netdev - Re: [PATCH v2 01/17] asm: simd context helper API

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Date:   Sun, 26 Aug 2018 08:18:15 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Andrew Lutomirski <luto@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>, linux-arch@...r.kernel.org,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2 01/17] asm: simd context helper API

On Sun, Aug 26, 2018 at 8:06 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > Do you mean to say you intend to make kernel_fpu_end() and
> > kernel_neon_end() only actually do something upon context switch, but
> > not when it's actually called? So that multiple calls to
> > kernel_fpu_begin() and kernel_neon_begin() can be made without
> > penalty?
>
> On context switch and exit to user. That allows to keep those code pathes
> fully preemptible. Still twisting my brain around the details.

Just to make sure we're on the same page, the goal is so that this code:

kernel_fpu_begin();
kernel_fpu_end();
kernel_fpu_begin();
kernel_fpu_end();
kernel_fpu_begin();
kernel_fpu_end();
kernel_fpu_begin();
kernel_fpu_end();
kernel_fpu_begin();
kernel_fpu_end();
kernel_fpu_begin();
kernel_fpu_end();
...

has the same performance as this code:

kernel_fpu_begin();
kernel_fpu_end();

(Unless of course the process is preempted or the like.)

Currently the present situation makes the performance of the above
wildly different, since kernel_fpu_end() does something immediately.

What about something like this:
- Add a tristate flag connected to task_struct (or in the global fpu
struct in the case that this happens in irq and there isn't a valid
current).
- On kernel_fpu_begin(), if the flag is 0, do the usual expensive
XSAVE stuff, and set the flag to 1.
- On kernel_fpu_begin(), if the flag is non-0, just set the flag to 1
and return.
- On kernel_fpu_end(), if the flag is non-0, set the flag to 2.
(Otherwise WARN() or BUG() or something.)
- On context switch / preemption / etc away from the task, if the flag
is non-0, XRSTOR and such.
- On context switch / preemption / etc back to the task, if the flag
is 1, XSAVE and such. If the flag is 2, set it to 0.

Jason