Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks

Waiman Long
2018-11-09T15:48:44Z

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