From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 529BB37AA86; Tue, 16 Jun 2026 14:32:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781620372; cv=none; b=hX8aBjYREP8roWsmm+fQrwbfVeOmCy4aYfENwjhjJv+sqTkZSHgFeIxK+skTgIv1zepPQgpNxEySFwkvtiFNtiWp+MatTaAq3JOXFdtRrPWikM3aqWkLrxfYQp3U7jKIdpM5dU6XeSwevHnS1xf7U3mQCFLc3rGpMVqH2GZBL5A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781620372; c=relaxed/simple; bh=NsJ1C3ZFkbjRPWxIw6nC2LMqqGkxqpl8vmqo6unR9YA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tm6L14DHxuzF36KKr0tkXdTU2dE3pQvbJsdhNlcrjiffHqiSOx5EYRsts7Uua8y7OFA6BvxxVTxAd7RB4gfyb3yjFt5uYQ6r7cYvkqfohfNNxRf0rr7vzPPTD2Bp8oiz18ABSyjtGCBTiVucwGmUkpfayp7ZL5L5w/tIv9NN/Rg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TFwIyjAa; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TFwIyjAa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9A4921F000E9; Tue, 16 Jun 2026 14:32:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781620371; bh=cQkPCepMFN/kNHAo58VQi2oFi/6dUIEFRl+CxxOmOyI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TFwIyjAaUrNaD/BiuHM4d1rxBj0xQboZQ0d82GzfKb09s8RRLBl/FKkA5qHYsOFUg krg3WUjkwyM5ELGTwiNGf/fQesE6VF4qAWBeC27S8jBTWnRJ/zbJFcxYOW6ctOi5to wo11Zl9VjXrXXAg8s3Ce39PxZVliu24mu3gXpl24D8xf8JlMtivD6fgSH74i05Dwjf qvXbPSPS24sbAuwGBBtqfNvQY/iA28aiG6DlLF7abKmF352GWRAbhY9I2tfv3Y4PFG VuOqEVVpmADNxBfBEmI3ySGz+k3e7u+wp5Cl2khC3qpTWsNES0i3liot3vqxXBiDyQ g1ETSoG4QMRPQ== Message-ID: <4d8ddf50-e032-44a9-84d4-ddf2cd0c5bf6@kernel.org> Date: Tue, 16 Jun 2026 16:32:46 +0200 Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/2] virtio-balloon: add stats push mode To: Gregory Price Cc: virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, mst@redhat.com, jasowang@redhat.com, xuanzhuo@linux.alibaba.com, eperezma@redhat.com, hannes@cmpxchg.org, surenb@google.com, peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com References: <20260513165006.2790857-1-gourry@gourry.net> <20260513165006.2790857-3-gourry@gourry.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/16/26 15:57, Gregory Price wrote: > On Tue, Jun 16, 2026 at 02:33:43PM +0200, David Hildenbrand (Arm) wrote: >> On 5/13/26 18:50, Gregory Price wrote: >>> >>> The pull model remains available and is the default. >> >> I don't quite see the big benefit here, really: either it's a timer in the >> hypervisor or a timer in the VM. A slow VM will, in either model, delay the >> update of stats. >> >> If you need some "liveness detection", is virtio-balloon stats updates really >> the right mechanism? >> >> I don't quite understand the "Latency-sensitive consumers" problem. If the VM is >> slow, it is slow and will mess with latency-sensitive consumers in either way? >> > > Latency sensitive here should probably be defined as "Does not like > blocking operations". This was prototyped in the context of > cloud-hypervisor [1] and an orchestrator trying poll 1000 VMs on a > single machine for stats. > > The poller couldn't determine the difference between "guest is slow" and > "guest is hung" and so had to block on the operation (I didn't see how > to solve this async). > > Similarly, having a single thread just round-robin poll the VMs is > bluntly inefficient and provides poor guarantees about the liveliness > of the stats (a couple slow guests can cause other guests' stats to > become stale for 10s of seconds). > > Definitely an RFC here because I'm not sure if I was missing something > that might help me solve the problem. Well, in QEMU we just run a timer internally that does the polling. Then, upper layers in the stack can ask QEMU for the latest stats. There, you just get the stats along with a "last-update" timestamp. -- Cheers, David