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

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Date:   Fri, 10 Sep 2021 09:28:00 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Prakash Sangappa <prakash.sangappa@...cle.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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RESEND RFC PATCH 0/3] Provide fast access to thread specific data

On Fri, Sep 10, 2021 at 9:13 AM Prakash Sangappa
<prakash.sangappa@...cle.com> wrote:
>
>
>
> > 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.

There could be a sysctl or some other mechanism that limits the amount
of memory pinned per mm (or per task). Having "options" hardcoded for
such a generally useful feature seems limiting...

>
> 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
> >
> > [...]
>