From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-32.mta0.migadu.com [91.218.175.32]) (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 982103B83E8 for ; Mon, 14 Sep 2026 07:34:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.32 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789371252; cv=none; b=uSnRto9UNumUHdGu+8jRu0gDgJ1NF6auBCI/CGFulMIoPpYoQKJORMhg8hM4vL4pXgP+TotRc7FFnLB3+xcYeLBChy4pu1SxAemnEaByvddfqNYXMlgiGEYJj4MaejjadepdW3xMOgfmpvlyCt1Muq10u1mc0QHfmuKX4oUKAb8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789371252; c=relaxed/simple; bh=T9yqexVIbUceZI9GQ7YgOL3+cqQQ9ZaJeJTzzuJBpjY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=DKGaJHEh8lwgrvA3GwoJ/BdC6qZSSoAT+PJiwdnDDEJ6vNJCzJ951pI1GlU3M3lui/3JSMDtqXOH9ten5IbTNpgkLcC7BBpu78MRrz8LKYeIruY4tN6couH7qIRTvjD+/NgFX+DeYHCe2PP4CcP07xo4dvQFWTzDQ4UijA1ELjg= 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=b6pCvLLd; arc=none smtp.client-ip=91.218.175.32 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="b6pCvLLd" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=T9yqexVIbUceZI9GQ7YgOL3+cqQQ9ZaJeJTzzuJBpjY=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1789371246; v=1; x=1789976046; b=b6pCvLLdw//ZztLIUFZYl/nNZFtefyttTJgOyQ/tY0LG1xLrz2Ean29fvXELz9cq70Wj62rX goxiJPaFkT8LEW6c/HYdrhCmmeWlQ6P7f+9rJ8RaZP3ahStCynGXk0PeexfvjQR31BRMcLP8TD9 ImvOh8B+ekkSEW2LQkpeyUhY= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id c11aff05638d77e2; Mon, 14 Sep 2026 07:34:06 +0000 X-Mizu-Trace-ID: c11aff05638d77e2 X-Migadu-Flow: FLOW_OUT From: Tao Cui To: tj@kernel.org, josef@toxicopanda.com, axboe@kernel.dk Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, andrii@kernel.org, ast@kernel.org, daniel@iogearbox.net, linux-kselftest@vger.kernel.org, cui.tao@linux.dev, cuitao@kylinos.cn Subject: [RFC PATCH v3 0/5] blk-iocost: BPF struct_ops cost model Date: Mon, 14 Sep 2026 15:33:51 +0800 Message-ID: <20260914073356.791518-1-cui.tao@linux.dev> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tao Cui This is v3 of the RFC. The changes since v2 are listed in the changelog at the bottom. Why a pluggable model at all ---------------------------- When iocost landed in 2019, its commit message already promised that "a later patch will also allow using bpf progs for cost models", and the code has carried the split for it ever since: calc_vtime_cost() is a dispatcher whose only implementation is calc_vtime_cost_builtin(). Seven years later the builtin linear model is still the only one. This series fills that slot, following the TCP congestion control model registration pattern: builtin algorithms remain the default while new ones can be prototyped in BPF. The measured problems --------------------- The builtin model prices each IO with a binary sequential/random base picked by a single per-cgroup cursor and a 16MB seek threshold, plus a per-page cost. On a virtio-blk device with the HDD autop profile, a 4k IO costs ~24us when judged sequential and ~2.7ms when judged random, a 112x spread, so a wrong judgement becomes a wrong price. Three classes of mispricing, all measured: 1. Heuristic rigidity. Two legitimate sequential readers in one cgroup (a database with multiple tablespaces, a threaded backup) ping-pong the single cursor and are all priced random: a measured 89x overcharge collapses throughput under the same weight. Random IO within a hot window smaller than the 16MB threshold is priced sequential: measured 107x undercharge, an accounting escape for hotspot workloads. No setting of the six builtin parameters seems able to fix this: telling the streams apart requires per-IO state tracking, which looks like logic rather than coefficients. 2. Device nonlinearity. SLC-cache phases, SMR band placement and shared controllers (multiple NVMe namespaces multiplexing one device) make the real cost of an identical IO vary by an order of magnitude over time or across namespaces. A static 6-parameter linear model has no way to express that. 3. Unpriced operations. Flush and zone append fall through to a cost of zero and bypass throttling entirely, and the same pattern extends to device quirks the builtin model was never taught. Mispricing feeds directly into the control loop: vtime budgets, surplus donation and the vrate feedback all consume the model's output, so a wrong model can skew the whole controller. How --- A bound BPF model fully owns pricing for every IO on the device: it is called from the bio charging path and prices every operation including flushes. The completion-time request sizing for the latency met/missed accounting still uses the builtin coefficients (the request's bio, and with it the issuing cgroup, is gone by then); extending the model there is an open question of this interface. The builtin cursor is not exposed; a model is expected to track its own stream state. Note that model state keyed by the blkcg alone is shared across every device the model is bound to, unlike the builtin cursor which is per (cgroup, device). u64 calc_cost(u64 opf, u64 nbytes, sector_t sector, struct blkcg *blkcg, u64 model_flags) opf is the full bio->bi_opf (the operation must be extracted with a mask, and the REQ_* flag bits, including PREFLUSH/FUA, are part of it); model_flags carries iocost-specific metadata which is not part of the bio operation flags, such as whether the cost calculation is for a merged request; the return value is vtime, clamped to 1 second of device time per IO. blkcg is passed so the model can key per-cgroup state; state stored in BPF_MAP_TYPE_CGRP_STORAGE follows the cgroup lifetime, and optional blkcg_online()/ blkcg_offline() callbacks mirror the css lifecycle for models which want eager setup or teardown. The registration and binding model follows the TCP congestion control model registration pattern: registering a struct_ops makes the model available by its name, while io.cost.model binds one registered model to a device with "model=" and restores the builtin model with "model=linear". Unregistering a model removes it from the registry so it can no longer be selected by name; devices already using the model keep using it until they are switched back to the builtin model, at which point the reference is released. A model which does not implement calc_cost is rejected at load. Sleepable models are rejected at verification, since calc_cost() runs under RCU read lock. Patch overview: 1/5: the BPF struct_ops cost model support: Kconfig, ops definition, name registry, registration, io.cost.model binding, unified dispatch and verifier checks 2/5: selftest with the 2x example model (the full builtin linear HDD formula at double cost) plus a runner and the selftest kernel config entries 3/5: add an iocost_ioc_tick tracepoint emitting the per-period controller state, so model quality can be evaluated without drgn (existing events are state-change driven and silent in steady state) 4/5: a second example model which replaces the single-cursor sequentiality heuristic with per-cgroup multi-stream detection keyed by the cgroup, the first consumer of the state interface 5/5: document the model= binding in cgroup-v2.rst Does it work ------------ Mechanism, verified functionally (QEMU, virtio-blk with the HDD profile, sequential-read workload from a 1%-weight cgroup, builtin vs the 2x example model): - per-IO charge: 2882us -> 5722us, a factor of 1.985x; the completed IO count halves and total cost.usage is conserved, i.e. the model output drives both charging and budgeting - edge cases: binding an unknown model name fails with ENOENT and nothing is applied; unregistering a bound model leaves the device correctly priced (2x) until it is switched back; the readback shows the bound model name; the selftest runner checks the write error and errno of every step, including the restoration Workload-shape verification added in this revision (same setup, 4k IOs at weight 1000, builtin vs the 2x example model): - flush-heavy workload (read/write/fsync alternating): priced 1.99x the builtin, i.e. flushes no longer reset the cursor and misjudge the following IO as random - non-page-multiple IO (6 KiB): priced ~2x, matching the builtin's truncating page count - first IO from a high LBA (past 16 MiB): priced 2.01x, i.e. a fresh cgroup's zero cursor no longer misjudges the first IO as random Payoff, demonstrated with the multi-stream example model (4/5) on the same setup, 4k IOs at weight 1000, builtin vs the model: - two sequential readers in one cgroup: priced 1961us/op by builtin (both judged random by the single cursor) and 23us/op by the model (each stream keeps its own slot); the completed IO count rises by two orders of magnitude - random IO inside an 8M window: priced 24us/op by builtin (undercharge, an accounting escape) and 2607us/op by the model - single-stream sequential and whole-disk random pricing are unchanged, so the model fixes both directions of mispricing without introducing a new one Non-interference, measured on enterprise NVMe: no measurable overhead when the BPF model is not attached. --- Changes in v3: - blkcg online/offline notifications follow the model binding, not the name registry; an unregistered but still-bound model keeps receiving them until its last reference is dropped - the io.cost.model write resolves the model name before applying anything and swaps the model pointer under ioc->lock, so a bad name rejects the whole write and concurrent writers cannot interleave a half-applied configuration - iocost_ioc_tick reports the period's own state, including its measured duration and the period number that just ended - a model which does not implement calc_cost is rejected at load (the CFI stub previously satisfied the check and priced every IO at 0) - CFI stubs are provided for blkcg_online/blkcg_offline, without which the verifier rejected any model trying to implement the optional lifecycle callbacks - the example models match the builtin cursor semantics: a zero cursor means no previous IO, the cursor is only advanced for bios the builtin prices (a dataless flush no longer resets it), the cursor advance truncates to whole sectors like bio_end_sector(), and the page count truncates like the builtin; stream updates are lockless like the builtin cursor, noted in the example - the selftest builds at every commit (the streams test moved to the patch adding the multi-stream model), includes , checks enable=1, skips instead of failing on devices without iocost, distinguishes the open() errno from the write() errno, and reports the restoration error Tao Cui (5): blk-iocost: add BPF struct_ops cost model support selftests/bpf: add iocost cost model test blk-iocost: add iocost_ioc_tick tracepoint for per-period device summary selftests/bpf: add multi-stream sequentiality example model docs: cgroup-v2: document io.cost model= binding Documentation/admin-guide/cgroup-v2.rst | 12 + block/Kconfig | 9 + block/Makefile | 1 + block/blk-cgroup.c | 3 + block/blk-iocost-bpf.c | 307 ++++++++++++++++++ block/blk-iocost.c | 229 +++++++++++-- include/linux/blk-iocost.h | 83 +++++ include/trace/events/iocost.h | 46 +++ tools/testing/selftests/bpf/config | 2 + .../selftests/bpf/prog_tests/iocost_model.c | 200 ++++++++++++ .../selftests/bpf/progs/iocost_model.c | 134 ++++++++ tools/testing/selftests/bpf/progs/iocost_ms.c | 156 +++++++++ 12 files changed, 1158 insertions(+), 24 deletions(-) create mode 100644 block/blk-iocost-bpf.c create mode 100644 include/linux/blk-iocost.h create mode 100644 tools/testing/selftests/bpf/prog_tests/iocost_model.c create mode 100644 tools/testing/selftests/bpf/progs/iocost_model.c create mode 100644 tools/testing/selftests/bpf/progs/iocost_ms.c -- 2.43.0