linux-ext4 - Re: Kernel Benchmarking

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Date:   Tue, 15 Sep 2020 12:24:52 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthieu Baerts <matthieu.baerts@...sares.net>
Cc:     Michael Larabel <Michael@...haellarabel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Amir Goldstein <amir73il@...il.com>,
        "Ted Ts'o" <tytso@...gle.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernel Benchmarking

On Tue, Sep 15, 2020 at 12:01 PM Matthieu Baerts
<matthieu.baerts@...sares.net> wrote:
>
> Earlier today, I got one trace with 'sysrq-T' but it is more than 1100
> lines. It is attached to this email also with a version from
> "decode_stacktrace.sh", I hope that's alright.

Yeah, there's nothing interesting there.

The only relevant tasks seem to be the packetdrill ones that are
blocked on the page lock. I don't see anything that looks even
*remotely* like it could be holding a page lock and be waiting for
anything else.

A couple of pipe readers, a number of parents waiting on their
children, one futex waiter, one select loop.. Nothing at all
unexpected or remotely suspicious.

The packetdrill ones look very similar.

> I forgot one important thing, I was on top of David Miller's net-next
> branch by reflex. I can redo the traces on top of linux-next if needed.

Not likely an issue.

I'll go stare at the page lock code again to see if I've missed
anything. I still suspect it's a latent ABBA deadlock that is just
much *much* easier to trigger with the synchronous lock handoff, but I
don't see where it is.

I guess this is all fairly theoretical since we apparently need to do
that hybrid "limited fairness" patch anyway, and it fixes your issue,
but I hate not understanding the problem.

              Linus