linux-kernel - Re: BUG: MAX_LOCKDEP_CHAIN

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Message-ID: <Y9LNoDtOK4nun968@boqun-archlinux>
Date:   Thu, 26 Jan 2023 10:59:44 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>, dsterba@...e.cz,
        Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Chris Murphy <lists@...orremedies.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!

On Thu, Jan 26, 2023 at 01:30:34PM -0500, Waiman Long wrote:
> On 1/26/23 12:38, Boqun Feng wrote:
> > [Cc lock folks]
> > 
> > On Thu, Jan 26, 2023 at 02:47:42PM +0500, Mikhail Gavrilov wrote:
> > > On Wed, Jan 25, 2023 at 10:21 PM David Sterba <dsterba@...e.cz> wrote:
> > > > On Wed, Jan 25, 2023 at 01:27:48AM +0500, Mikhail Gavrilov wrote:
> > > > > On Tue, Jul 26, 2022 at 9:47 PM David Sterba <dsterba@...e.cz> wrote:
> > > > > > On Tue, Jul 26, 2022 at 05:32:54PM +0500, Mikhail Gavrilov wrote:
> > > > > > > Hi guys.
> > > > > > > Always with intensive writing on a btrfs volume, the message "BUG:
> > > > > > > MAX_LOCKDEP_CHAIN_HLOCKS too low!" appears in the kernel logs.
> > > > > > Increase the config value of LOCKDEP_CHAINS_BITS, default is 16, 18
> > > > > > tends to work.
> > > > > Hi,
> > > > > Today I was able to get the message "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too
> > > > > low!" again even with LOCKDEP_CHAINS_BITS=18 and kernel 6.2-rc5.
> > > > > 
> > > > > ❯ cat /boot/config-`uname -r` | grep LOCKDEP_CHAINS_BITS
> > > > > CONFIG_LOCKDEP_CHAINS_BITS=18
> > > > > 
> > > > > [88685.088099] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
> > > > > [88685.088124] turning off the locking correctness validator.
> > > > > [88685.088133] Please attach the output of /proc/lock_stat to the bug report
> > > > > [88685.088142] CPU: 14 PID: 1749746 Comm: mv Tainted: G        W    L
> > > > >    -------  ---  6.2.0-0.rc5.20230123git2475bf0250de.38.fc38.x86_64 #1
> > > > > [88685.088154] Hardware name: System manufacturer System Product
> > > > > Name/ROG STRIX X570-I GAMING, BIOS 4408 10/28/2022
> > > > > 
> > > > > What's next? Increase this value to 19?
> > > > Yes, though increasing the value is a workaround so you may see the
> > > > warning again.
> > > Is there any sense in this WARNING if we would ignore it and every
> > > time increase the threshold value?
> > Lockdep uses static allocated array to track lock holdings chains to
> > avoid dynmaic memory allocation in its own code. So if you see the
> > warning it means your test has more combination of lock holdings than
> > the array can record. In other words, you reach the resource limitation,
> > and in that sense it makes sense to just ignore it and increase the
> > value: you want to give lockdep enough resource to work, right?
> > 
> > > May Be set 99 right away? Or remove such a check condition?
> > That requires having 2^99 * 5 * sizeof(u16) memory for lock holding
> > chains array..
> 
> Note that every increment of LOCKDEP_CHAINS_BITS double the storage space.
> With 99, that will likely exceed the total amount of memory you have in your
> system.
> 
> Boqun, where does the 5 figure come from. It is just a simple u16 array of

	#define MAX_LOCKDEP_CHAINS_BITS	CONFIG_LOCKDEP_CHAINS_BITS
	#define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)

	#define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5)

I think the last one means we think the average length of a lock chain
is 5, in other words, in average, a task hold at most 5 locks. I don't
know where the 5 came from either, but it's there ;-)

Regards,
Boqun

> size MAX_LOCKDEP_CHAIN_HLOCKS. The chain_hlocks array stores the lock chains
> that show up in the lockdep splats and in the /proc/lockdep* files. Each
> chain is variable size. As we add new lock into the chain, we have to
> repeatedly deallocate and reallocate a larger chain buffer. That will cause
> fragmentation in the chain_hlocks[]. So if we have a very long lock chain,
> the allocation may fail because the largest free block is smaller than the
> requested chain length. There may be enough free space in chain_hlocks, but
> it is just too fragmented to be useful.
> 
> Maybe we should figure out a better way to handle this fragmentation. In the
> mean time, the easiest way forward is just to increase the
> LOCKDEP_CHAINS_BITS by 1.
> 
> > 
> > However, a few other options we can try in lockdep are:
> > 
> > *	warn but not turn off the lockdep: the lock holding chain is
> > 	only a cache for what lock holding combination lockdep has ever
> > 	see, we also record the dependency in the graph. Without the
> > 	lock holding chain, lockdep can still work but just slower.
> > 
> > *	allow dynmaic memory allocation in lockdep: I think this might
> > 	be OK since we have lockdep_recursion to avoid lockdep code ->
> > 	mm code -> lockdep code -> mm code ... deadlock. But maybe I'm
> > 	missing something. And even we allow it, the use of memory
> > 	doesn't change, you will still need that amout of memory to
> > 	track lock holding chains.
> 
> It is not just the issue of calling the memory allocator. There is also the
> issue of copying data from old chain_hlocks to new one while the old one may
> be updated during the copying process unless we can freeze everything else.
> 
> Cheers,
> Longman
>