[patch 00/10] PI-futex: -V1

[patch 00/10] PI-futex: -V1

[Ingo Molnar]

We are pleased to announce "lightweight userspace priority inheritance" 
(PI) support for futexes. The following patchset and glibc patch 
implements it, ontop of the robust-futexes patchset which is included in 
2.6.16-mm1.

We are calling it lightweight for 3 reasons:

 - in the user-space fastpath a PI-enabled futex involves no kernel work 
   (or any other PI complexity) at all. No registration, no extra kernel
   calls - just pure fast atomic ops in userspace.

 - in the slowpath (in the lock-contention case), the system call and 
   scheduling pattern is in fact better than that of normal futexes,
   due to the 'integrated' nature of FUTEX_LOCK_PI. [more about that 
   further down]

 - the in-kernel PI implementation is streamlined around the mutex
   abstraction, with strict rules that keep the implementation
   relatively simple: only a single owner may own a lock (i.e. no
   read-write lock support), only the owner may unlock a lock, no
   recursive locking, etc.

Priority Inheritance - why, oh why???
-------------------------------------

Many of you heard the horror stories about the evil PI code circling 
Linux for years, which makes no real sense at all and is only used by 
buggy applications and which has horrible overhead. Some of you have 
dreaded this very moment, when someone actually submits working PI code 
;-)

So why would we like to see PI support for futexes?

We'd like to see it done purely for technological reasons. We dont think 
it's a buggy concept, we think it's useful functionality to offer to 
applications, which functionality cannot be achieved in other ways. We 
also think it's the right thing to do, and we think we've got the right 
arguments and the right numbers to prove that. We also believe that we 
can address all the counter-arguments as well. For these reasons (and 
the reasons outlined below) we are submitting this patch-set for 
upstream kernel inclusion.

What are the benefits of PI?

The short reply:
----------------

User-space PI helps achieving/improving determinism for user-space 
applications. In the best-case, it can help achieve determinism and 
well-bound latencies. Even in the worst-case, PI will improve the 
statistical distribution of locking related application delays.

The longer reply:
-----------------

Firstly, sharing locks between multiple tasks is a common programming 
technique that often cannot be replaced with lockless algorithms. As we 
can see it in the kernel [which is a quite complex program in itself], 
lockless structures are rather the exception than the norm - the current 
ratio of lockless vs. locky code for shared data structures is somewhere 
between 1:10 and 1:100. Lockless is hard, and the complexity of lockless 
algorithms often endangers to ability to do robust reviews of said code.  
I.e. critical RT apps often choose lock structures to protect critical 
data structures, instead of lockless algorithms. Furthermore, there are 
cases (like shared hardware, or other resource limits) where lockless 
access is mathematically impossible.

Media players (such as Jack) are an example of reasonable application 
design with multiple tasks (with multiple priority levels) sharing 
short-held locks: for example, a highprio audio playback thread is 
combined with medium-prio construct-audio-data threads and low-prio 
display-colory-stuff threads. Add video and decoding to the mix and 
we've got even more priority levels.

So once we accept that synchronization objects (locks) are an 
unavoidable fact of life, and once we accept that multi-task userspace 
apps have a very fair expectation of being able to use locks, we've got 
to think about how to offer the option of a deterministic locking 
implementation to user-space.

Most of the technical counter-arguments against doing priority 
inheritance only apply to kernel-space locks. But user-space locks are 
different, there we cannot disable interrupts or make the task 
non-preemptible in a critical section, so the 'use spinlocks' argument 
does not apply (user-space spinlocks have the same priority inversion 
problems as other user-space locking constructs). Fact is, pretty much 
the only technique that currently enables good determinism for userspace 
locks (such as futex-based pthread mutexes) is priority inheritance:

Currently (without PI), if a high-prio and a low-prio task shares a lock 
[this is a quite common scenario for most non-trivial RT applications], 
even if all critical sections are coded carefully to be deterministic 
(i.e. all critical sections are short in duration and only execute a 
limited number of instructions), the kernel cannot guarantee any 
deterministic execution of the high-prio task: any medium-priority task 
could preempt the low-prio task while it holds the shared lock and 
executes the critical section, and could delay it indefinitely.

Implementation:
---------------

As mentioned before, the userspace fastpath of PI-enabled pthread 
mutexes involves no kernel work at all - they behave quite similarly to 
normal futex-based locks: a 0 value means unlocked, and a value==TID 
means locked. (This is the same method as used by list-based robust 
futexes.) Userspace uses atomic ops to lock/unlock these mutexes without 
entering the kernel.

To handle the slowpath, we have added two new futex ops:

  FUTEX_LOCK_PI
  FUTEX_UNLOCK_PI

If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to 
TID fails], then FUTEX_LOCK_PI is called. The kernel does all the 
remaining work: if there is no futex-queue attached to the futex address 
yet then the code looks up the task that owns the futex [it has put its 
own TID into the futex value], and attaches a 'PI state' structure to 
the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, 
kernel-based synchronization object. The 'other' task is made the owner 
of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the 
futex value. Then this task tries to lock the rt-mutex, on which it 
blocks. Once it returns, it has the mutex acquired, and it sets the 
futex value to its own TID and returns. Userspace has no other work to 
perform - it now owns the lock, and futex value contains 
FUTEX_WAITERS|TID.

If the unlock side fastpath succeeds, [i.e. userspace manages to do a 
TID -> 0 atomic transition of the futex value], then no kernel work is 
triggered.

If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), 
then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the 
behalf of userspace - and it also unlocks the attached 
pi_state->rt_mutex and thus wakes up any potential waiters.

Note that under this approach, contrary to other PI-futex approaches, 
there is no prior 'registration' of a PI-futex. [which is not quite 
possible anyway, due to existing ABI properties of pthread mutexes.]

Also, under this scheme, 'robustness' and 'PI' are two orthogonal 
properties of futexes, and all four combinations are possible: futex, 
robust-futex, PI-futex, robust+PI-futex.

glibc support:
--------------

Ulrich Drepper and Jakub Jelinek have written glibc support for 
PI-futexes (and robust futexes), enabling robust and PI 
(PTHREAD_PRIO_INHERIT) POSIX mutexes. (PTHREAD_PRIO_PROTECT support will 
be added later on too, no additional kernel changes are needed for 
that). [NOTE: The glibc patch is obviously inofficial and unsupported 
without matching upstream kernel functionality.]

the patch-queue and the glibc patch can also be downloaded from:

  http://redhat.com/~mingo/PI-futex-patches/

a diffstat is attached below. The patch-queue is against 2.6.16-mm1, 
plus the following small updates to -mm1:

 lightweight-robust-futexes-updates.patch
 lightweight-robust-futexes-updates-2.patch
 itimer-validate-uservalue.patch
 hrtimer-generic-sleeper.patch
 futex-timeval-check.patch

all have been sent to Andrew and are independent of PI-futexes.

many thanks go to the people who helped us create this kernel feature: 
Steven Rostedt, Esben Nielsen, Benedikt Spranger, Daniel Walker, John 
Cooper, Arjan van de Ven, Oleg Nesterov and others. Credits for related 
prior projects goes to Dirk Grambow, Inaky Perez-Gonzalez, Bill Huey and 
many others.

	Ingo Molnar, Thomas Gleixner

--

 Documentation/rtmutex.txt                    |   60 +
 arch/i386/mm/pageattr.c                      |    4 
 include/linux/futex.h                        |   11 
 include/linux/init_task.h                    |    2 
 include/linux/mm.h                           |   11 
 include/linux/plist.h                        |  226 ++++++
 include/linux/rtmutex.h                      |  119 +++
 include/linux/rtmutex_internal.h             |  187 +++++
 include/linux/sched.h                        |   34 
 include/linux/syscalls.h                     |    4 
 init/Kconfig                                 |    5 
 kernel/Makefile                              |    3 
 kernel/exit.c                                |    9 
 kernel/fork.c                                |    6 
 kernel/futex.c                               |  929 +++++++++++++++++++++----
 kernel/futex_compat.c                        |   11 
 kernel/rtmutex-debug.c                       |  511 +++++++++++++
 kernel/rtmutex-debug.h                       |   32 
 kernel/rtmutex-tester.c                      |  436 +++++++++++
 kernel/rtmutex.c                             |  997 +++++++++++++++++++++++++++
 kernel/rtmutex.h                             |   28 
 kernel/sched.c                               |  136 +++
 lib/Kconfig                                  |    6 
 lib/Kconfig.debug                            |   20 
 lib/Makefile                                 |    1 
 lib/plist.c                                  |   72 +
 mm/page_alloc.c                              |    4 
 mm/slab.c                                    |    3 
 scripts/rt-tester/check-all.sh               |   21 
 scripts/rt-tester/rt-tester.py               |  222 ++++++
 scripts/rt-tester/t2-l1-2rt-sameprio.tst     |  101 ++
 scripts/rt-tester/t2-l1-pi.tst               |   84 ++
 scripts/rt-tester/t2-l1-signal.tst           |   79 ++
 scripts/rt-tester/t2-l2-2rt-deadlock.tst     |   91 ++
 scripts/rt-tester/t3-l1-pi-1rt.tst           |   95 ++
 scripts/rt-tester/t3-l1-pi-2rt.tst           |   96 ++
 scripts/rt-tester/t3-l1-pi-3rt.tst           |   95 ++
 scripts/rt-tester/t3-l1-pi-signal.tst        |   98 ++
 scripts/rt-tester/t3-l1-pi-steal.tst         |   99 ++
 scripts/rt-tester/t3-l2-pi.tst               |   95 ++
 scripts/rt-tester/t4-l2-pi-deboost.tst       |  127 +++
 scripts/rt-tester/t5-l4-pi-boost-deboost.tst |  148 ++++
 42 files changed, 5138 insertions(+), 180 deletions(-)

Re: [patch 00/10] PI-futex: -V1

[Bill Huey]

On Sat, Mar 25, 2006 at 07:45:28PM +0100, Ingo Molnar wrote:
> We are pleased to announce "lightweight userspace priority inheritance" 
> (PI) support for futexes. The following patchset and glibc patch 
> implements it, ontop of the robust-futexes patchset which is included in 
> 2.6.16-mm1.
... 
> Priority Inheritance - why, oh why???
> -------------------------------------
...
> The longer reply:
> -----------------

[comments on app usages of priority inheritance deleted]

You'll need to do priority ceiling emulation as well. I've been using that in an
application as a manual means of preventing preemption of key critical sections,
like a spinlock/preempt_disable(), to prevent priority inversion from happening.
Raising the priority of threads using that lock/critical section is a pretty
effective means of partitioning program logical sections of the application into
threads that are higher and lower priority than the resource. It's cool stuff.

bill

Re: [patch 00/10] PI-futex: -V1

[Ingo Molnar]

* Bill Huey <billh@gnuppy.monkey.org> wrote:

> You'll need to do priority ceiling emulation as well. [...]

i mentioned it further down in the text - PRIO_PROTECT support (which is 
priority ceiling) is planned for pthread mutexes. It needs no further 
kernel changes, it's a pure userspace thing.

	Ingo

Re: [patch 00/10] PI-futex: -V1

[Esben Nielsen]

On Sun, 26 Mar 2006, Ingo Molnar wrote:

>
> * Bill Huey <billh@gnuppy.monkey.org> wrote:
>
> > You'll need to do priority ceiling emulation as well. [...]
>
> i mentioned it further down in the text - PRIO_PROTECT support (which is
> priority ceiling) is planned for pthread mutexes. It needs no further
> kernel changes, it's a pure userspace thing.
>

Wouldn't this always include a call to sched_setscheduler() even for the
fast path? And it would also involve assigning a priority to all locks up
front.

There are only 2 good reasons to choose this, as far as I can see. One is
that it is more deterministic: The "fast path" is almost as slow as the slow
path. So you will not be surprised by a sudden increase CPU use because
timing is moved slightly. This is on the other hand something which can
happen with PI.
On UP there is usually no congestion with this mechanism if you avoid blocking
when you have a lock as the task holding the lock will have higher
priority than any other task interested in the lock.

Esben


> 	Ingo
> -
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Re: [patch 00/10] PI-futex: -V1

[Ingo Molnar]

* Esben Nielsen <simlo@phys.au.dk> wrote:

> On Sun, 26 Mar 2006, Ingo Molnar wrote:
> 
> > i mentioned it further down in the text - PRIO_PROTECT support (which is
> > priority ceiling) is planned for pthread mutexes. It needs no further
> > kernel changes, it's a pure userspace thing.
> 
> Wouldn't this always include a call to sched_setscheduler() even for 
> the fast path? And it would also involve assigning a priority to all 
> locks up front.

correct. Priority ceiling based boosting can be called "manual PI" or 
"open-coded PI". It is quite error-prone for more complex apps, because 
not only has each thread its own priority (which must be well-designed), 
but also every lock used by these threads must be assigned a maximum 
priority that it will boost threads to. That priority is redundant and 
can be mis-assigned.

priority ceiling locks also involve two additional syscalls per critical 
section, so they are significantly slower than the atomic-ops based 
PI-futex solution. [and thus it could degrade worst-case behavior by 
virtue of higher locking overhead alone.]

> There are only 2 good reasons to choose this, as far as I can see. One 
> is that it is more deterministic: The "fast path" is almost as slow as 
> the slow path. So you will not be surprised by a sudden increase CPU 
> use because timing is moved slightly. This is on the other hand 
> something which can happen with PI.

correct. That on the other hand is also a disadvantage: PI is in essence 
'optional boosting on an as-needed basis', while priority-ceiling is 
unconditional boosting. Unconditional boosting will hurt average 
latencies, which are often just as important as the worst-case 
latencies.

For example: there is a high-prio, a medium-prio and a low-prio task.  
There is a critical section lock, only used by the high-prio and the 
low-prio task.

Under the priority ceiling logic, we manually assign 'high-prio' to the 
lock [which step is redundant information, and which is an additional 
source of coding/design bugs]. When the low-prio task takes the lock, 
that will unconditionally boost it to high-prio, and that task might 
delay the medium-prio task - even if there is no high-prio task running 
right then.

If PI is used, then the kernel will automatically boost the low-prio 
task to high-prio, if the high-prio task wants to take that lock too.  
Otherwise the low-prio task will remain on low-prio - not delaying the 
medium-prio task. So the average delays of the medium-prio task improve.  

[ Furthermore, theoretically, if the application has a workflow pattern 
  in which the medium-prio and high-prio tasks will never run at the 
  same time, then priority ceiling logic would degrade the worst-case 
  latency of the medium-prio task too. ]

[ for completeness: PI is transitive as well, so if at such a moment the 
  highprio task preempts the medium prio task (which preempted the 
  lowprio task holding the lock), and the highprio task wants to take 
  the lock, then the lowprio task will be boosted to high-prio and can 
  finish the critical section to hand over the lock to the highprio 
  task. ]

i.e. PI, as implemented by this patchset, is clearly superior to 
priority ceiling logic.

furthermore, there's a usability advantage to PI too: programmers often 
find it more natural to assign priorities to a given 'workflow' 
(task/thread), and not to every lock. I.e. i think PI is more natural. 
(and hence easier to design for)

but nevertheless, the PRIO_PROTECT API for POSIX mutexes does exist, and 
we can (and will) support it from userspace. All that it needed was to 
make sure that setscheduler() is fully PI-aware: it must not decrease 
the priority of a task to below the highest-priority waiter, and it must 
'remember' the setscheduler() priority [and restore to it] after PI 
effects wear off. This is implemented by our PI code anyway, so we get 
correct priority ceiling (PRIO_PROTECT) behavior 'for free'.

	Ingo

Re: [patch 00/10] PI-futex: -V1

[Bill Huey]

On Sun, Mar 26, 2006 at 04:25:39PM +0200, Ingo Molnar wrote:
> If PI is used, then the kernel will automatically boost the low-prio 
> task to high-prio, if the high-prio task wants to take that lock too.  
> Otherwise the low-prio task will remain on low-prio - not delaying the 
> medium-prio task. So the average delays of the medium-prio task improve.  
> 
> [ Furthermore, theoretically, if the application has a workflow pattern 
>   in which the medium-prio and high-prio tasks will never run at the 
>   same time, then priority ceiling logic would degrade the worst-case 
>   latency of the medium-prio task too. ]
> 
> [ for completeness: PI is transitive as well, so if at such a moment the 
>   highprio task preempts the medium prio task (which preempted the 
>   lowprio task holding the lock), and the highprio task wants to take 
>   the lock, then the lowprio task will be boosted to high-prio and can 
>   finish the critical section to hand over the lock to the highprio 
>   task. ]
> 
> i.e. PI, as implemented by this patchset, is clearly superior to 
> priority ceiling logic.
> 
> furthermore, there's a usability advantage to PI too: programmers often 
> find it more natural to assign priorities to a given 'workflow' 
> (task/thread), and not to every lock. I.e. i think PI is more natural. 
> (and hence easier to design for)

It's not quite as a simple as that. The use of a ceiling is a more aggressive
method of controlling priority in an application. The use it can control the
thread preeemption, with regard to thread priority, primarily below the ceiling
by prevent the occurance of priority inversion and the for the need for simple
priority inheritance in the first place. It just simplifies what's going on in
the app as wel as what can happen.

Apps using ceilings don't have to consider the potential of a PI boost chain
in the first place if it was allowed to complete without the effects of floating
priority. Certain RT app needs depend on this kind of ceiling behavior and the
timing that goes with it. I'm sure Esben and company will correctly me if I'm
wrong, but his is how I understand it and I believe this to be correct.

> but nevertheless, the PRIO_PROTECT API for POSIX mutexes does exist, and 
> we can (and will) support it from userspace. All that it needed was to 
> make sure that setscheduler() is fully PI-aware: it must not decrease 
> the priority of a task to below the highest-priority waiter, and it must 
> 'remember' the setscheduler() priority [and restore to it] after PI 
> effects wear off. This is implemented by our PI code anyway, so we get 
> correct priority ceiling (PRIO_PROTECT) behavior 'for free'.

Talking about nearly free things. If your userpace implementation has a crude
notion of TCB, then you might able to a lazy user to kernel space sync of a
thread's priority at an in-kernel preemption point such as an interrupt exit
or some kind of preemption checks. This would maintain the run correctness of
that thraed since it is already running in the first place. Userspace
implementations of pthreead_{get,set}schedparam would just use the cached
userspace value directly in from the TCB instead of a kernel call.

I mention this because this could be a decent method of optimizing out the
opening priority boost and associated setscheduler() with a ceiling. The demotion
hit for a ceiling is unavoiable, but the PI demotion case suffers this as well
if it's contended. That's how I understand the problem and I'm just forwarding
some ideas with regard to the issue, but this is starting to push into scheduler
activations which just about everybody has abandoned these days (Solaris, etc...). 

bill

Re: [patch 00/10] PI-futex: -V1

[Bill Huey]

On Sun, Mar 26, 2006 at 03:10:00PM -0800, Bill Huey wrote:
> On Sun, Mar 26, 2006 at 04:25:39PM +0200, Ingo Molnar wrote:
> 
> It's not quite as a simple as that. The use of a ceiling is a more aggressive
> method of controlling priority in an application. The use [of] it can control [-the-]
> thread preeemption, with regard to thread priority, [-primarily-] below the ceiling
> by prevent the occurance of priority inversion and the[re]for[e] the need for simple
> priority inheritance in the first place. It just simplifies what's going on in
> the app as wel[l] as what can happen [if priorities in an app are complex].

[with some grammar and spelling corrections]

Sorry, I think you folks get the idea, but typing a technical email just getting out of bed
without contacts is a bit challenging. :)
 
bill