[PATCH 1/3] drm/xe/gt_stats: Detect and log overflow

[Lucas De Marchi]

On Tue, Feb 25, 2025 at 09:41:15AM -0600, Lucas De Marchi wrote:
>On Tue, Feb 25, 2025 at 02:17:04PM +0100, Francois Dugast wrote:
>>Check if the atomic counter limit has been reached and if that is the
>>case, set the counter value to the new increment to prevent keeping
>>a garbage value, and notify it in the kernel log as below:
>>
>>   [drm:xe_gt_stats_incr [xe]] GT0: stats vma_pagefault_bytes overflow, resetting
>>
>>Signed-off-by: Francois Dugast <francois.dugast at intel.com>
>>---
>>drivers/gpu/drm/xe/xe_gt_stats.c | 18 ++++++++++++------
>>1 file changed, 12 insertions(+), 6 deletions(-)
>>
>>diff --git a/drivers/gpu/drm/xe/xe_gt_stats.c b/drivers/gpu/drm/xe/xe_gt_stats.c
>>index 2e9879ea4674..753c081b68a6 100644
>>--- a/drivers/gpu/drm/xe/xe_gt_stats.c
>>+++ b/drivers/gpu/drm/xe/xe_gt_stats.c
>>@@ -8,8 +8,15 @@
>>#include <drm/drm_print.h>
>>
>>#include "xe_gt.h"
>>+#include "xe_gt_printk.h"
>>#include "xe_gt_stats.h"
>>
>>+static const char *const stat_description[__XE_GT_STATS_NUM_IDS] = {
>>+	"tlb_inval_count",
>>+	"vma_pagefault_count",
>>+	"vma_pagefault_bytes",
>>+};
>>+
>>/**
>> * xe_gt_stats_incr - Increments the specified stats counter
>> * @gt: GT structure
>>@@ -24,13 +31,12 @@ void xe_gt_stats_incr(struct xe_gt *gt, const enum xe_gt_stats_id id, int incr)
>>		return;
>>
>>	atomic_add(incr, &gt->stats.counters[id]);
>>-}
>>
>>-static const char *const stat_description[__XE_GT_STATS_NUM_IDS] = {
>>-	"tlb_inval_count",
>>-	"vma_pagefault_count",
>>-	"vma_pagefault_bytes",
>>-};
>>+	if (atomic_read(&gt->stats.counters[id]) < 0) {
>
>why are we not using unsigned logic? it stops being atomic when you
>update/read/set.

Also see "TYPES (signed vs unsigned)" in Documentation/atomic_t.txt
as I don't think what you did here really works.

Lucas De Marchi

>
>Lucas De Marchi
>
>>+		xe_gt_dbg(gt, "stats %s overflow, resetting\n", stat_description[id]);
>>+		atomic_set(&gt->stats.counters[id], incr);
>>+	}
>>+}
>>
>>/**
>> * xe_gt_stats_print_info - Print the GT stats
>>-- 
>>2.43.0
>>