Messages in this thread | | | Date | Mon, 21 Jul 2014 23:19:02 -0400 | From | Waiman Long <> | Subject | Re: [RFC PATCH 5/5] futex, doc: add a document on how to use the spinning futexes |
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On 07/21/2014 11:45 AM, Randy Dunlap wrote: > On 07/21/2014 08:24 AM, Waiman Long wrote: >> This patch adds a new document file on how to use the spinning futexes. >> >> Signed-off-by: Waiman Long<Waiman.Long@hp.com> >> --- >> Documentation/spinning-futex.txt | 109 ++++++++++++++++++++++++++++++++++++++ >> 1 files changed, 109 insertions(+), 0 deletions(-) >> create mode 100644 Documentation/spinning-futex.txt >> >> diff --git a/Documentation/spinning-futex.txt b/Documentation/spinning-futex.txt >> new file mode 100644 >> index 0000000..e3cb5a2 >> --- /dev/null >> +++ b/Documentation/spinning-futex.txt >> @@ -0,0 +1,109 @@ >> +Started by: Waiman Long<waiman.long@hp.com> >> + >> +Spinning Futex >> +-------------- >> + >> +There are two main problems for a wait-wake futex (FUTEX_WAIT and >> +FUTEX_WAKE) when used for creating user-space lock primitives: >> + >> + 1) With a wait-wake futex, tasks waiting for a lock are put to sleep >> + in the futex queue to be woken up by the lock owner when it is done >> + with the lock. Waking up a sleeping task, however, introduces some >> + additional latency which can be large especially if the critical >> + section protected by the lock is relatively short. This may cause >> + a performance bottleneck on large systems with many CPUs running >> + applications that need a lot of inter-thread synchronization. >> + >> + 2) The performance of the wait-wake futex is currently >> + spinlock-constrained. When many threads are contending for a >> + futex in a large system with many CPUs, it is not unusual to have >> + spinlock contention accounting for more than 90% of the total >> + CPU cycles consumed at various points in time. >> + >> +Spinning futex is a solution to both the wakeup latency and spinlock >> +contention problems by optimistically spinning on a locked futex >> +when the lock owner is running within the kernel until the lock is >> +free. This is the same optimistic spinning mechanism used by the kernel >> +mutex and rw semaphore implementations to improve performance. The >> +optimistic spinning was done without taking any lock. > is done > >> + >> +Implementation >> +-------------- >> + >> +Like the PI and robust futexes, a lock acquirer has to atomically >> +put its thread ID (TID) into the lower 30 bits of the 32-bit futex >> +which should has an original value of 0. If it succeeds, it will be > have > >> +the owner of the futex. Otherwise, it has to call into the kernel >> +using the new FUTEX_SPIN_LOCK futex(2) syscall. >> + >> +The kernel will use the setting of the most significant bit >> +(FUTEX_WAITERS) in the futex value to indicate one or more waiters >> +are sleeping and need to be woken up later on. >> + >> +When it is time to unlock, the lock owner has to atomically clear >> +the TID portion of the futex value. If the FUTEX_WAITERS bit is set, >> +it has to issue a FUTEX_SPIN_UNLOCK futex system call to wake up the >> +sleeping task. >> + >> +A return value of 1 from the FUTEX_SPIN_UNLOCK futex(2) syscall >> +indicates a task has been woken up. The syscall returns 0 if no >> +sleeping task is found or spinners are present to take the lock. >> + >> +The error number returned by a FUTEX_SPIN_UNLOCK call on an empty >> +futex can be used to decide if the spinning futex functionality is >> +implemented in the kernel. If it is present, the returned error number >> +should be ESRCH. Otherwise it will be ENOSYS. >> + >> +Currently, only the first and the second arguments (the futex address >> +and the opcode) of the futex(2) syscall is used. All the other > are used. > >> +arguments must be set to 0 or NULL to avoid forward compatibility >> +problem. >> + >> +The spinning futex requires the kernel to have support for the cmpxchg >> +functionality. For architectures that don't support cmpxchg, spinning >> +futex will not be supported as well. >> + >> +Usage Scenario >> +-------------- >> + >> +A spinning futex can be used as an exclusive lock to guard a critical >> +section which are unlikely to go to sleep in the kernel. The spinners > is > >> +in a spinning futex, however, will fall back to sleep in a wait queue >> +if the lock owner isn't running. Therefore, it can also be used when >> +the critical section is long and prone to sleeping. However, it may >> +not have the performance benefit when compared with a wait-wake futex >> +in this case. >> + >> +Sample Code >> +----------- >> + >> +The following are sample code to implement a simple lock and unlock > is > >> +function. >> + >> +__thread int tid; /* Thread ID */ >> + >> +void mutex_lock(int *faddr) >> +{ >> + if (cmpxchg(faddr, 0, tid) == 0) >> + return; >> + for (;;) >> + if (futex(faddr, FUTEX_SPIN_LOCK, ...) == 0) >> + break; >> +} >> + >> +void mutex_unlock(int *faddr) >> +{ >> + int old, fval; >> + >> + if ((fval = cmpxchg(faddr, tid, 0)) == tid) >> + return; >> + /* Clear only the TID portion of the futex */ >> + for (;;) { >> + old = fval; >> + fval = cmpxchg(faddr, old, old& ~FUTEX_TID_MASK); >> + if (fval == old) >> + break; >> + } >> + if (fval& FUTEX_WAITERS) >> + futex(faddr, FUTEX_SPIN_UNLOCK, ...); >> +} >> >
Thank for the grammar corrections. Will apply those to the documents.
-Longman
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