linux-kernel - Re: [PATCH] um: time-travel: fix time going backwards

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Date:   Wed, 25 Oct 2023 22:41:38 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Vincent Whitchurch <Vincent.Whitchurch@...s.com>,
        "anton.ivanov@...bridgegreys.com" <anton.ivanov@...bridgegreys.com>,
        "richard@....at" <richard@....at>
Cc:     "linux-um@...ts.infradead.org" <linux-um@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>
Subject: Re: [PATCH] um: time-travel: fix time going backwards

On Wed, 2023-10-25 at 22:02 +0200, Johannes Berg wrote:
> On Wed, 2023-10-25 at 21:51 +0200, Johannes Berg wrote:
> > On Wed, 2023-10-25 at 11:55 +0000, Vincent Whitchurch wrote:
> > > On Mon, 2023-10-23 at 09:33 +0200, Johannes Berg wrote:
> > > > Do you have a specific workload that tends to reproduce this?
> > > 
> > > I've been seeing it when running roadtest, but it's easily reproducible
> > > without that by using the attached config and the following program as
> > > init.
> > > 
> > >   cp repro.config .config
> > >   make ARCH=um olddefconfig all
> > >   gcc -Wall -static -o repro repro.c
> > >   ./linux time-travel init=$PWD/repro rootfstype=hostfs
> 
> Ohhh.
> 
> Pure "time-travel" is actually something I hardly think about these
> days, we mostly use time-travel=inf-cpu (or =ext).
> 
> That makes sense, here you actually *can* get interrupted. I'll need to
> dig into what happens though.

I just sent a patch, please take a look. It does seem to fix it - at
least I got bored of waiting after running your test program for a few
minutes :)

johannes