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

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Message-ID: <b03383e3-cb10-8b70-a042-dbc2170ee3ec@kernel.dk>
Date:   Tue, 24 Jan 2023 09:06:08 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>
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 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."

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?

-- 
Jens Axboe