From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5112982B.1020901@xenomai.org> Date: Wed, 06 Feb 2013 18:51:39 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <51128CE4.4020303@siemens.com> <51128E3E.808@xenomai.org> <511293EB.1080502@siemens.com> <5112945F.8080102@xenomai.org> <51129599.3080709@siemens.com> <51129693.1040400@xenomai.org> <5112974A.8050008@siemens.com> In-Reply-To: <5112974A.8050008@siemens.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] ipipe/x86: do not restore during context switch List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai On 02/06/2013 06:47 PM, Jan Kiszka wrote: > On 2013-02-06 18:44, Gilles Chanteperdrix wrote: >> On 02/06/2013 06:40 PM, Jan Kiszka wrote: >> >>> On 2013-02-06 18:35, Gilles Chanteperdrix wrote: >>>> On 02/06/2013 06:33 PM, Jan Kiszka wrote: >>>> >>>>> On 2013-02-06 18:09, Gilles Chanteperdrix wrote: >>>>>> On 02/06/2013 06:03 PM, Jan Kiszka wrote: >>>>>> >>>>>>> Gilles, >>>>>>> >>>>>>> do you remember if this core-3.4 change was a performance optimization >>>>>>> or a necessary fix? Also, I'm not yet understanding why we need all the >>>>>>> #ifdefs except for the first one which forces fpu.preload to 0. >>>>>> >>>>>> >>>>>> It is a performance optimization, without it, we systematically hit the >>>>>> maximum latency when the timer would tick during a context switch which >>>>>> restores the FPU. Note that if you change that, you will probably break >>>>>> -forge. >>>>> >>>>> According to the Intel folks who introduced eagerfpu, xsave, or at least >>>>> xsaveopt (which I didn't implemented yet) is now faster than serializing >>>>> clts/stts. On the other hand, the worst case is a full SSE + AVX restore >>>>> while the target RT task is not depending on the FPU. >>>> >>>> >>>> Without xsave, we never restore fpu if the RT task never used it. This >>>> changes with xsave? >>> >>> This would change with eagerfpu which depends on xsave. The kernel >>> sticks with lazy switching in the absence of xsaveopt. >> >> >> I am not sure you understand what I mean, so, I am going to reformulate. >> Without xsave, Linux uses lazy fpu restore, and Xenomai uses eager fpu >> restore. But Xenomai eager fpu restore is a nop if the RT task never >> used FPU since its inception (and all the parents from which it is >> cloned never used FPU either). Does Linux eager switching mean the same >> thing? > > eagerfpu means: always call xsaveopt/xrstor, it will optimize the case > that the FPU was unused by the source/destination. And no fiddling with > TS anymore, at no time. I still do not understand this sentence then: "the worst case is a full SSE + AVX restore while the target RT task is not depending on the FPU." If the RT task does not depend on the FPU, why would xsaveopt/xrstor restore SSE and AVX context? -- Gilles.