registering/unregistering. > >> >> > > > > > BTW can callbacks for multiple VQs run on multiple CPUs at the moment? > >> >> > > > > I'm not sure I understand the question. > >> >> > > > > > >> >> > > > > I do think we can have multiple CPUs that are executing some portion of > >> >> > > > > virtio_ccw_int_handler(). So I guess the answer is yes. Connie what do you think? > >> >> > > > > > >> >> > > > > On the other hand we could also end up serializing synchronize_cbs() > >> >> > > > > calls for different devices if they happen to use the same airq_info. But > >> >> > > > > this probably was not your question > >> >> > > > > >> >> > > > I am less concerned about synchronize_cbs being slow and more about > >> >> > > > the slowdown in interrupt processing itself. > >> >> > > > > >> >> > > > > > this patch serializes them on a spinlock. > >> >> > > > > > > >> >> > > > > Those could then pile up on the newly introduced spinlock. > >> > >> How bad would that be in practice? IIUC, we hit on the spinlock when > >> - doing synchronize_cbs (should be rare) > >> - processing queue interrupts for devices using per-device indicators > >> (which is the non-preferred path, which I would basically only expect > >> when running on an ancient or non-standard hypervisor) > > > > this one is my concern. I am worried serializing everything on a single lock > > will drastically regress performance here. > > Yeah, that case could get much worse. OTOH, how likely is it that any > setup that runs a recent kernel will actually end up with devices using > per-device indicators? Anything running under a QEMU released in the > last couple of years is unlikely to not use airqs, I think. Halil, do > you think that the classic indicator setup would be more common on any > non-QEMU hypervisors? > I really don't know. My opinion is that, two stages indicators are kind of recommended for anybody who cares about notifications performance. > IOW, how much effort is it worth spending on optimizing this case? We > certainly should explore any simple solutions, but I don't think we need > to twist ourselves into pretzels to solve it. > Frankly, I would be fine with an rwlock based solution as proposed by Jason. My rationale is: we recommend two stage indicators, and the two stage indicators are already encumbered by an rwlock on the interrupt path. Yes, the coalescence of adapter interrupts is architecturally different, and so it is with GISA (without GISA, I'm not even sure), so this rwlock end up being worse than the one for 2 stage. But my feeling is, that it should be fine. On the other hand, I don't feel comfortable with plain spinlock, and I am curious about a more advanced solution. But my guess is that rwlock + some testing for the legacy indicator case just to double check if there is a heavy regression despite of our expectations to see none should do the trick. Regards, Halil > > > > > >> - configuration change interrupts (should be rare) > >> - during setup, reset, etc. (should not be a concern) >