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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Nov 2023 11:44:10 -0500
Message-ID: <CAHk-=whLbJ7vvB1ACVC6t44zjihX8w7GMY2y584+Fm83rsmaKg@mail.gmail.com>
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression
To:     Borislav Petkov <bp@alien8.de>
Cc:     David Howells <dhowells@redhat.com>,
        kernel test robot <oliver.sang@intel.com>,
        oe-lkp@lists.linux.dev, lkp@intel.com,
        linux-kernel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>,
        David Laight <David.Laight@aculab.com>, ying.huang@intel.com,
        feng.tang@intel.com, fengwei.yin@intel.com
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On Fri, 17 Nov 2023 at 11:32, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The test case seems to be (from the profile) just a simple

Just to clarify: that's the test robot test case. David's test-case is
just the simple KUnit test.

> do_iter_readv_writev ->
>   shmem_file_write_iter ->
>     generic_perform_write ->
>       copy_page_from_iter_atomic ->
>         memcpy_from_iter_mc

.. and more details: this is *not* the actual normal "write()" path,
which would copy memory from user space, and that uses our
"copy_user()" function etc.

No, the path leading up to this seems to be

  worker_thread ->
    process_one_work ->
      loop_process_work ->
        lo_write_simple ->
         do_iter_write

which is why it uses a regular memcpy (ie it's a kernel buffer due to
loop block device).

So the test robot load is kind of odd.

Not that I think that David's KUnit test is necessarily much of a real
load either. so...

                  Linus