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From:   Prakash Sangappa <prakash.sangappa@oracle.com>
To:     Greg KH <gregkh@linuxfoundation.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "mingo@redhat.com" <mingo@redhat.com>
Subject: Re: [RESEND RFC PATCH 1/3] Introduce per thread user-kernel shared
 structure
Thread-Topic: [RESEND RFC PATCH 1/3] Introduce per thread user-kernel shared
 structure
Thread-Index: AQHXpQ4EAwEPTwjbx06EdbuOOXayyqubQN+AgADKu4A=
Date:   Thu, 9 Sep 2021 18:45:29 +0000
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> On Sep 8, 2021, at 11:39 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
>=20
> On Wed, Sep 08, 2021 at 05:10:23PM -0700, Prakash Sangappa wrote:
>> A structure per thread is allocated from a page that is shared mapped
>> between user space and kernel as means for faster communication. This wi=
ll
>> facilitate sharing information, Ex: per thread stats shared between kern=
el
>> and user space, that can be read by applications without the need for
>> making frequent system calls in latency sensitive code path.
>>=20
>> A new system call is added, which will allocate the shared structure and
>> return its mapped user address. Multiple such structures will be allocat=
ed
>> on a page to accommodate requests from different threads of a multithrea=
ded
>> process. Available space on a page is managed using a bitmap. When a thr=
ead
>> exits, the shared structure is freed and can get reused for another thre=
ad
>> that requests the shared structure. More pages will be allocated and use=
d
>> as needed based on the number of threads requesting use of shared
>> structures. These pages are all freed when the process exits.
>>=20
>> Each of these shared structures are rounded to 128 bytes. Available spac=
e
>> 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 an=
d
>> managed by the kernel. Although, space in shared structure for sharing a=
ny
>> kind of stats or state should be sparingly used. Therefore shared struct=
ure
>> 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 futu=
re
>> enhancements/changes without breaking the API.
>>=20
>> Signed-off-by: Prakash Sangappa <prakash.sangappa@oracle.com>
>=20
> Where is the Documentation/ABI/ entries for this new user/kernel abi
> that you are creating and must maintain for the next 20+ years?
=20

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_shar=
ed.h.
Man page for the system call will need to be added.

Thanks,
>=20
> thanks,
>=20
> greg k-h