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Sat, 09 Nov 2019 11:17:12 -0800 (PST) To: Jens Axboe , io-uring@vger.kernel.org References: From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH v3] io_uring: limit inflight IO Message-ID: <8d4610ab-48b0-8f55-27f0-ca760ff5be5f@gmail.com> Date: Sat, 9 Nov 2019 22:16:53 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="OrScgFeg6WSHVa0XElPmR6J8HhxrGAA1S" Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --OrScgFeg6WSHVa0XElPmR6J8HhxrGAA1S Content-Type: multipart/mixed; boundary="OUK9XpncAAGyFc3GUHchY4ROUXqdskUN3"; protected-headers="v1" From: Pavel Begunkov To: Jens Axboe , io-uring@vger.kernel.org Message-ID: <8d4610ab-48b0-8f55-27f0-ca760ff5be5f@gmail.com> Subject: Re: [PATCH v3] io_uring: limit inflight IO References: In-Reply-To: --OUK9XpncAAGyFc3GUHchY4ROUXqdskUN3 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 09/11/2019 18:21, Jens Axboe wrote: > With unbounded request times, we can potentially have a lot of IO > inflight. As we provide no real backpressure unless > IORING_SETUP_CQ_NODROP is set, and even there there's quite some delay > between overflows and backpressure being applied, let's put some safety= > in place to avoid going way overboard. >=20 > This limits the maximum number of inflight IO for any given io_ring_ctx= > to twice the CQ ring size. This is a losely managed limit, we only chec= k > for every SQ ring size number of events. That should be good enough to > achieve our goal, which is to prevent massively deep queues. If these > are async requests, they would just be waiting for an execution slot > anyway. >=20 > We return -EBUSY if we can't queue anymore IO. The caller should reap > some completions and retry the operation after that. Note that this is > a "should never hit this" kind of condition, as driving the depth into > CQ overflow situations is unreliable. >=20 > Signed-off-by: Jens Axboe >=20 > --- >=20 > Changes since v2: >=20 > - Check upfront if we're going over the limit, use the same kind of > cost amortization logic except something that's appropriate for > once-per-batch. >=20 > - Fold in with the backpressure -EBUSY logic >=20 > - Use twice setup CQ ring size as the rough limit >=20 > diff --git a/fs/io_uring.c b/fs/io_uring.c > index 81457913e9c9..9dd0f5b5e5b2 100644 > --- a/fs/io_uring.c > +++ b/fs/io_uring.c > @@ -744,7 +744,7 @@ static struct io_kiocb *io_get_req(struct io_ring_c= tx *ctx, > struct io_kiocb *req; > =20 > if (!percpu_ref_tryget(&ctx->refs)) > - return NULL; > + return ERR_PTR(-ENXIO); > =20 > if (!state) { > req =3D kmem_cache_alloc(req_cachep, gfp); > @@ -790,7 +790,7 @@ static struct io_kiocb *io_get_req(struct io_ring_c= tx *ctx, > if (req) > goto got_it; > percpu_ref_put(&ctx->refs); > - return NULL; > + return ERR_PTR(-EBUSY); > } > =20 > static void io_free_req_many(struct io_ring_ctx *ctx, void **reqs, int= *nr) > @@ -2992,6 +2992,30 @@ static bool io_get_sqring(struct io_ring_ctx *ct= x, struct sqe_submit *s) > return false; > } > =20 > +static bool io_sq_over_limit(struct io_ring_ctx *ctx, unsigned to_subm= it) > +{ > + unsigned inflight; > + > + if ((ctx->flags & IORING_SETUP_CQ_NODROP) && > + !list_empty(&ctx->cq_overflow_list)) > + return true; > + > + /* > + * This doesn't need to be super precise, so only check every once > + * in a while. > + */ > + if ((ctx->cached_sq_head & ctx->sq_mask) !=3D > + ((ctx->cached_sq_head + to_submit) & ctx->sq_mask)) > + return false; ctx->sq_mask =3D sq_entries - 1, e.g. 0x0000...ffff. Thus the code above is modular arithmetic (% sq_entries) and equivalent to: if (to_submit !=3D sq_entries) return false; =20 I suggest, the intention was: if ((sq_head & ~mask) =3D=3D ((sq_head + to_submit) & ~mask)) return false; > + > + /* > + * Limit us to 2x the CQ ring size > + */ > + inflight =3D ctx->cached_sq_head - > + (ctx->cached_cq_tail + atomic_read(&ctx->cached_cq_overflow)); > + return inflight > 2 * ctx->cq_entries; > +} > + > static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr, > struct file *ring_file, int ring_fd, > struct mm_struct **mm, bool async) > @@ -3002,8 +3026,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx= , unsigned int nr, > int i, submitted =3D 0; > bool mm_fault =3D false; > =20 > - if ((ctx->flags & IORING_SETUP_CQ_NODROP) && > - !list_empty(&ctx->cq_overflow_list)) > + if (unlikely(io_sq_over_limit(ctx, nr))) > return -EBUSY; > =20 > if (nr > IO_PLUG_THRESHOLD) { > @@ -3016,9 +3039,9 @@ static int io_submit_sqes(struct io_ring_ctx *ctx= , unsigned int nr, > unsigned int sqe_flags; > =20 > req =3D io_get_req(ctx, statep); > - if (unlikely(!req)) { > + if (unlikely(IS_ERR(req))) { > if (!submitted) > - submitted =3D -EAGAIN; > + submitted =3D PTR_ERR(req); > break; > } > if (!io_get_sqring(ctx, &req->submit)) { > @@ -3039,8 +3062,10 @@ static int io_submit_sqes(struct io_ring_ctx *ct= x, unsigned int nr, > if (link && (sqe_flags & IOSQE_IO_DRAIN)) { > if (!shadow_req) { > shadow_req =3D io_get_req(ctx, NULL); > - if (unlikely(!shadow_req)) > + if (unlikely(IS_ERR(shadow_req))) { > + shadow_req =3D NULL; > goto out; > + } > shadow_req->flags |=3D (REQ_F_IO_DRAIN | REQ_F_SHADOW_DRAIN); > refcount_dec(&shadow_req->refs); > } >=20 --=20 Pavel Begunkov --OUK9XpncAAGyFc3GUHchY4ROUXqdskUN3-- --OrScgFeg6WSHVa0XElPmR6J8HhxrGAA1S Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl3HEKoACgkQWt5b1Glr +6VAvQ/9GtwRyUdQbXdoJK7tvDBFmKwE/YVBgyvjJg/C7bH5et4ZmkWs3MLyLjvd 8VGD+J3+51GxyOWHJyUZDi07bvwumr3gLWrribQc8Ch30tCO98JWAO+GjU4awLtC NlxkqtS9u4t3QkToIEc42e/543ElBHdkf++KifkSHtmWMhAEJ6MW+blxyLRAwsNS AHiFzdXSequaqTRCpEfjCQyyXuSFJbwPF9h3Cz81KAdOkNUc0rnq1OZUtNAWBdVq 8GhImlLIBWlIja9E+f71BxAOwxM68oK/nxXBwe9AqZqNzt0J8/K4/Md7CIN5GQjO DRgCmXQ2XPuqszzHmSudTLcwQR9jqeRJ+E+n5tKMmiyiAH6sGEsa6vSD7wdtSp5S AqdqTwy0J1DQEGIPdUC7Q6CHQlxQCxO7TwkuIhn2cC5OoLCJktjSUFU4KmdxMwDh SmgKnItprqp1+yZFn2+HinSn0bCal80DEdpUck1Ygv+ddhYQjQRYZlxc8sCLIWu/ xlBtHSx1uP/U5FrQTZ2Ut08+WkIgEVbUugxFqC0/o3YED5StqcKp8wLZiYCgPu2F FcG1ENJ70b/OxQmfXKhm6TyASyVB9oCDwPCe077rl6KwaB1Rdx4TFEBhyFGFP6Ns dvkrndTWaN9PTj2AAogFeXW6e0izyHKDu4Zx4f/TchP353Xjtfw= =BISr -----END PGP SIGNATURE----- --OrScgFeg6WSHVa0XElPmR6J8HhxrGAA1S--