linux-kernel - Re: This is the fourth time I’ve tried to find what led to the regression of outgoing network speed and each time I find the merge commit 8c94ccc7cd691472461448f98e2372c75849406c

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Message-ID: <Zb/30qOGYAH4j6Mn@cae.in-ulm.de>
Date: Sun, 4 Feb 2024 21:47:14 +0100
From: "Christian A. Ehrhardt" <lk@...e.de>
To: Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>
Cc: Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
	linux-netdev@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
Subject: Re: This is the fourth time I’ve tried to find what led to the regression of outgoing network speed and each time I find the merge
 commit 8c94ccc7cd691472461448f98e2372c75849406c

Hi,

[ sorry, replying to myself ]

On Sat, Feb 03, 2024 at 07:20:47PM +0100, Christian A. Ehrhardt wrote:
> On Sat, Feb 03, 2024 at 06:02:15AM +0500, Mikhail Gavrilov wrote:
> > Hi,
> > I'm trying to find the first bad commit that led to a decreased
> > network outgoing speed.
> > And every time I come to a huge merge [Merge tag 'usb-6.8-rc1' of
> > git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb]
> > I have already triple-checked all my answers and speed measurements.
> > I don't understand where I'm making a mistake.
> > 
> > Let's try to figure it out together.
> > 
> > Input data:
> > Two computers connected 1Gbps link.
> > Both have the same hardware.
> > Network: RTL8125 2.5GbE Controller (rev 05)
> > 
> > When I copy files from one computer to another and kernel snapshot
> > builded from commit 296455ade1fd I have 97-110MB/sec which is almost
> > max speed of 1Gbps link.
> > When I move to commit 9d1694dc91ce I have only 66-70MB/sec which is
> > significantly slower.
> > 
> > I bisected the issue by measuring network speed on each step.
> > I save all results to file [1]
> > 
> > [1] file is attached as a zip archive.
> > 
> > # first bad commit: [8c94ccc7cd691472461448f98e2372c75849406c] Merge
> > tag 'usb-6.8-rc1' of
> > git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb
> 
> So (simplified) the change history looks something like this:
> [ ... ]

Sorry, I was looking at the wrong merge commit and when using
the commit pinpointed by your bisect your log shows
that _both_ parents of the bad merge commit are marked as good
which is somewhat strange.

However, it should be possible to bisect further if you do a rebase
like this:

$ git cat-file -p
8c94ccc7cd691472461448f98e2372c75849406c  | head -n 3
tree d3907cad2a1fbbbcf71847274fdbdcf5a2aeb9a2
parent bd736f38c014ba70ba7ec3bdc6af6fe5368d6612
parent 933bb7b878ddd0f8c094db45551a7daddf806e00
$ git branch m bd736f38c014ba70ba7ec3bdc6af6fe5368d6612
$ git branch d933bb7b878ddd0f8c094db45551a7daddf806e000
$ git checkout d 
Updating files: 100% (11666/11666), done.
Switched to branch 'd'
$ git rebase m 
Successfully rebased and updated refs/heads/d.

Now, "m" must be good as per your bisect log and "d" must be bad
because it is the same tree as the bad merge commit (8c94ccc7cd69).

Due to the rebase there's a liner history between the two, thus
starting a bisect like this might yield more information:

$ git bisect good m
$ git bisect bad d

     regards   Christian