Re: [RESEND RFC PATCH 1/3] Introduce per thread user-kernel shared structure

On Sep 8, 2021, at 11:39 PM, Greg KH [off-list ref] wrote:

On Wed, Sep 08, 2021 at 05:10:23PM -0700, Prakash Sangappa wrote:

quoted

A structure per thread is allocated from a page that is shared mapped
between user space and kernel as means for faster communication. This will
facilitate sharing information, Ex: per thread stats shared between kernel
and user space, that can be read by applications without the need for
making frequent system calls in latency sensitive code path.

A new system call is added, which will allocate the shared structure and
return its mapped user address. Multiple such structures will be allocated
on a page to accommodate requests from different threads of a multithreaded
process. Available space on a page is managed using a bitmap. When a thread
exits, the shared structure is freed and can get reused for another thread
that requests the shared structure. More pages will be allocated and used
as needed based on the number of threads requesting use of shared
structures. These pages are all freed when the process exits.

Each of these shared structures are rounded to 128 bytes. Available space
in this structure can be used to accommodate additional per thread stats,
state etc as needed. In future, if more space beyond 128 bytes, is
needed, multiple such shared structures per thread could be allocated and
managed by the kernel. Although, space in shared structure for sharing any
kind of stats or state should be sparingly used. Therefore shared structure
layout is not exposed to user space. the system call will return the
mapped user address of a specific member or nested structure within the
shared structure corresponding to stats requested, This would allow future
enhancements/changes without breaking the API.

Signed-off-by: Prakash Sangappa <redacted>

Where is the Documentation/ABI/ entries for this new user/kernel abi
that you are creating and must maintain for the next 20+ years?

 

I can include the documentation in subsequent version of patchset.

The shared structure size and layout will not be exposed to user space.
The ABI details of supported struct members will be in uapi/linux/task_shared.h.
Man page for the system call will need to be added.

Thanks,

thanks,

greg k-h