Re: [PATCH v2] ACPI: processor_idle: Fix invalid comparison with insertion sort for latency

Mario Limonciello
2024-07-01T15:17:21Z

On 6/30/2024 23:42, 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@gmail.com>
> Closes: https://lore.kernel.org/lkml/70674dc7-5586-4183-8953-8095567e73df@gmail.com/
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
> v1 -> v2:
> - Avoid swapping if arr[i] is an invalid element.
>
> I do not have the appropriate AMD hardware to reproduce this issue and
> test the patch.

Even if you had hardware, the behavior stems from a BIOS bug with the
_CST entries. I know it's been fixed in the BIOS on most platforms in
that era, but if I recall correctly from a few years back Julian's
system was EoL already.

> However, if the aforementioned reason is indeed the
> source of the problem, I believe this patch might help.

Thanks!

Assuming this patch works for Julian, I believe you can also drop the
#include <linux/sort.h>

from this file as well.

I do think this should be cc to @stable too in that case.

>
> drivers/acpi/processor_idle.c | 35 ++++++++++++++---------------------
> 1 file changed, 14 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index bd6a7857ce05..813c718b9108 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -386,25 +386,21 @@ 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++) {
> + if (!arr[i].valid)
> + continue;
> + 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 +445,7 @@ 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);