linux-kernel - RE: BUG: in squashfs_xz_uncompress() (Was: RCU stalls in squashfs

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Date:   Fri, 18 Nov 2022 16:51:26 +0000
From:   "Elliott, Robert (Servers)" <elliott@....com>
To:     Phillip Lougher <phillip@...ashfs.org.uk>,
        Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
CC:     "phillip.lougher@...il.com" <phillip.lougher@...il.com>,
        Thorsten Leemhuis <regressions@...mhuis.info>
Subject: RE: BUG: in squashfs_xz_uncompress() (Was: RCU stalls in
 squashfs_readahead())



> -----Original Message-----
> From: Phillip Lougher <phillip@...ashfs.org.uk>
> Sent: Friday, November 18, 2022 12:11 AM
> To: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>; LKML <linux-
> kernel@...r.kernel.org>; Paul E. McKenney <paulmck@...nel.org>
> Cc: phillip.lougher@...il.com; Thorsten Leemhuis
> <regressions@...mhuis.info>
> Subject: Re: BUG: in squashfs_xz_uncompress() (Was: RCU stalls in
> squashfs_readahead())
> 
> On 17/11/2022 23:05, Mirsad Goran Todorovac wrote:
> > Hi,
> >
> > While trying to bisect, I've found another bug that predated the
> > introduction of squashfs_readahead(), but it has
> > a common denominator in squashfs_decompress() and
> squashfs_xz_uncompress().
> 
> Wrong, the stall is happening in the XZ decompressor library, which
> is *not* in Squashfs.
> 
> This reported stall in the decompressor code is likely a symptom of you
> deliberately thrashing your system.  When the system thrashes everything
> starts to happen very slowly, and the system will spend a lot of
> its time doing page I/O, and the CPU will spend a lot of time in
> any CPU intensive code like the XZ decompressor library.
> 
> So the fact the stall is being hit here is a symptom and not
> a cause.  The decompressor code is likely running slowly due to
> thrashing and waiting on paged-out buffers.  This is not indicative
> of any bug, merely a system running slowly due to overload.
> 
> As I said, this is not a Squashfs issue, because the code when the
> stall takes place isn't in Squashfs.
> 
> The people responsible for the rcu code should have a lot more insight
> about what happens when the system is thrashing, and how this will
> throw up false positives.  In this I believe this is an instance of
> perfectly correct code running slowly due to thrashing incorrectly
> being flagged as looping.
> 
> CC'ing Paul E. McKenney <paulmck@...nel.org>
> 
> Phillip

How big can these readahead sizes be? Should one of the loops include
cond_resched() calls?