linux-kernel - Expense of read

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Date:   Thu, 7 Jan 2021 15:11:25 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Eric Sandeen <esandeen@...hat.com>,
        Dave Chinner <dchinner@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        Wang Jianchao <jianchao.wan9@...il.com>,
        "Kani, Toshi" <toshi.kani@....com>,
        "Norton, Scott J" <scott.norton@....com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@....com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-nvdimm@...ts.01.org
Subject: Expense of read_iter

On Thu, Jan 07, 2021 at 08:15:41AM -0500, Mikulas Patocka wrote:
> I'd like to ask about this piece of code in __kernel_read:
> 	if (unlikely(!file->f_op->read_iter || file->f_op->read))
> 		return warn_unsupported...
> and __kernel_write:
> 	if (unlikely(!file->f_op->write_iter || file->f_op->write))
> 		return warn_unsupported...
> 
> - It exits with an error if both read_iter and read or write_iter and 
> write are present.
> 
> I found out that on NVFS, reading a file with the read method has 10% 
> better performance than the read_iter method. The benchmark just reads the 
> same 4k page over and over again - and the cost of creating and parsing 
> the kiocb and iov_iter structures is just that high.

Which part of it is so expensive?  Is it worth, eg adding an iov_iter
type that points to a single buffer instead of a single-member iov?

+++ b/include/linux/uio.h
@@ -19,6 +19,7 @@ struct kvec {
 
 enum iter_type {
        /* iter types */
+       ITER_UBUF = 2,
        ITER_IOVEC = 4,
        ITER_KVEC = 8,
        ITER_BVEC = 16,
@@ -36,6 +36,7 @@ struct iov_iter {
        size_t iov_offset;
        size_t count;
        union {
+               void __user *buf;
                const struct iovec *iov;
                const struct kvec *kvec;
                const struct bio_vec *bvec;

and then doing all the appropriate changes to make that work.