linux-kernel - Re: irq_fpu_usable() is irreliable

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Message-ID: <CAHmME9opFmC2494O7-5BL+Fct4CE1k5Rtc5=j3j+qWoZajtRxA@mail.gmail.com>
Date:	Wed, 18 Nov 2015 13:16:09 +0100
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	hpa@...or.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: irq_fpu_usable() is irreliable

Hi Ingo,

Thanks for looking into this.

On Wed, Nov 18, 2015 at 7:55 AM, Ingo Molnar <mingo@...nel.org> wrote:
> Is this 'problem' a performance problem (code not being able to use the FPU
> occasionally and hence sporadically performing poorly), or some sort of actual
> stability/correctness problem?

More of the performance variety; I want the FPU but sometimes don't
have it, that coy mistress. This happens in the ndo_start_xmit() path
of a network driver, which executes with a non-zero softirq_count().
This means that in_interrupt() will be true and
interrupted_user_mode() will be false (confirmed by my tests and by
Thomas' assertions). What isn't clear is why
interrupted_kernel_fpu_idle() is false. In a strange twist of fate,
interrupted_kernel_fpu_idle() is true and thus irq_fpu_usable() is
true when sending TCP packets, but interrupted_kernel_fpu_idle() is
false and thus irq_fpu_usable() is false when sending UDP packets. I
haven't found anything along the UDP path that might result in the FPU
being used, leaving me a bit flummoxed.

So, my inquiries have lead in two directions:
1. Why would interrupted_kernel_fpu_idle() be false here? And does
interrupted_kernel_fpu_idle() depend on what userspace is doing? Or is
it entirely limited to behavior inside the kernel?
2. Most of the time ndo_start_xmit() is reached via a syscall
(sys_write or similar). I know there's a softirq_count() for all sorts
of reasons involving the networking stack, but pretty please - can't
there be some way for irq_fpu_usable() to always be true when the
entry point is a syscall?

> Also, what CPU does the test system have, Intel or AMD? The FPU behavior can be
> very different in the two cases.

Intel 3820QM, but inside VMWare Workstation 12.

> Third, could you post such a problematic stack trace?

Sure: https://paste.kde.org/pfhhdchs9/7mmtvb

Regards,
Jason
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