Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Pavel Begunkov <asml.silence@gmail.com>
To:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org
Subject: [PATCH 1/2] iov_iter: optimise iov_iter_npages for bvec
Date:   Thu, 19 Nov 2020 15:29:42 +0000
Message-Id: <b52c2ad9e2aaf6129d60aac595df86231c8373ef.1605799583.git.asml.silence@gmail.com>
In-Reply-To: <cover.1605799583.git.asml.silence@gmail.com>
References: <cover.1605799583.git.asml.silence@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Block layer spend quite a while in iov_iter_npages(), but for bvecs
number of pages is already known and stored in iter->nr_segs, so it can
be returned directly as an optimisation

Running an io_uring benchmark with registered buffers (i.e. bvec) perf
showed ~1.5-2.0% total cycle was spent there, and that dropped to ~0.2%
after applying this patch.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 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))) {
 		struct pipe_inode_info *pipe = i->pipe;
@@ -1614,11 +1616,9 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
 			- p / PAGE_SIZE;
 		if (npages >= maxpages)
 			return maxpages;
-	0;}),({
-		npages++;
-		if (npages >= maxpages)
-			return maxpages;
-	}),({
+	0;}),
+		0 /* bvecs are handled above */
+	,({
 		unsigned long p = (unsigned long)v.iov_base;
 		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
 			- p / PAGE_SIZE;
-- 
2.24.0