linux-kernel - Re: [RESEND RFC PATCH 0/3] Provide fast access to thread specific data

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Message-ID: <3591AC6D-45D2-476A-80B1-46BFA1742602@oracle.com>
Date:   Fri, 10 Sep 2021 16:13:52 +0000
From:   Prakash Sangappa <prakash.sangappa@...cle.com>
To:     Peter Oskolkov <posk@...gle.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Paul Turner <pjt@...gle.com>,
        Jann Horn <jannh@...gle.com>, Peter Oskolkov <posk@...k.io>
Subject: Re: [RESEND RFC PATCH 0/3] Provide fast access to thread specific
 data



> On Sep 10, 2021, at 8:18 AM, Peter Oskolkov <posk@...gle.com> wrote:
> 
> On Wed, Sep 8, 2021 at 5:16 PM Prakash Sangappa
> <prakash.sangappa@...cle.com> wrote:
>> 
>> Including liunx-kernel..
>> 
>> Resending RFC. This patchset is not final. I am looking for feedback on
>> this proposal to share thread specific data for us in latency sensitive
>> codepath.
> 
> Hi Prakash,


> 
> I'd like to add here that Jann and I have been discussing a similar
> feature for my UMCG patchset:
> 
> https://lore.kernel.org/lkml/CAG48ez0mgCXpXnqAUsa0TcFBPjrid-74Gj=xG8HZqj2n+OPoKw@mail.gmail.com/

Hi Peter,

I will take  a look.

> 
> In short, due to the need to read/write to the userspace from
> non-sleepable contexts in the kernel it seems that we need to have some
> form of per task/thread kernel/userspace shared memory that is pinned,
> similar to what your sys_task_getshared does.

Exactly. For this reason wanted kernel to allocate the pinned memory.
Didn’t want to deal with files etc as a large number threads will be using
the shared structure mechanism. 

> 
> Do you think your sys_task_getshared can be tweaked to return an
> arbitrarily-sized block of memory (subject to overall constraints)
> rather than a fixed number of "options"?

I suppose it could. How big of a size? We don’t want to hold on to 
arbitrarily large  amount of pinned memory. The preference would 
be for the kernel to decide what is going to be shared based on
what functionality/data sharing is supported. In that sense the size 
is pre defined not something the userspace/application can ask. 

I have not looked at your use case.

> 
> On a more general note, we have a kernel extension internally at
> Google, named "kuchannel", that is similar to what you propose here:
> per task/thread shared memory with counters and other stat fields that
> the kernel populates and the userspace reads (and some additional
> functionality that is not too relevant to the discussion).

We have few other use cases for this we are looking at, which I can 
describe later. 

-Prakash

> 
> Thanks,
> Peter
> 
> [...]