linux-kernel - Re: [RFC PATCH v3 6/7] scripts/sorttable: Add ORC unwind tables sort concurrently

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Date:   Fri, 15 Nov 2019 10:07:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Shile Zhang <shile.zhang@...ux.alibaba.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [RFC PATCH v3 6/7] scripts/sorttable: Add ORC unwind tables sort
 concurrently

On Fri, Nov 15, 2019 at 02:47:49PM +0800, Shile Zhang wrote:

> +#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.

> +/**
> + * 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 = (num/2 - 1) * size, n = num * size, c, r;
> +
> +	/* heapify */
> +	for ( ; i >= 0; i -= size) {
> +		for (r = i; r * 2 + size < n; r  = c) {
> +			c = r * 2 + size;
> +			if (c < n - size &&
> +					cmp_func(base + c, base + c + size) < 0)
> +				c += size;
> +			if (cmp_func(base + r, base + c) >= 0)
> +				break;
> +			swap_func(base + r, base + c, size);
> +		}
> +	}
> +
> +	/* sort */
> +	for (i = n - size; i > 0; i -= size) {
> +		swap_func(base, base + i, size);
> +		for (r = 0; r * 2 + size < i; r = c) {
> +			c = r * 2 + size;
> +			if (c < i - size &&
> +					cmp_func(base + c, base + c + size) < 0)
> +				c += size;
> +			if (cmp_func(base + r, base + c) >= 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.