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Sat, 03 Oct 2026 02:37:00 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Sat, 03 Oct 2026 09:36:59 +0000 Message-Id: Cc: , , , "Kaitao Cheng" Subject: Re: [RFC v3 0/3] block: Introduce a BPF-based I/O scheduler From: "Alexei Starovoitov" To: "Kaitao Cheng" , "Jens Axboe" In-Reply-To: <20261003042748.33795-1-kaitao.cheng@linux.dev> References: <20261003042748.33795-1-kaitao.cheng@linux.dev> X-Mailer: mkdraft (claude review draft; edit before sending) Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, Oct 03, 2026 at 12:27 PM Kaitao Cheng wrote: > PFQ gives us a concrete policy to explore how well the UFQ interface > supports more involved scheduling decisions and to guide further work on > the framework. It has not yet been used in production, and further testing > and workload evaluation are needed. Third version in six months and still not a single number. v2 got replies from the bots only. Without a solid use case there is no point in polishing this. > In particular, I would appreciate suggestions on the boundary between the > UFQ framework and BPF policies, the struct_ops interface, and request > ownership and fallback handling. One global ufq_ops for all disks is not the best shape. I'd do it like bpf_qdisc. The ownership is the bigger problem. The request sits in ctx->rq_lists and in a bpf map at the same time and the kernel relies on the prog to keep the two in sync. The prog holds rq->ref. rq holds q_usage_counter until __blk_mq_free_request(). One request that the prog didn't return from dispatch_req or left in a map and blk_mq_freeze_queue() waits forever. sched_ext has a watchdog that kicks the bpf scheduler out. Something like that is necessary here too. pw-bot: cr