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(-)
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