From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-167.mta0.migadu.com [91.218.175.167]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F17943BE633 for ; Mon, 21 Sep 2026 07:27:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.167 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789975647; cv=none; b=b4LOK/8X6FQgyNTJml66xLBxiu2/Wmm9tybhopxZhiKsU7LRtEtOZH7ukRiG8hkt0OxwkFysg2229tTKb7IxPYPjiI/Fmr8hyu0PnRriWsn+2MN3dcsdg8lpOX2u1wb0N4b9zBQgv9sW9WFNjzhCviGROEuai3atNQwGL1WKp+s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789975647; c=relaxed/simple; bh=IXoexxxwMj9ZHbqMM/S/tzSKGTmKj2ZFLeJiOwKWMgU=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=r7vOGP2W78UpIEdZOwHFj6zpKNPdD3x7WraUmb8OB2wxOFfjpUbXuvMf3cu5rSi1lS6e2gjkS040aIn8ZEPxyAaax6WEi8FEy6FX/vJ+sNZPv3wf6NLzCzJZL+eEYH+ZXWe6W+xAqMrU1OBzMRcE7RPc1fFWt1Lrbvh+YwrZS9g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=KYZAW2Ba; arc=none smtp.client-ip=91.218.175.167 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="KYZAW2Ba" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=IXoexxxwMj9ZHbqMM/S/tzSKGTmKj2ZFLeJiOwKWMgU=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1789975640; v=1; x=1790580440; b=KYZAW2BaT7BOilkJs6DE8Xrqj7Pl+P8wU/ntuSXDngs4dAhuEd3IcQO/o+tC3an4ztPbf0Qs W5j/3gIOb4xpd2oxwwKxZFRInNytNJK43PVprtE2G7WtCpOQxZ0Aac1FRXldw3INfSOqCoj7EX9 /a988H9Z7iBO03z84jGT8IpM= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 10294fe07906a545; Mon, 21 Sep 2026 07:27:20 +0000 X-Mizu-Trace-ID: 10294fe07906a545 X-Migadu-Flow: FLOW_OUT Message-ID: Date: Mon, 21 Sep 2026 15:27:10 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: cui.tao@linux.dev, linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, cuitao@kylinos.cn Subject: Re: [RFC PATCH 0/1] block: charge passthrough requests to the submitter's cgroup To: axboe@kernel.dk, kbusch@kernel.org, tj@kernel.org, hch@lst.de, sagi@grimberg.me, yukuai@fygo.io References: <20260921070647.1928289-1-cui.tao@linux.dev> From: Tao Cui In-Reply-To: <20260921070647.1928289-1-cui.tao@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi, 在 2026/9/21 15:06, Tao Cui 写道: > From: Tao Cui > > While looking at cgroup IO control coverage, I stumbled into a hole > that is probably familiar to people who have worked on passthrough: > SG_IO, bsg, and the NVMe ioctl / uring command paths build requests > directly and dispatch them through blk_execute_rq{,_nowait}(), > never passing through submit_bio(). The mapped bio carries no blkcg > association, so the bytes never show up in cgroup io.stat, and none > of the rq_qos policies (iocost, iolatency, wbt) see these commands; > blk-throttle, hooking submit_bio_noacct() directly, is equally > blind. On a scsi_debug device with iocost enabled and vrate pinned > to its 1% floor, a direct writer in a cgroup is throttled ~10x while > the same cgroup issuing sg_dd writes runs at full device speed with > io.stat staying at zero. > > This RFC is a prototype asking for feedback on one possible > direction, not a proposal I am attached to: is charging passthrough > at dispatch time the right layer and the right semantics at all? > > The patch associates the mapped bio with the submitter's blkcg in > blk_execute_rq{,_nowait}(), runs the regular bio accounting > (blk_cgroup_bio_start()) and the rq_qos throttle path with it, and > gates by opcode (READ/WRITE/DRV_IN/DRV_OUT, mapping the latter two > to READ/WRITE for io.stat classification) and to queues that already > have a gendisk so probing stays exempt. On linux-next, with > scsi_debug + sg_dd and qemu emulated nvme + an NVME_IOCTL_IO64_CMD > loop, the transferred bytes are fully accounted in the issuing > cgroup, and with vrate clamped the pacing bites: the same 128MB > SG_IO run takes 0.025s unthrottled and 31.7s at the 1% floor, with > the issuing task sleeping on the iocost waitqueue like any bio > submitter. > > Questions I am unsure about: > > 1. Is always-on the right default, or should this hide behind the > queue/iostats_passthrough opt-in from the passthrough iostats > series, keeping default behavior unchanged? > 2. The throttle may sleep; all callers I found (ioctl / uring_cmd > submit, target and error handling kthreads) are sleepable, and > kthread-issued charges land on root and turn into no-ops - but > I may have missed paths, and I'd rather ask than assume. > 3. What should happen on nvme multipath failover, where the bio is > re-routed to another queue but carries the original queue's > blkg? Untested; guidance welcome. > 4. wbt also gains passthrough throttling through the same rq_qos > walk - probably benign since wbt targets buffered writeback, > but it is a behavior change I did not set out to make. > 5. blk-throttle does not benefit (it is not an rq_qos policy, and > its queue-and-resubmit model does not fit bios already bound to > a dispatched request); would a synchronous wait variant there > be welcome as follow-up work, or is leaving it alone preferred? > > Reproduction notes in case anyone tries: sg_dd coalesces into very > large single commands by default (use bpt=1), and a short burst from > a single cgroup rides the initial budget, so sustained transfers > are what show the pacing. > The cover letter summarized the motivation briefly, so let me add some background on how this came up and what the prototype is trying to address. How this was found ------------------ This came from stress testing blk-iocost on linux-next with mixed IO workloads and cgroup churn. With the known bio-path accounting gaps (flush, zone append) already carrying posted fixes, the remaining missing cases had one property in common: they build requests directly and enter through blk_execute_rq(), bypassing submit_bio() entirely. The invariant being violated is simple to state: any IO that consumes device bandwidth but bypasses submit_bio() currently bypasses blkcg IO accounting. Passthrough commands, including SG_IO, bsg, nvme ioctls and uring commands, currently fall into this category: they have no blkcg association, do not show up in io.stat, and do not participate in blkcg-aware rq_qos policies such as iocost and iolatency. The observable difference ------------------------- On the same scsi_debug device, with the same cgroup and iocost enabled with vrate pinned at its 1% floor: bio writer (fio) throttled ~10x SG_IO writer (sg_dd) full device speed, io.stat remains zero The NVMe ioctl path shows the same behavior: passthrough provides a path that bypasses the existing accounting boundary. With the prototype patch ------------------------ The mapped bio is associated with the submitting blkcg in blk_execute_rq{,_nowait}(), and goes through the normal bio accounting and rq_qos paths. With the same setup: - a 128MB SG_IO write (bs=1M, bpt=1) is accounted to the issuing cgroup's io.stat, with wbytes/wios matching the transfer; - the same workload takes 0.025s without throttling and 31.7s with vrate at the 1% floor, with the task sleeping in the iocost throttle path. Two reproduction notes: sg_dd combines transfers into large commands by default, so bpt=1 is needed for scsi_debug. Also, short bursts can be covered by the initial budget; sustained transfers make the difference visible. Relation to passthrough iostats ------------------------------- This RFC builds on Keith's passthrough iostats work. That series made disk-level passthrough accounting possible and provided block-device association for passthrough bios where it is available. This RFC is about the next layer: whether passthrough IO should also participate in blkcg accounting and enforcement. The questions from the cover letter still stand. In particular, I am not sure whether attaching blkcg/accounting at this layer is the right abstraction, or whether this should remain an opt-in behavior like passthrough iostats. Thanks, Tao > > Tao Cui (1): > block: charge passthrough requests to the submitter's cgroup > > block/blk-mq.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 72 insertions(+) >