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From: Ingo Molnar <mingo@kernel.org>
To: Waiman Long <longman@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	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: Mon, 12 Nov 2018 06:15:37 +0100	[thread overview]
Message-ID: <20181112051537.GB123204@gmail.com> (raw)
In-Reply-To: <1fcaa330-a4be-0f8a-7974-7b17f0ce01ad@redhat.com>


* Waiman Long <longman@redhat.com> wrote:

> > 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.

Thanks!

> >> 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.

Yeah.

> [...] 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.

Of course!

> 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.

The one worry I have is that this interim solution removes the benefit of 
a proper static analysis method.

But if you promise to make a serious effort on the static analysis 
tooling as well (which should have awesome performance results and 
automate the manual markup), then I have no fundamental objections to the 
interim approach either.

If static analysis works as well as I expect it to then in principle we 
might even be able to have lockdep enabled in production kernels: it 
would only add overhead to locks that are overly complex - which would 
create incentives to improve those dependencies.

Thanks,

	Ingo

  reply	other threads:[~2018-11-12  5:15 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
2018-11-12  5:15     ` Ingo Molnar [this message]
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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