linux-kernel - Re: HPET regression in 2.6.26 versus 2.6.25 -- revert for 2.6.26-rc1 failed

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Date:	Thu, 14 Aug 2008 03:36:35 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Bill Fink" <billfink@...dspring.com>
Cc:	"David Witbrodt" <dawitbro@...global.net>,
	linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, netdev <netdev@...r.kernel.org>
Subject: Re: HPET regression in 2.6.26 versus 2.6.25 -- revert for 2.6.26-rc1 failed

On Thu, Aug 14, 2008 at 3:04 AM, Bill Fink <billfink@...dspring.com> wrote:
> Hi David,
>
> On Wed, 13 Aug 2008, David Witbrodt wrote:
>
>> [Yinghai, please note that I did not request a patch to revert the
>> problem commit.  I was merely experimenting -- on my own time, so
>> you folks would not have to bother -- to see if I could make it
>> work.  I should have made that more clear!  Having said that, I am
>> glad to test changes of any kind on my machine:  reverts, code for
>> debugging or info, experiments, etc.]
>
> I'm not sure Yinghai's revert patch is completely equivalent to
> a revert of the original problematic commit, by a side-by-side
> comparison of the original commit with his recent revert patch,
> but then I don't really know that code at all.
>
> In the original code there was a section (in e820_reserve_resources()):
>
> #ifdef CONFIG_KEXEC
>                   if (crashk_res.start != crashk_res.end)
>                           request_resource(res, &crashk_res);
> #endif
>
> If you don't have CONFIG_KEXEC defined in your .config, which is
> probably the case, then you would never request a crashk_res resource.
> But in the code after the original commit, it unconditionally calls
> (in reserve_crashkernel()):
>
>           crashk_res.start = crash_base;
>           crashk_res.end   = crash_base + crash_size - 1;
>           insert_resource(&iomem_resource, &crashk_res);
>
> And after Yinghai's revert patch it still does (in reserve_crashkernel()):
>
>        crashk_res.start = crash_base;
>        crashk_res.end   = crash_base + crash_size - 1;
>        crashk_res_ptr = &crashk_res;
>
> and (in setup_arch()):
>
>        num_res = 3;
>        if (crashk_res_ptr) {
>                res_kernel[num_res] = crashk_res_ptr;
>                num_res++;
>        }
>        e820_reserve_resources(res_kernel, num_res);
>
> then (in e820_reserve_resources()):
>
>                        for (j = 0; j < nr_res_k; j++) {
>                                if (!res_kernel[j])
>                                        continue;
>                                request_resource(res, res_kernel[j]);
>                        }
>
> which for j == 3 is:
>
>        request_resource(res, &crashk_res);
>
> Now it would appear that the new:
>
>        insert_resource(&iomem_resource, &crashk_res);
>
> or new:
>
>        request_resource(res, &crashk_res);
>
> should be noops.  But if for any reason crash_size is not zero,
> then there could be a difference.  I have no idea if this is at all
> significant, but I thought I'd point it out just in case.

why oops ?
if not valid crash kernel size etc is input, crashk_res_ptr will be null

>        if (crashk_res_ptr) {
>                res_kernel[num_res] = crashk_res_ptr;
>                num_res++;
>        }

it that is not appended to res_kernel...

YH
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