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From: Waiman Long <longman@redhat.com>
To: Ingo Molnar <mingo@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
	linux-mm@kvack.org, Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Tejun Heo <tj@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks
Date: Fri, 9 Nov 2018 10:48:39 -0500	[thread overview]
Message-ID: <1fcaa330-a4be-0f8a-7974-7b17f0ce01ad@redhat.com> (raw)
In-Reply-To: <20181109080412.GC86700@gmail.com>

On 11/09/2018 03:04 AM, Ingo Molnar wrote:
> * Waiman Long <longman@redhat.com> wrote:
>
>> The purpose of this patchset is to add a new class of locks called
>> terminal locks and converts some of the low level raw or regular
>> spinlocks to terminal locks. A terminal lock does not have forward
>> dependency and it won't allow a lock or unlock operation on another
>> lock. Two level nesting of terminal locks is allowed, though.
>>
>> Only spinlocks that are acquired with the _irq/_irqsave variants or
>> acquired in an IRQ disabled context should be classified as terminal
>> locks.
>>
>> Because of the restrictions on terminal locks, we can do simple checks on
>> them without using the lockdep lock validation machinery. The advantages
>> of making these changes are as follows:
>>
>>  1) The lockdep check will be faster for terminal locks without using
>>     the lock validation code.
>>  2) It saves table entries used by the validation code and hence make
>>     it harder to overflow those tables.
>>
>> In fact, it is possible to overflow some of the tables by running
>> a variety of different workloads on a debug kernel. I have seen bug
>> reports about exhausting MAX_LOCKDEP_KEYS, MAX_LOCKDEP_ENTRIES and
>> MAX_STACK_TRACE_ENTRIES. This patch will help to reduce the chance
>> of overflowing some of the tables.
>>
>> Performance wise, there was no statistically significant difference in
>> performanace when doing a parallel kernel build on a debug kernel.
> Could you please measure a locking intense workload instead, such as:
>
>    $ perf stat --null --sync --repeat 10 perf bench sched messaging
>
> and profile which locks used there could be marked terminal, and measure 
> the before/after performance impact?

I will run the test. It will probably be done after the LPC next week.

>> Below were selected output lines from the lockdep_stats files of the
>> patched and unpatched kernels after bootup and running parallel kernel
>> builds.
>>
>>   Item                     Unpatched kernel  Patched kernel  % Change
>>   ----                     ----------------  --------------  --------
>>   direct dependencies           9732             8994          -7.6%
>>   dependency chains            18776            17033          -9.3%
>>   dependency chain hlocks      76044            68419         -10.0%
>>   stack-trace entries         110403           104341          -5.5%
> That's pretty impressive!
>
>> There were some reductions in the size of the lockdep tables. They were
>> not significant, but it is still a good start to rein in the number of
>> entries in those tables to make it harder to overflow them.
> Agreed.
>
> BTW., if you are interested in more radical approaches to optimize 
> lockdep, we could also add a static checker via objtool driven call graph 
> analysis, and mark those locks terminal that we can prove are terminal.
>
> This would require the unified call graph of the kernel image and of all 
> modules to be examined in a final pass, but that's within the principal 
> scope of objtool. (This 'final pass' could also be done during bootup, at 
> least in initial versions.)
>
> Note that beyond marking it 'terminal' such a static analysis pass would 
> also allow the detection of obvious locking bugs at the build (or boot) 
> stage already - plus it would allow the disabling of lockdep for 
> self-contained locks that don't interact with anything else.
>
> I.e. the static analysis pass would 'augment' lockdep and leave only 
> those locks active for runtime lockdep tracking whose dependencies it 
> cannot prove to be correct yet.

It is a pretty interesting idea to use objtool to scan for locks. The
list of locks that I marked as terminal in this patch was found by
looking at /proc/lockdep for those that only have backward dependencies,
but no forward dependency. I focused on those with a large number of BDs
and check the code to see if they could marked as terminal. This is a
rather labor intensive process and is subject to error. It would be nice
if it can be done by an automated tool. So I am going to look into that,
but it won't be part of this initial patchset, though.

I sent this patchset out to see if anyone has any objection to it. It
seems you don't have any objection to that. So I am going to move ahead
to do more testing and performance analysis.

Thanks,
Longman


  reply	other threads:[~2018-11-09 15:48 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-08 20:34 Waiman Long
2018-11-08 20:34 ` [RFC PATCH 01/12] locking/lockdep: Rework lockdep_set_novalidate_class() Waiman Long
2018-11-10 14:14   ` Peter Zijlstra
2018-11-11  0:26     ` Waiman Long
2018-11-11  1:28       ` Peter Zijlstra
2018-11-08 20:34 ` [RFC PATCH 02/12] locking/lockdep: Add a new terminal lock type Waiman Long
2018-11-10 14:17   ` Peter Zijlstra
2018-11-11  0:28     ` Waiman Long
2018-11-08 20:34 ` [RFC PATCH 03/12] locking/lockdep: Add DEFINE_TERMINAL_SPINLOCK() and related macros Waiman Long
2018-11-08 20:34 ` [RFC PATCH 04/12] printk: Make logbuf_lock a terminal lock Waiman Long
2018-11-08 20:34 ` [RFC PATCH 05/12] debugobjects: Mark pool_lock as " Waiman Long
2018-11-08 20:34 ` [RFC PATCH 06/12] debugobjects: Move printk out of db lock critical sections Waiman Long
2018-11-08 20:34 ` [RFC PATCH 07/12] locking/lockdep: Add support for nested terminal locks Waiman Long
2018-11-10 14:20   ` Peter Zijlstra
2018-11-11  0:30     ` Waiman Long
2018-11-11  1:30       ` Peter Zijlstra
2018-11-08 20:34 ` [RFC PATCH 08/12] debugobjects: Make object hash locks " Waiman Long
2018-11-08 20:34 ` [RFC PATCH 09/12] lib/stackdepot: Make depot_lock a terminal spinlock Waiman Long
2018-11-08 20:34 ` [RFC PATCH 10/12] locking/rwsem: Mark rwsem.wait_lock as a terminal lock Waiman Long
2018-11-08 20:34 ` [RFC PATCH 11/12] cgroup: Mark the rstat percpu lock as terminal Waiman Long
2018-11-08 20:34 ` [RFC PATCH 12/12] mm/kasan: Make quarantine_lock a terminal lock Waiman Long
2018-11-09  8:04 ` [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks Ingo Molnar
2018-11-09 15:48   ` Waiman Long [this message]
2018-11-12  5:15     ` Ingo Molnar
2018-11-10 14:10   ` Peter Zijlstra
2018-11-10 23:35     ` Waiman Long
2018-11-12  5:10       ` Ingo Molnar
2018-11-12  5:53         ` Josh Poimboeuf
2018-11-12  6:30           ` Ingo Molnar
2018-11-12 22:22             ` Josh Poimboeuf
2018-11-12 22:56               ` Waiman Long

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