linux-kernel - Re: [PATCH] ACPI: processor_idle: Fix invalid comparison with insertion sort for latency

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Date: Mon, 1 Jul 2024 05:13:42 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: rafael@...nel.org
Cc: lenb@...nel.org, mario.limonciello@....com, akpm@...ux-foundation.org,
	jserv@...s.ncku.edu.tw, alexdeucher@...il.com, belegdol@...il.com,
	linux-acpi@...r.kernel.org, regressions@...ts.linux.dev,
	regressions@...mhuis.info, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ACPI: processor_idle: Fix invalid comparison with
 insertion sort for latency

+ linux-kernel@...r.kernel.org

On Mon, Jul 01, 2024 at 05:08:09AM +0800, Kuan-Wei Chiu wrote:
> The acpi_cst_latency_cmp comparison function currently used for sorting
> C-state latencies does not satisfy transitivity, causing incorrect
> sorting results. Specifically, if there are two valid acpi_processor_cx
> elements A and B and one invalid element C, it may occur that A < B,
> A = C, and B = C. Sorting algorithms assume that if A < B and A = C,
> then C < B, leading to incorrect ordering.
> 
> Given the small size of the array (<=8), we replace the library sort
> function with a simple insertion sort that properly ignores invalid
> elements and sorts valid ones based on latency. This change ensures
> correct ordering of the C-state latencies.
> 
> Fixes: 65ea8f2c6e23 ("ACPI: processor idle: Fix up C-state latency if not ordered")
> Reported-by: Julian Sikorski <belegdol@...il.com>
> Closes: https://lore.kernel.org/lkml/70674dc7-5586-4183-8953-8095567e73df@gmail.com/
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> I do not have the appropriate AMD hardware to reproduce this issue and
> test the patch. However, if the aforementioned reason is indeed the
> source of the problem, I believe this patch might help.
> 
>  drivers/acpi/processor_idle.c | 33 ++++++++++++---------------------
>  1 file changed, 12 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index bd6a7857ce05..d58a7c64d80b 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -386,25 +386,19 @@ static void acpi_processor_power_verify_c3(struct acpi_processor *pr,
>  	acpi_write_bit_register(ACPI_BITREG_BUS_MASTER_RLD, 1);
>  }
>  
> -static int acpi_cst_latency_cmp(const void *a, const void *b)
> +static void acpi_cst_latency_sort(struct acpi_processor_cx *arr, size_t length)
>  {
> -	const struct acpi_processor_cx *x = a, *y = b;
> +	int i, j, k;
>  
> -	if (!(x->valid && y->valid))
> -		return 0;
> -	if (x->latency > y->latency)
> -		return 1;
> -	if (x->latency < y->latency)
> -		return -1;
> -	return 0;
> -}
> -static void acpi_cst_latency_swap(void *a, void *b, int n)
> -{
> -	struct acpi_processor_cx *x = a, *y = b;
> -
> -	if (!(x->valid && y->valid))
> -		return;
> -	swap(x->latency, y->latency);
> +	for (i = 1; i < length; i++) {
> +		for (j = i - 1, k = i; j >= 0; j--) {
> +			if (!arr[j].valid)
> +				continue;
> +			if (arr[j].latency > arr[k].latency)
> +				swap(arr[j].latency, arr[k].latency);
> +			k = j;
> +		}
> +	}
>  }
>  
>  static int acpi_processor_power_verify(struct acpi_processor *pr)
> @@ -449,10 +443,7 @@ static int acpi_processor_power_verify(struct acpi_processor *pr)
>  
>  	if (buggy_latency) {
>  		pr_notice("FW issue: working around C-state latencies out of order\n");
> -		sort(&pr->power.states[1], max_cstate,
> -		     sizeof(struct acpi_processor_cx),
> -		     acpi_cst_latency_cmp,
> -		     acpi_cst_latency_swap);
> +		acpi_cst_latency_sort(&pr->power.states[1], max_cstate);
>  	}
>  
>  	lapic_timer_propagate_broadcast(pr);
> -- 
> 2.34.1
>