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Date: Tue, 26 May 2020 14:58:50 -0400
From: Qian Cai <cai@....pw>
To: Waiman Long <longman@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/lockdep: Increase MAX_LOCKDEP_ENTRIES by half
On Tue, May 26, 2020 at 01:43:49PM -0400, Waiman Long wrote:
> It was found by Qian Cai that lockdep splat sometimes appears with the
> "BUG: MAX_LOCKDEP_ENTRIES too low" message on linux-next. On a 32-vcpu VM
> guest with a v5.7-rc7 based kernel, I looked at how many of the various
> table entries were being used after bootup and after a parallel kernel
> build (make -j32). The tables below show the usage statistics.
I think this approach could help the situatin on this AMD server.
However, I don't understand why this particular system starts to exceed
the MAX_LOCKDEP_ENTRIES. All other Intel, powerpc, amd64 and s390
running the same workload have only < 20k at the end, but on this AMD
server,
After system boot: ~13000 (which is similar to other arches.)
After LTP MM +2000
After LTP Syscalls: +5000
After reading all sysfs [1] (except debugfs): +10000
[1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/fs/read_all/read_all.c
# read_all -d /sys -q -r 10
I still don't understand why reading all sysfs files on this system
could increase that much, but here is the lockdep file after
running sysfs read to see if you could spot anything obviously,
https://cailca.github.io/files/lockdep.txt
lock-classes: 2114 [max: 8192]
direct dependencies: 28568 [max: 32768]
indirect dependencies: 126882
all direct dependencies: 304192
dependency chains: 38269 [max: 65536]
dependency chain hlocks used: 141480 [max: 327680]
dependency chain hlocks lost: 0
in-hardirq chains: 166
in-softirq chains: 818
in-process chains: 37285
stack-trace entries: 175558 [max: 524288]
number of stack traces: 8355
number of stack hash chains: 6545
combined max dependencies: 804750782
hardirq-safe locks: 80
hardirq-unsafe locks: 1048
softirq-safe locks: 169
softirq-unsafe locks: 957
irq-safe locks: 183
irq-unsafe locks: 1048
hardirq-read-safe locks: 4
hardirq-read-unsafe locks: 943
softirq-read-safe locks: 12
softirq-read-unsafe locks: 940
irq-read-safe locks: 12
irq-read-unsafe locks: 943
uncategorized locks: 255
unused locks: 0
max locking depth: 19
max bfs queue depth: 1136
chain lookup misses: 43195
chain lookup hits: 3151662027
cyclic checks: 44009
redundant checks: 0
redundant links: 0
find-mask forwards checks: 7726
find-mask backwards checks: 8095
hardirq on events: 3503795225
hardirq off events: 3503795224
redundant hardirq ons: 453772
redundant hardirq offs: 1856554342
softirq on events: 51104551
softirq off events: 51104551
redundant softirq ons: 0
redundant softirq offs: 0
debug_locks: 1
zapped classes: 1101
zapped lock chains: 1117
large chain blocks: 1
>
> After bootup:
>
> Table Used Max %age
> ----- ---- --- ----
> lock_classes[] 1834 8192 22.4
> list_entries[] 15646 32768 47.7
> lock_chains[] 20873 65536 31.8
> chain_hlocks[] 83199 327680 25.4
> stack_trace[] 146177 524288 27.9
>
> After parallel kernel build:
>
> Table Used Max %age
> ----- ---- --- ----
> lock_classes[] 1864 8192 22.8
> list_entries[] 17134 32768 52.3
> lock_chains[] 25196 65536 38.4
> chain_hlocks[] 106321 327680 32.4
> stack_trace[] 158700 524288 30.3
>
> Percentage-wise, it can be seen that the list_entries for direct
> dependencies are used much more than the other tables. So it is also
> the table that is mostly likely to run out of space when running a
> compex workload.
>
> To reduce the likelihood of running out of table entries, we can increase
> MAX_LOCKDEP_ENTRIES by 50% from 16384/32768 to 24576/49152. On a 64-bit
> architecture, that represents an increase in memory consumption of
> 917504 bytes. With that change, the percentage usage of list_entries[]
> will fall to 31.8% and 34.9% respectively to make them more in line
> with the other tables.
>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
> kernel/locking/lockdep_internals.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
> index baca699b94e9..6108d2fbe775 100644
> --- a/kernel/locking/lockdep_internals.h
> +++ b/kernel/locking/lockdep_internals.h
> @@ -89,12 +89,12 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
> * table (if it's not there yet), and we check it for lock order
> * conflicts and deadlocks.
> */
> -#define MAX_LOCKDEP_ENTRIES 16384UL
> +#define MAX_LOCKDEP_ENTRIES 24576UL
> #define MAX_LOCKDEP_CHAINS_BITS 15
> #define MAX_STACK_TRACE_ENTRIES 262144UL
> #define STACK_TRACE_HASH_SIZE 8192
> #else
> -#define MAX_LOCKDEP_ENTRIES 32768UL
> +#define MAX_LOCKDEP_ENTRIES 49152UL
>
> #define MAX_LOCKDEP_CHAINS_BITS 16
>
> --
> 2.18.1
>
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