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(version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-606-T24G6uPfO7OewlqHwZ9_pA-1; Wed, 27 Apr 2022 05:27:05 -0400 X-MC-Unique: T24G6uPfO7OewlqHwZ9_pA-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EC1722999B53; Wed, 27 Apr 2022 09:27:04 +0000 (UTC) Received: from localhost (dhcp-192-194.str.redhat.com [10.33.192.194]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AD406C44AE9; Wed, 27 Apr 2022 09:27:04 +0000 (UTC) From: Cornelia Huck To: "Michael S. Tsirkin" Cc: Jason Wang , Halil Pasic , 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 Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs() In-Reply-To: <20220426124243-mutt-send-email-mst@kernel.org> Organization: Red Hat GmbH 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> User-Agent: Notmuch/0.34 (https://notmuchmail.org) Date: Wed, 27 Apr 2022 11:27:03 +0200 Message-ID: <87ilqu7u6w.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.85 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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: >>=20 >> > 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: >> >> >=20 >> >> > =E5=9C=A8 2022/4/26 11:38, Michael S. Tsirkin =E5=86=99=E9=81=93: >> >> > > On Mon, Apr 25, 2022 at 11:35:41PM -0400, Michael S. Tsirkin wrot= e: >> >> > > > 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: >> >> > > > >=20 >> >> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wro= te: >> >> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" wrote: >> >> > > > > > > > On Mon, Apr 25, 2022 at 10:44:15AM +0800, Jason Wang wr= ote: >> >> > > > > > > > > This patch tries to implement the synchronize_cbs() f= or ccw. For the >> >> > > > > > > > > vring_interrupt() that is called via virtio_airq_hand= ler(), the >> >> > > > > > > > > synchronization is simply done via the airq_info's lo= ck. For the >> >> > > > > > > > > vring_interrupt() that is called via virtio_ccw_int_h= andler(), a per >> >> > > > > > > > > device spinlock for irq is introduced ans used in the= synchronization >> >> > > > > > > > > method. >> >> > > > > > > > >=20 >> >> > > > > > > > > Cc: Thomas Gleixner >> >> > > > > > > > > Cc: Peter Zijlstra >> >> > > > > > > > > Cc: "Paul E. McKenney" >> >> > > > > > > > > Cc: Marc Zyngier >> >> > > > > > > > > Cc: Halil Pasic >> >> > > > > > > > > Cc: Cornelia Huck >> >> > > > > > > > > Signed-off-by: Jason Wang >> >> > > > > > > >=20 >> >> > > > > > > > This is the only one that is giving me pause. Halil, Co= rnelia, >> >> > > > > > > > should we be concerned about the performance impact her= e? >> >> > > > > > > > Any chance it can be tested? >> >> > > > > > > We can have a bunch of devices using the same airq struct= ure, and the >> >> > > > > > > sync cb creates a choke point, same as registering/unregi= stering. >> >> > > > > > BTW can callbacks for multiple VQs run on multiple CPUs at = the moment? >> >> > > > > I'm not sure I understand the question. >> >> > > > >=20 >> >> > > > > 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. Conni= e what do you think? >> >> > > > >=20 >> >> > > > > On the other hand we could also end up serializing synchroniz= e_cbs() >> >> > > > > calls for different devices if they happen to use the same ai= rq_info. But >> >> > > > > this probably was not your question >> >> > > >=20 >> >> > > > I am less concerned about synchronize_cbs being slow and more = about >> >> > > > the slowdown in interrupt processing itself. >> >> > > >=20 >> >> > > > > > this patch serializes them on a spinlock. >> >> > > > > >=20 >> >> > > > > Those could then pile up on the newly introduced spinlock. >>=20 >> 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 l= ock > 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? 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. > > >> - configuration change interrupts (should be rare) >> - during setup, reset, etc. (should not be a concern)