From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5112AF72.3020201@xenomai.org> Date: Wed, 06 Feb 2013 20:30:58 +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> <5112982B.1020901@xenomai.org> <5112A06A.7030809@siemens.com> <5112A175.5010002@xenomai.org> <5112A269.40609@siemens.com> <5112A392.3050302@xenomai.org> <5112AD78.5080308@siemens.com> In-Reply-To: <5112AD78.5080308@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 08:22 PM, Jan Kiszka wrote: > On 2013-02-06 19:40, Gilles Chanteperdrix wrote: >> On 02/06/2013 07:35 PM, Jan Kiszka wrote: >> >>> On 2013-02-06 19:31, Gilles Chanteperdrix wrote: >>>> On 02/06/2013 07:26 PM, Jan Kiszka wrote: >>>> >>>>> On 2013-02-06 18:51, Gilles Chanteperdrix wrote: >>>>>> 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? >>>>> >>>>> Switching between two tasks that both use the full state space defines >>>>> the maximum latency of the FPU save/restore step. We cannot interrupt >>>>> xsave or xrstor instructions, but we couldn't interrupt fxsave either. >>>>> >>>>> What we can do, though, is to ensure that we have at least an preemption >>>>> point between both. Do we have such thing so far, a chance to handle a >>>>> Xenomai IRQ between some FPU save for Linux task A and a FPU restore for >>>>> the following task B? If not, the discussion is mood and we are just >>>>> shifting probabilities of the very same worst case. >>>> >>>> >>>> We can implement unlocked context switch support on x86 as we do on >>>> other platforms. I tried that on atom actually and it did not really >>>> improve latencies. You do not answer my question though, why would >>>> xsave/xrstor do anything if the RT thread has not used FPU (and all its >>>> parents have not used fpu) ? >>> >>> We first of all would have to wait for the unrelated switch between >>> those two Linux tasks before we could handle the IRQ and switch to the >>> FPU-free RT task. __switch_to is atomic, also for Linux->Linux, no? >> >> >> Only the *IP and *SP switch need to be atomic, the whole __switch_to can >> be split in several atomic sections, this is what I tested on atom. But >> as I said, it did not lead to any latency improvement. > > Ok, so back to the patch about which this discussion started: It > enforced that Linux only saves the FPU state on switches, never directly > restores it but enforces lazy restoring, right? To ensure that > save+restore for Linux tasks is always interruptible in the middle. > However, that sounds pretty expensive when applying FPU/SSE/etc. load on > Linux. To the contrary, the overhead is the cost of the fault (with the user/kernel and kernel/user switches), so, the larger the context switch, the smaller the overhead in proportion. > > Instead of always doing stts for the new task, we could do the restore > later, after the hard_local_irq_enable of __ipipe_switch_tail. That > should allow the eager model for Linux as well without making > save+restore of Linux-Linux switches atomic. That could be done, but it is probably simpler to implement unlocked context switch, and split __switch_to into several atomic sections. Anyway, any change in this area will probably break the work done for kthreads on -forge, so, can't we postpone this? -- Gilles.