From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6D1D5C63777 for ; Fri, 20 Nov 2020 13:29:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1DD4E222BA for ; Fri, 20 Nov 2020 13:29:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725890AbgKTN3T convert rfc822-to-8bit (ORCPT ); Fri, 20 Nov 2020 08:29:19 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([207.82.80.151]:22419 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725801AbgKTN3T (ORCPT ); Fri, 20 Nov 2020 08:29:19 -0500 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-264-OD_kpoYPO02kqUFppqEYAg-1; Fri, 20 Nov 2020 13:29:14 +0000 X-MC-Unique: OD_kpoYPO02kqUFppqEYAg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Fri, 20 Nov 2020 13:29:14 +0000 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Fri, 20 Nov 2020 13:29:14 +0000 From: David Laight To: 'Pavel Begunkov' , "linux-fsdevel@vger.kernel.org" , Alexander Viro CC: Jens Axboe , "linux-block@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec Thread-Topic: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec Thread-Index: AQHWvsvDuqyPv2EDYECI7EFyRCltYanRAzeA Date: Fri, 20 Nov 2020 13:29:14 +0000 Message-ID: References: In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org From: Pavel Begunkov > Sent: 19 November 2020 23:25 > > The block layer spends quite a while in iov_iter_npages(), but for the > bvec case the number of pages is already known and stored in > iter->nr_segs, so it can be returned immediately as an optimisation > > Perf for an io_uring benchmark with registered buffers (i.e. bvec) shows > ~1.5-2.0% total cycle count spent in iov_iter_npages(), that's dropped > by this patch to ~0.2%. > > Reviewed-by: Jens Axboe > Signed-off-by: Pavel Begunkov > --- > lib/iov_iter.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > index 1635111c5bd2..0fa7ac330acf 100644 > --- a/lib/iov_iter.c > +++ b/lib/iov_iter.c > @@ -1594,6 +1594,8 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages) > return 0; > if (unlikely(iov_iter_is_discard(i))) > return 0; > + if (unlikely(iov_iter_is_bvec(i))) > + return min_t(int, i->nr_segs, maxpages); > > if (unlikely(iov_iter_is_pipe(i))) { Is it worth putting an extra condition around these three 'unlikely' cases. ie: if (unlikely((iov_iter_type(i) & (ITER_DISCARD | ITER_BVEC | ITER_PIPE)) { if (iov_iter_is_discard(i)) return 0; if (iov_iter_is_bvec(i)) return min_t(int, i->nr_segs, maxpages); /* Must be ITER_PIPE */ David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)