# Patent

## Title
CPU/GPU synchronization mechanism

## Abstract

A thread on one processor may be used to enable another processor to lock or release a mutex. For example, a central processing unit thread may be used by a graphics processing unit to secure a mutex for a shared memory.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation application claiming priority to U.S. patent application Ser. No. 13/193,779 filed Jul. 29, 2011 hereby expressly incorporated by reference herein.


BACKGROUND
This relates generally to computers that have general purpose processors and graphics processing units.
The memory used by user applications running on the general purpose or central processing unit and the memory used by a graphics processing unit are typically separated. A graphics processing unit driver copies data from the user space into driver memory for processing on a graphics processing unit. In a shared virtual memory model, data is not copied to the graphics processing unit, but, instead, it is shared between the graphics processing unit and the central processing unit.
Currently, in multithreaded applications, shared data is protected by locks called mutexes. Each thread that wants to access shared data must first lock a corresponding mutex to prevent other threads from accessing that mutex. This locking can be done through “spinning” on lock, but this technique is not efficient from power and performance points of view.
To optimize the central processing unit, the operating system provides system calls that allow a thread to sleep until a mutex is available and then notifies other threads when a mutex is unlocked. But this mechanism works only for threads that run on central processing unit cores.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic depiction of one embodiment of the present invention;
FIG. 2 is extended thread and memory model in accordance with one embodiment of the present invention;
FIG. 3 is a flow chart for page fault handling in accordance with one embodiment of the present invention; and
FIG. 4 is a system depiction for one embodiment.



DETAILED DESCRIPTION
For each thread group running on a graphics processor, an auxiliary shadow thread running on the central processing unit may be created, in one embodiment. When a thread running on the graphics processing unit wants to lock a mutex, if it wants to wait until the thread is freed by another task, the graphics thread sends a request to a shadow thread on the central processing unit. The shadow thread on the central processing unit issues the corresponding system called to the operating system. When the operating system gives a lock to the shadow thread, the shadow thread sends the notification to the thread on the graphics processing unit.
While the term graphics processing unit is used in the present application, it should be understood that the graphics processing unit may or may not be a separate integrated circuit. The present invention is applicable to situations where the graphics processing unit and the central processing unit are integrated into one integrated circuit. As used herein, a processor or processing unit may be a processor, controller, or coprocessor.
Referring to FIG. 1, a host/central processing unit 16 communicates with the graphics processing unit 18 in a processor-based system 10. The host/central processing unit 16 may be part of any processor-based system, including a hardwired or mobile device. Examples of mobile devices include cellular telephones, laptop computers, tablet computers, and mobile Internet devices, to mention a few examples. The host central processing unit 16 includes user applications 20 which provide control information to a shadow thread 22. The shadow thread 22 then communicates synchronization on sync and control information to the graphics processing unit driver 26. A shadow thread also communicates with the host operating system 24.
As shown in FIG. 1, the user level 12 includes a shadow thread 22 and the user applications 20, while the kernel level 14 includes a host operating system 24, and the graphics processing unit driver 26. The graphics processing unit driver 26 is a driver for the graphics processing unit even though that driver is resident in the central processing unit 16.
The graphics processing unit 18 includes, in user level 12, a gthread 28 which sends control and synchronization messages to the operating system (pOS) 30 and receives messages from the operating system 30. A gthread is user code that runs on the graphics processing unit, sharing virtual memory with the parent thread running on the central processing unit. The operating system 30 may be a relatively small operating system, running on the graphics processing unit, that is responsible for graphics processing unit exceptions. It is a small relative to the host operating system 24, as one example.
User applications 20 are any user process that runs on the central processing unit 16. The user applications 20 spawn threads on the graphics processing unit 18.
An eXtended Threaded Library or XTL is an extension to create and manage user threads on the graphics processing unit. This library creates the shadow thread for each gthread and has library functions for synchronization.
User applications offload computations to the graphics processing unit using an extension of a traditional multithreaded model such as:

    
    
        xthread_create (thread, attr, gpu_worker,arg).
    
    


The gthread or worker thread created on the graphics processing unit shares virtual memory with the parent thread. It behaves in the same way as a regular thread in that all standard inter-process synchronization mechanisms, such as Mutex and semaphore, can be used. At the same time, a new shadow thread is created on the host central processing unit 16. This shadow thread works as a proxy for exception handling units and synchronization between threads on the central processing unit and the graphics processing unit.
In some embodiments, the parent thread, the host shadow thread and the graphics processing unit worker threads may share virtual memory as shown in FIG. 2. Host/central processing unit 16 includes the parent thread 32 that generates the xthread_create( ) for the shadow thread 22. The shadow thread 22 accesses the shadow stack which is a private address space in the process address space 36. The parent thread 32 also accesses the memory descriptors 34 and the main stack, which is a private address space within the process address space 36. The memory descriptors 34 may also communicate with the gthread worker 28. The gthread worker 28 can access the gthread code within the process space 36 as well as the shared data section and the private gthread stack. The material in the upper blocks corresponds to the process model 38 and the lower blocks correspond to the memory model 40.
Referring to FIG. 3, the synchronization algorithms may be implemented in hardware, software and/or firmware. In software embodiments, the algorithms may be implemented as computer executable instructions stored on a non-transitory computer readable medium such as an optical, semiconductor or magnetic memory. In FIG. 3, the flows for the host operating system 24, the shadow thread 22, driver 26 of the central processing unit 16, and the operating system 30 and ring 3 57 in the graphics processing unit 18 are shown as parallel vertical flow paths with interactions between them indicated by generally horizontal arrows.
Continuing in FIG. 3, in one embodiment, at ring 3, the gthread 28 tries to acquire a mutex from the user space by spinning for a short period of time, as indicated in block 60, in column 57. If the mutex is successfully acquired, as determined in diamond 61, the program continues, as indicated in block 62. If the mutex was not acquired because it was already locked by another thread, a call to the operating system 30 (“SYSCALL”) is made with an ACQUIRE op code.
In the operating system 30, the SYSCALL is received. The operating system 30 sends a message to the driver called PassHostCommand. The PassHostCommand includes an op code of SYSCALL and a data address plus operation, as indicated in block 50. Then the operating system 30 puts the calling thread to sleep, as indicated in block 54. The calling thread then goes to idle, as indicated in block 56.
The driver 26 in the central processing unit 16 transfers the message to the Xthread 22 using a transfer command. In the Xthread 22, the transfer command is received. The xthread waited for this message from the driver, as indicated at block 44. Then the Xthread performs the mutex lock, as indicated in block 46 and may include an SYSCALL to the host operating system 24. The operating system 24 waits until the mutex is acquired, (i.e. there is a context switch), as indicated at 42. After the mutex was acquired, the Xthread sends a resume execution command to the driver 26 and then waits for the next message from the driver, as indicated in block 48. The driver 26 receives the resume execution command and sends a message PassGpuCommand to the operating system 30 in the graphics processing unit. The PassGpuCommand may include an op code of resume execution with no data, as indicated in block 52.
The operating system 30 wakes up the thread, as indicated in block 58, and sends a resume from SYSCAL to the gthread 28. As a result, the gthread 28 continues working, as indicated in block 62. The shadow thread may also be used to release a mutex.
The computer system 130, shown in FIG. 4, may include a hard drive 134 and a removable medium 136, coupled by a bus 104 to a chipset core logic 110. A keyboard and mouse 120, or other conventional components, may be coupled to the chipset core logic via bus 108. The core logic may couple to the graphics processor 112, via a bus 105, and the central processor 100 in one embodiment. The graphics processor 112 may also be coupled by a bus 106 to a frame buffer 114. The frame buffer 114 may be coupled by a bus 107 to a display screen 118. In one embodiment, a graphics processor 112 may be a multi-threaded, multi-core parallel processor using single instruction multiple data (SIMD) architecture.
In the case of a software implementation, the pertinent code may be stored in any suitable semiconductor, magnetic, or optical memory, including the main memory 132 (as indicated at 139) or any available memory within the graphics processor. Thus, in one embodiment, the code to perform the sequences of FIG. 3 may be stored in a non-transitory machine or computer readable medium, such as the memory 132, and/or the graphics processor 112, and/or the central processor 100 and may be executed by the processor 100 and/or the graphics processor 112 in one embodiment.
FIG. 3 is a flow chart. In some embodiments, the sequences depicted in this flow chart may be implemented in hardware, software, or firmware. In a software embodiment, a non-transitory computer readable medium, such as a semiconductor memory, a magnetic memory, or an optical memory may be used to store instructions and may be executed by a processor to implement the sequences shown in FIG. 3.
The graphics processing techniques described herein may be implemented in various hardware architectures. For example, graphics functionality may be integrated within a chipset. Alternatively, a discrete graphics processor may be used. As still another embodiment, the graphics functions may be implemented by a general purpose processor, including a multicore processor.
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

## Claims

1. A processing apparatus comprising:
a host central processing unit (CPU) to execute a first thread;
a graphics processing unit (GPU) coupled to the host CPU, the host CPU and the GPU to share access to a shared virtual address space, the first thread to synchronize access to the shared virtual address space;
wherein the first thread is to synchronize access between a second thread to execute on the host CPU and a third thread to execute on the GPU; and
wherein the first thread is to synchronize access via an acquire operation and a release operation, the acquire operation and release operation to be performed in response to a request received from the third thread.

2. The processing apparatus as in claim 1, wherein the third thread is to access data in the shared virtual address space via the acquire operation and to wait to access the data until the second thread is to release the data.

3. The processing apparatus as in claim 2, wherein the third thread is to release the data after the access via the release operation, the release to enable the second thread to access the data.

4. The processing apparatus as in claim 1, wherein the GPU is to wake the third thread when a signal message is received from the host CPU.

5. The processing apparatus as in claim 4, wherein the first thread is to enable the signal message between the host CPU and the GPU.

6. The processing apparatus as in claim 1, wherein the first thread is additionally to synchronize access to virtual memory shared between the third thread and the first thread.

7. The processing apparatus as in claim 1, wherein the host CPU and the GPU are integrated within one integrated circuit.

8. The processing apparatus as in claim 7, wherein the host CPU includes multiple processing cores.

9. The processing apparatus as in claim 8, wherein the GPU is a multi-threaded, multi-core parallel processor.

10. A heterogeneous processing system comprising:
a plurality of heterogeneous processors including a host CPU and a GPU coupled to the host CPU;
a memory shared by the host CPU and the GPU, wherein the memory includes a shared virtual address space; and
a first thread to execute on the host CPU to synchronize memory accesses by a second thread on the host CPU and a third thread on the GPU, wherein the first thread is to synchronize access via an acquire operation and a release operation, the acquire operation and release operation to be performed in response to a request received from the third thread.

11. The heterogeneous processing system as in claim 10, the third thread on the GPU to request to acquire access to data in the shared virtual address space and wait to acquire access to the data until the data is released by the first thread.

12. The heterogeneous processing system as in claim 10, the GPU to wake the third thread when a signal message is received from the host CPU, the signal message to enable the third thread to acquire access to the data.

13. The heterogeneous processing system as in claim 10, the first thread to synchronize access between the third thread and the first thread.

14. The heterogeneous processing system as in claim 10, the plurality of heterogeneous processors integrated within one integrated circuit.

15. The heterogeneous processing system as in claim 10, additionally including a driver associated with the GPU, the driver to execute on the host CPU to transfer synchronization messages between the first thread and the third thread.

16. The heterogeneous processing system as in claim 10, wherein the host CPU includes multiple processing cores.

17. The heterogeneous processing system as in claim 16, wherein the GPU is a multi-threaded, multi-core parallel processor.

18. A data processing system comprising:
a plurality of heterogeneous processors including a host CPU and a GPU coupled to the host CPU, the plurality of heterogeneous processors integrated within one integrated circuit;
a display device to display output from the GPU;
a memory shared by the host CPU and the GPU, wherein the memory includes a shared virtual address space; and
a first thread to execute on the host CPU to synchronize memory accesses by a second thread on the host CPU and a third thread on the GPU, wherein the first thread is to synchronize access via an acquire operation and a release operation, the acquire operation and release operation to be performed in response to a request received from the third thread.

19. The data processing system as in claim 18, the third thread on the GPU to request to acquire access to data in the shared virtual address space, wait to acquire access to the data until the data is released by the first thread, and the GPU to wake the third thread when a signal message is received from the host CPU, the signal message to enable the third thread to acquire access to the data.

20. The data processing system as in claim 18, the first thread to synchronize access between the third thread and the first thread.

21. The data processing system as in claim 18, additionally including a driver associated with the GPU, the driver to execute on the host CPU to transfer synchronization messages between the first thread and the third thread.

22. The data processing system as in claim 18, wherein the host CPU includes multiple processing cores and the GPU is a multi-threaded, multi-core parallel processor.