linux-kernel - Re: [PATCH] x86/fpu: don't set TIF_NEED_FPU_LOAD for PF_IO

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Date:   Tue, 24 Jan 2023 17:23:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/fpu: don't set TIF_NEED_FPU_LOAD for PF_IO_WORKER
 threads

On Tue, Jan 24, 2023 at 09:06:08AM -0700, Jens Axboe wrote:
> On 1/24/23 8:40 AM, Peter Zijlstra wrote:
> > On Tue, Jan 24, 2023 at 08:23:20AM -0700, Jens Axboe wrote:
> >> We don't set it on PF_KTHREAD threads as they never return to userspace,
> >> and PF_IO_WORKER threads are identical in that regard. As they keep
> >> running in the kernel until they die, skip setting the FPU flag on them.
> > 
> > No objection to the actual patch; but this changelog fails to tell us
> > why this is important.
> > 
> > What made you get up and write this patch :-) Presumably this is a
> > performance issue? If so, can you quantify how much?
> 
> You snipped the part where that was explained, but arguably that should
> probably go into the commit message itself:
> 
> "Not urgent, more of a cosmetic thing that was found while debugging and
> issue and pondering why the FPU flag is set on these threads."

Duh, I stopped reading at the --- just like a patch tool.. :/

> So it's not really a performance issue, it was just something odd that
> got me scratching my head when debugging another issue and poking at
> the flags.
> 
> Want a resend of it, or will you just augment the commit message?

I think tglx typically takes fpu patches, but sure can do.