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

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Message-ID: <Y8/8CFwtpmjzdK9c@hirez.programming.kicks-ass.net>
Date:   Tue, 24 Jan 2023 16:40:56 +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 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?