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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [RFC PATCH v3 6/7] scripts/sorttable: Add ORC unwind tables sort concurrently
Date:   Fri, 15 Nov 2019 09:24:07 -0800
Message-Id: <A4F85A0C-C8A8-4226-A334-276F9D0C2679@amacapital.net>
References: <9594afbc-52bc-5ae7-4a19-8fc4b36a1abd@linux.alibaba.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org
In-Reply-To: <9594afbc-52bc-5ae7-4a19-8fc4b36a1abd@linux.alibaba.com>
To:     Shile Zhang <shile.zhang@linux.alibaba.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 15, 2019, at 1:43 AM, Shile Zhang <shile.zhang@linux.alibaba.com> w=
rote:
>=20
> =EF=BB=BF
>=20
>> On 2019/11/15 17:07, Peter Zijlstra wrote:
>>> On Fri, Nov 15, 2019 at 02:47:49PM +0800, Shile Zhang wrote:
>>>=20
>>> +#if defined(SORTTABLE_64) && defined(UNWINDER_ORC_ENABLED)
>>> +/* ORC unwinder only support X86_64 */
>>> +#include <errno.h>
>>> +#include <pthread.h>
>>> +#include <linux/types.h>
>>> +
>>> +#define ORC_REG_UNDEFINED    0
>>> +#define ERRSTRING_MAXSZ        256
>>> +
>>> +struct orc_entry {
>>> +    s16        sp_offset;
>>> +    s16        bp_offset;
>>> +    unsigned    sp_reg:4;
>>> +    unsigned    bp_reg:4;
>>> +    unsigned    type:2;
>>> +    unsigned    end:1;
>>> +} __attribute__((packed));
>>> +
>>> +struct orctable_info {
>>> +    size_t    orc_size;
>>> +    size_t    orc_ip_size;
>>> +} orctable;
>> There's ./arch/x86/include/asm/orc_types.h for this. Please don't
>> duplicate. objtool uses that same header.
> Good catch! Thanks for your kindly reminder! I'll remove it.
>>> +/**
>>> + * sort - sort an array of elements
>>> + * @base: pointer to data to sort
>>> + * @num: number of elements
>>> + * @size: size of each element
>>> + * @cmp_func: pointer to comparison function
>>> + * @swap_func: pointer to swap function
>>> + *
>>> + * This function does a heapsort on the given array. You may provide a
>>> + * swap_func function optimized to your element type.
>>> + *
>>> + * Sorting time is O(n log n) both on average and worst-case. While
>>> + * qsort is about 20% faster on average, it suffers from exploitable
>>> + * O(n*n) worst-case behavior and extra memory requirements that make
>>> + * it less suitable for kernel use.
>>> + *
>>> + * This code token out of /lib/sort.c.
>>> + */
>>> +static void sort(void *base, size_t num, size_t size,
>>> +      int (*cmp_func)(const void *, const void *),
>>> +      void (*swap_func)(void *, void *, int size))
>>> +{
>>> +    /* pre-scale counters for performance */
>>> +    int i =3D (num/2 - 1) * size, n =3D num * size, c, r;
>>> +
>>> +    /* heapify */
>>> +    for ( ; i >=3D 0; i -=3D size) {
>>> +        for (r =3D i; r * 2 + size < n; r  =3D c) {
>>> +            c =3D r * 2 + size;
>>> +            if (c < n - size &&
>>> +                    cmp_func(base + c, base + c + size) < 0)
>>> +                c +=3D size;
>>> +            if (cmp_func(base + r, base + c) >=3D 0)
>>> +                break;
>>> +            swap_func(base + r, base + c, size);
>>> +        }
>>> +    }
>>> +
>>> +    /* sort */
>>> +    for (i =3D n - size; i > 0; i -=3D size) {
>>> +        swap_func(base, base + i, size);
>>> +        for (r =3D 0; r * 2 + size < i; r =3D c) {
>>> +            c =3D r * 2 + size;
>>> +            if (c < i - size &&
>>> +                    cmp_func(base + c, base + c + size) < 0)
>>> +                c +=3D size;
>>> +            if (cmp_func(base + r, base + c) >=3D 0)
>>> +                break;
>>> +            swap_func(base + r, base + c, size);
>>> +        }
>>> +    }
>>> +}
>> Do we really need to copy the heapsort implementation? That is, why not
>> use libc's qsort() ? This is userspace after all.
>=20
> Yes, I think qsort is better choice than copy-paste here. But qsort does n=
ot support customized swap func, which is needed for ORC unwind swap two tab=
les together.
> I think it's hard to do with qsort, so I used sort same with original orc u=
nwind table sort.

One solution is to make an array of indices 0, 1, 2, etc, and sort that usin=
g a comparison function that compares i,j by actually comparing source[i], s=
ource[j]. (Or use pointers, but indices are probably faster on a 64-bit mach=
ine if you can use 32-bit indices.) Then, after sorting, permute the origina=
l array using the now-sorted indices. In the case where swapping is expensiv=
e, this is actually faster, since it does exactly n moves instead of O(n log=
 n).

>=20