Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets

From	David Miller <davem@davemloft.net>
Newsgroups	linux.kernel
Subject	Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets
Date	2015-11-16 21:40 +0100
Message-ID	<qvyJb-3Ft-9@gated-at.bofh.it> (permalink)
References	<qvy6u-3b3-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Mon, 16 Nov 2015 20:52:28 +0100

> This works fine with, say, iperf3 in TCP mode. The AVX performance
> is great. However, when using iperf3 in UDP mode, irq_fpu_usable()
> is mostly false! I added a dump_stack() call to see why, except
> nothing looks strange; the initial call in the stack trace is
> entry_SYSCALL_64_fastpath. Why would irq_fpu_usable() return false
> when we're in a syscall? Doesn't that mean this is in process
> context?

Network device driver transmit executes with software interrupts
disabled.

Therefore on x86, you cannot use the FPU.
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Thread

irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-16 21:00 +0100
  Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets David Miller <davem@davemloft.net> - 2015-11-16 21:40 +0100
    Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-16 22:00 +0100
      Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets David Miller <davem@davemloft.net> - 2015-11-16 22:20 +0100
        Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-16 22:30 +0100
          Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-16 22:40 +0100
          Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets David Miller <davem@davemloft.net> - 2015-11-16 22:40 +0100
      Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets Hannes Frederic Sowa <hannes@stressinduktion.org> - 2015-11-16 23:30 +0100
        Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-17 01:00 +0100
          Re: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets "Jason A. Donenfeld" <Jason@zx2c4.com> - 2015-11-17 01:10 +0100
    RE: irq_fpu_usable() is false in ndo_start_xmit() for UDP packets David Laight <David.Laight@ACULAB.COM> - 2015-11-17 13:40 +0100

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