Date: Thu, 03 Jun 2010 11:32:57 -0500 From: Alan Cox <alc@cs.rice.edu> To: "C. Jayachandran" <c.jayachandran@gmail.com> Cc: src-committers@freebsd.org, "Jayachandran C." <jchandra@freebsd.org>, svn-src-all@freebsd.org, Randall Stewart <rrs@lakerest.net>, Konstantin Belousov <kostikbel@gmail.com>, svn-src-head@freebsd.org Subject: Re: svn commit: r208589 - head/sys/mips/mips Message-ID: <4C07D939.6010008@cs.rice.edu> In-Reply-To: <AANLkTil2gE1niUWCHnsTlQvibhxBh7QYwD0TTWo0rj5c@mail.gmail.com> References: <201005271005.o4RA5eVu032269@svn.freebsd.org> <4C058145.3000707@cs.rice.edu> <AANLkTil955Ek-a3tek4Ony9NqrK5l2j7lNA4baRVPBzb@mail.gmail.com> <4C05F9BC.40409@cs.rice.edu> <AANLkTikdnvXsTwm8onl3MZPQmVfV-2GovB9--KMNnMgC@mail.gmail.com> <4C0731A5.7090301@cs.rice.edu> <AANLkTimIa3jmBPMhWIOcY6DenGpZ2ZYmqwDTWspVx0-u@mail.gmail.com> <AANLkTil2gE1niUWCHnsTlQvibhxBh7QYwD0TTWo0rj5c@mail.gmail.com>
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C. Jayachandran wrote: > On Thu, Jun 3, 2010 at 10:20 AM, C. Jayachandran > <c.jayachandran@gmail.com> wrote: > >> On Thu, Jun 3, 2010 at 10:07 AM, Alan Cox <alc@cs.rice.edu> wrote: >> >>> C. Jayachandran wrote: >>> >>>> On Wed, Jun 2, 2010 at 11:57 AM, Alan Cox <alc@cs.rice.edu> wrote: >>>> >>>> >>>>> C. Jayachandran wrote: >>>>> >>>>> >>>>>> On Wed, Jun 2, 2010 at 3:23 AM, Alan Cox <alc@cs.rice.edu> wrote: >>>>>> >>>>>> >>>>>> >>>>>>> On 5/27/2010 5:05 AM, Jayachandran C. wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> Author: jchandra >>>>>>>> Date: Thu May 27 10:05:40 2010 >>>>>>>> New Revision: 208589 >>>>>>>> URL: http://svn.freebsd.org/changeset/base/208589 >>>>>>>> >>>>>>>> Log: >>>>>>>> Call VM_WAIT in pmap_ptpgzone_allocf() if M_WAITOK is set. >>>>>>>> Removed unused variable. >>>>>>>> >>>>>>>> Approved by: rrs (mentor) >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> I'm afraid that this will work some of the time, but not all of the >>>>>>> time. >>>>>>> Specifically, there is no guarantee that any of the available free (or >>>>>>> cached) pages after the VM_WAIT will fall within the range of suitable >>>>>>> physical addresses. Moreover, and perhaps most worrisome, is that this >>>>>>> function could do a lot of spinning. Every time this function sleeps >>>>>>> and >>>>>>> a >>>>>>> single page is freed (or cached) by someone else, this function will be >>>>>>> reawakened. With a little bad luck, you could spin indefinitely. >>>>>>> >>>>>>> For essentially this reason, contigmalloc(), kmem_alloc_contig(), and >>>>>>> kmem_alloc_attr() don't use VM_WAIT, but instead a function called >>>>>>> vm_contig_grow_cache(). >>>>>>> >>>>>>> >>>>>>> >>>>>> I had seen the vm_contig_grow_cache() usage, but could not use it as >>>>>> it was defined as a static function. >>>>>> >>>>>> I did not want to use contigmalloc()/kmem_alloc_contig() either, >>>>>> because the pages in that memory region are already direct mapped and >>>>>> setting up another mapping is not necessary. The overall idea is to >>>>>> allocate pages in the direct mapped region for the page table entries >>>>>> so that we don't take a TLB exception while accessing page tables >>>>>> (which is costly as MIPS has a software TLB exception handler). >>>>>> >>>>>> Can you suggest the right way to do this? I will make the changes and >>>>>> send a patch for approval. >>>>>> >>>>>> >>>>>> >>>>> I would encourage you to make vm_contig_grow_cache() an extern function >>>>> and >>>>> use it. (vm/vm_pageout.h is probably the best place for the function >>>>> prototype.) >>>>> >>>>> >>>> I'll create a patch for this and related changes in mips/mips/pmap.c >>>> >>>> >>>> >>> Great. Thanks! >>> >>> >>>>> If you're interested in taking this a small step further, it would make >>>>> sense to add two parameters to vm_contig_grow_cache() and >>>>> vm_contig_launder(): a "low" and a "high" physical address. >>>>> vm_contig_launder() could then skip pages that are outside the desired >>>>> range. >>>>> >>>>> >>>> I am looking at this now, part of the logic which >>>> vm_phys_alloc_contig() uses to check pages address should work here. >>>> Will send out changes for this, if it works out. >>>> >>>> >>>> >>> I suspect that you'll just need to add two or three lines to >>> vm_contig_launder(). If something doesn't make sense, just e-mail me. >>> >> The current changes I have is attached - still testing it. >> > > I've attached the patch for review, but I was not able trigger the > condition in which vm_contig_grow_cache() is called during testing so > far. > > But if you are okay with the patch I can commit it. > The patch looks good. Please go ahead and commit it. Thanks, Alan
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