Re: [RFC PATCH 5/5] futex, doc: add a document on how to use the spinning futexes

Randy Dunlap
2014-07-21T15:45:25Z

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, ...);
> +}
>

--
~Randy