Date: Sat, 19 Jun 2010 23:28:31 +0530 From: "Jayachandran C." <c.jayachandran@gmail.com> To: Alan Cox <alc@cs.rice.edu> Cc: freebsd-mips@freebsd.org Subject: Re: Merging 64 bit changes to -HEAD - part 2 Message-ID: <AANLkTinXryNTq7T60X9V97l5Yqer5oa6Tup_zEVSJ0Cx@mail.gmail.com> In-Reply-To: <4C1CEDFB.9070802@cs.rice.edu> References: <20100617.100235.195066307596264499.imp@bsdimp.com> <AANLkTimkF47RlysFOrma0YhWNDw2w5Lcp9SB1bBoPuxW@mail.gmail.com> <4B66E1A4-E2A5-471F-9FA4-38B506797272@lakerest.net> <20100617.110504.200754750200158040.imp@bsdimp.com> <763BEBBB-B85A-44CE-BFEE-0BADEFF3C185@lakerest.net> <AANLkTikpOaSmTYVfqavtz8T4vZoHWc2CtXIy4SbLlGje@mail.gmail.com> <AANLkTimFjAoXuA-QDwXAOJ0mXK0okIME9SF3841T49zj@mail.gmail.com> <AANLkTil9mqyeWkSUzasZqwfZL4hmeIq-29FXVddc4Q33@mail.gmail.com> <4C1CEDFB.9070802@cs.rice.edu>
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On Sat, Jun 19, 2010 at 9:49 PM, Alan Cox <alc@cs.rice.edu> wrote: > On 6/19/2010 5:18 AM, Juli Mallett wrote: >> >> On Fri, Jun 18, 2010 at 12:41, Neel Natu<neelnatu@gmail.com> wrote: >> >>> >>> Hi JC, >>> >>> But what you really want here is to eliminate the intr_disable() and >>> intr_restore() and keep sched_pin() and sched_unpin(). >>> >> >> Are you sure? I'm not. By disabling interrupts we only have to >> ensure that the fault path on any address we might access within those >> routines doesn't need to use the large memory map. This isn't >> trivial, but I think we can acquire a reasonable confidence about it. >> If we merely pin, we have to ensure that nothing else that can run >> (including interrupts and threads that run via preemption) that would >> access the large memory map — given that this includes routines like >> pmap_zero_page, I think there's good reason for caution. Disabling >> interrupts is more conservative, but I think rightly-so. I may be >> mistaken. >> > > You're not mistaken. See, for example, the i386 pmap_zero_page(). Pinning > by itself is insufficient because a pinned thread can be preempted, and the > thread that then runs (on the same processor) could call pmap_zero_page(). > So, pinning must be combined with a per-processor mutex. > > I can imagine that blocking interrupts on mips is cheaper than the > combination of pinning and a mutex. However, do you want to have interrupts > blocked for the time it takes to read 4KB from DRAM and write 4KB to DRAM > for pmap_copy_page()? Ultimately, that's the question that you need to > answer. The original implementation was to lock and sched_pin(). As Neel noted, the intr_disable was added later as a temporary fix. I think we can go back to lock and pin. If there are no objections, I will post a patch for this. Thanks, JC.help
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