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[2003:cf:d73b:4190:ba24:b3c:c06d:9990]) by smtp.gmail.com with ESMTPSA id o6-20020a05600c510600b0040d5ae2905asm26442134wms.30.2024.01.02.08.55.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Jan 2024 08:55:54 -0800 (PST) Content-Type: multipart/alternative; boundary="------------zEzhaS0d0eLVCit0nm0XjfLf" Message-ID: Date: Tue, 2 Jan 2024 17:55:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 0/3] aio-posix: call ->poll_end() when removing AioHandler Content-Language: en-US To: Paolo Bonzini Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, Kevin Wolf , qemu-block@nongnu.org, Fam Zheng , Fiona Ebner References: <20231213211544.1601971-1-stefanha@redhat.com> <142d6078-1bb9-4116-ac87-7daac16f12d8@redhat.com> From: Hanna Czenczek In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.178, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org This is a multi-part message in MIME format. --------------zEzhaS0d0eLVCit0nm0XjfLf Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 02.01.24 16:53, Paolo Bonzini wrote: > On Tue, Jan 2, 2024 at 4:24 PM Hanna Czenczek wrote: >> I’ve attached the preliminary patch that I didn’t get to send (or test >> much) last year. Not sure if it has the same CPU-usage-spike issue >> Fiona was seeing, the only functional difference is that I notify the vq >> after attaching the notifiers instead of before. > I think the patch makes sense and cleaning up the logic of aio_poll > (which is one of those functions that grew and grew without much > clarity into who did what) can be done on top. > > Just one small thing, the virtio_queue_notify_vq() call is required > because the virtqueue interrupt and eventfd are edge-triggered rather > than level-triggered; so perhaps it should be placed in the > function(s) that establish the handlers, > virtio_queue_aio_attach_host_notifier() and > virtio_queue_aio_attach_host_notifier_no_poll()? Neither > virtio_blk_drained_end() nor virtio_scsi_drained_end() are > particularly special, and the comment applies just as well: > > /* > * We will have ignored notifications about new requests from the guest > * while handlers were not attached, so "kick" the virt queue to process > * those requests now. > */ I wasn’t entirely whether we want to call notify_vq() if we have instated the handlers for the first time (in which case someone ought to look for existing unprocessed requests anyway), so I decided to limit it to drained_end. But considering that it must be safe to call notify_vq() right after instating the handlers (virtio_queue_host_notifier_read() may then be called after all), we might as well do so every time, yes. Hanna --------------zEzhaS0d0eLVCit0nm0XjfLf Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
On 02.01.24 16:53, Paolo Bonzini wrote:
On Tue, Jan 2, 2024 at 4:24 PM Hanna Czenczek <hreitz@redhat.com> wrote:

I’ve attached the preliminary patch that I didn’t get to send (or test
much) last year.  Not sure if it has the same CPU-usage-spike issue
Fiona was seeing, the only functional difference is that I notify the vq
after attaching the notifiers instead of before.

I think the patch makes sense and cleaning up the logic of aio_poll
(which is one of those functions that grew and grew without much
clarity into who did what) can be done on top.

Just one small thing, the virtio_queue_notify_vq() call is required
because the virtqueue interrupt and eventfd are edge-triggered rather
than level-triggered; so perhaps it should be placed in the
function(s) that establish the handlers,
virtio_queue_aio_attach_host_notifier() and
virtio_queue_aio_attach_host_notifier_no_poll()? Neither
virtio_blk_drained_end() nor virtio_scsi_drained_end() are
particularly special, and the comment applies just as well:

    /*
     * We will have ignored notifications about new requests from the guest
     * while handlers were not attached, so "kick" the virt queue to process
     * those requests now.
     */

I wasn’t entirely whether we want to call notify_vq() if we have instated the handlers for the first time (in which case someone ought to look for existing unprocessed requests anyway), so I decided to limit it to drained_end.

But considering that it must be safe to call notify_vq() right after instating the handlers (virtio_queue_host_notifier_read() may then be called after all), we might as well do so every time, yes.

Hanna
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