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[199.203.229.89]) by smtp.gmail.com with ESMTPSA id y13sm4183768wrg.8.2019.09.17.02.13.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Sep 2019 02:14:00 -0700 (PDT) From: Avi Kivity To: Jens Axboe Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org Subject: [PATCH v1] io_uring: reserve word at cqring tail+4 for the user Date: Tue, 17 Sep 2019 12:13:58 +0300 Message-Id: <20190917091358.3652-1-avi@scylladb.com> X-Mailer: git-send-email 2.21.0 MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org In some applications, a thread waits for I/O events generated by the kernel, and also events generated by other threads in the same application. Typically events from other threads are passed using in-memory queues that are not known to the kernel. As long as the threads is active, it polls for both kernel completions and inter-thread completions; when it is idle, it tells the other threads to use an I/O event to wait it up (e.g. an eventfd or a pipe) and then enters the kernel, waiting for such an event or an ordinary I/O completion. When such a thread goes idle, it typically spins for a while to avoid the kernel entry/exit cost in case an event is forthcoming shortly. While it spins it polls both I/O completions and inter-thread queues. The x86 instruction pair UMONITOR/UMWAIT allows waiting for a cache line to be written to. This can be used with io_uring to wait for a wakeup without spinning (and wasting power and slowing down the other hyperthread). Other threads can also wake up the waiter by doing a safe write to the tail word (which triggers the wakeup), but safe writes are slow as they require an atomic instruction. To speed up those wakeups, reserve a word after the tail for user writes. A thread consuming an io_uring completion queue can then use the following sequences: - while busy: - pick up work from the completion queue and from other threads, and process it - while idle: - use UMONITOR/UMWAIT to wait on completions and notifications from other threads for a short period - if no work is picked up, let other threads know you will need a kernel wakeup, and use io_uring_enter to wait indefinitely Signed-off-by: Avi Kivity --- fs/io_uring.c | 5 +++-- include/uapi/linux/io_uring.h | 4 ++++ 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/fs/io_uring.c b/fs/io_uring.c index cfb48bd088e1..4bd7905cee1d 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -77,12 +77,13 @@ #define IORING_MAX_ENTRIES 4096 #define IORING_MAX_FIXED_FILES 1024 struct io_uring { - u32 head ____cacheline_aligned_in_smp; - u32 tail ____cacheline_aligned_in_smp; + u32 head ____cacheline_aligned; + u32 tail ____cacheline_aligned; + u32 reserved_for_user; // for cq ring and UMONITOR/UMWAIT (or similar) wakeups }; /* * This data is shared with the application through the mmap at offset * IORING_OFF_SQ_RING. diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h index 1e1652f25cc1..1a6a826a66f3 100644 --- a/include/uapi/linux/io_uring.h +++ b/include/uapi/linux/io_uring.h @@ -103,10 +103,14 @@ struct io_sqring_offsets { */ #define IORING_SQ_NEED_WAKEUP (1U << 0) /* needs io_uring_enter wakeup */ struct io_cqring_offsets { __u32 head; + // tail is guaranteed to be aligned on a cache line, and to have the + // following __u32 free for user use. This allows using e.g. + // UMONITOR/UMWAIT to wait on both writes to head and writes from + // other threads to the following word. __u32 tail; __u32 ring_mask; __u32 ring_entries; __u32 overflow; __u32 cqes;