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Tsirkin" , Jason Wang Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs() In-Reply-To: <20220425235415-mutt-send-email-mst@kernel.org> Organization: Red Hat GmbH References: <20220425024418.8415-1-jasowang@redhat.com> <20220425024418.8415-7-jasowang@redhat.com> <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> User-Agent: Notmuch/0.34 (https://notmuchmail.org) Date: Tue, 26 Apr 2022 17:47:17 +0200 Message-ID: <87o80n7soq.fsf@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.85 on 10.11.54.9 Cc: paulmck@kernel.org, lulu@redhat.com, peterz@infradead.org, maz@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Halil Pasic , 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localhost (unknown [10.39.192.48]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0FD6B54F8A1; Tue, 26 Apr 2022 15:47:18 +0000 (UTC) From: Cornelia Huck To: "Michael S. Tsirkin" , Jason Wang Cc: 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: <20220425235415-mutt-send-email-mst@kernel.org> Organization: Red Hat GmbH References: <20220425024418.8415-1-jasowang@redhat.com> <20220425024418.8415-7-jasowang@redhat.com> <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> User-Agent: Notmuch/0.34 (https://notmuchmail.org) Date: Tue, 26 Apr 2022 17:47:17 +0200 Message-ID: <87o80n7soq.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 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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: >> >=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 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: >> > > > >=20 >> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote: >> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" w= rote: >> > > > > > > > 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_hand= ler(), a per >> > > > > > > > > device spinlock for irq is introduced ans used in the sy= nchronization >> > > > > > > > > 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, Corne= lia, >> > > > > > > > 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/unregiste= ring. >> > > > > > 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 por= tion of >> > > > > virtio_ccw_int_handler(). So I guess the answer is yes. Connie w= hat do you think? >> > > > >=20 >> > > > > On the other hand we could also end up serializing synchronize_c= bs() >> > > > > calls for different devices if they happen to use the same airq_= info. But >> > > > > this probably was not your question >> > > >=20 >> > > > I am less concerned about synchronize_cbs being slow and more abo= ut >> > > > 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. 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) - configuration change interrupts (should be rare) - during setup, reset, etc. (should not be a concern) >> > > > >=20 >> > > > > Regards, >> > > > > Halil >> > > > Hmm yea ... not good. >> > > Is there any other way to synchronize with all callbacks? >> >=20 >> >=20 >> > Maybe using rwlock as airq handler? >> >=20 >> > Thanks >> >=20 >>=20 >> rwlock is still a shared cacheline bouncing between CPUs and >> a bunch of ordering instructions. >> Maybe something per-cpu + some IPIs to run things on all CPUs instead? > > ... and I think classic and device interrupts are different enough > here ... You mean classic (per-device) and adapter interrupts, right?