Re: [PATCH 2/8] xor: improve the runtime selection benchmark

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] A division by zero kernel panic will occur during boot on systems using a coarse clocksource when calculating the benchmark speed.
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--- Patch [2]: [PATCH 2/8] xor: improve the runtime selection benchmark ---

commit 69c8bf90e6d6fa234ebb61b5b5b7c4dfd2d8e9c1
Author: Christoph Hellwig [off-list ref]

xor: improve the runtime selection benchmark

This commit updates the XOR benchmark to use ktime_get_ns() for timing,
increases the workload to 4 source disks, and calculates throughput based
on the data disks.

[ ... ]

[ ... ]

-static void __init
-do_xor_speed(struct xor_block_template *tmpl, void *b1, void *b2)
+static void __init do_xor_speed(struct xor_block_template *tmpl, void *dest,
+		void *srcs[NR_SRCS])
 {

[ ... ]

+	t = ktime_get_ns();
+	for (i = 0; i < REPS; i++) {
 		mb(); /* prevent loop optimization */
-		tmpl->xor_gen(b1, srcs, 1, BENCH_SIZE);
+		tmpl->xor_gen(dest, srcs, NR_SRCS, BENCH_SIZE);
 		mb();
-	} while (reps++ < REPS || (t0 = ktime_get()) == start);
-	min = ktime_sub(t0, start);
-
+	}
+	t = ktime_get_ns() - t;
 	preempt_enable();
 
-	// bytes/ns == GB/s, multiply by 1000 to get MB/s [not MiB/s]
-	speed = (1000 * reps * BENCH_SIZE) / (unsigned int)ktime_to_ns(min);
-	tmpl->speed = speed;
+	/* bytes/ns == GB/s, multiply by 1000 to get MB/s [not MiB/s] */
+	tmpl->speed = div64_u64((u64)BENCH_SIZE * REPS * NR_SRCS * 1000, t);

[Severity: High]
Could this cause a division by zero panic during boot on systems with a
coarse clocksource?

If the 800 loop iterations execute very quickly (within the same clock tick)
on a platform without high-resolution timers, or during early boot phases
where jiffies are used as the clocksource, the elapsed time t will evaluate
to 0.

The previous code avoided this by looping until the clock advanced before
and after the benchmark:

    while ((start = ktime_get()) == t0)

Since t is now used directly as the divisor in div64_u64(), a 0 value will
trigger an unhandled divide-by-zero hardware exception.

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Sashiko AI review · https://sashiko.dev/#/patchset/20260708090740.1433685-1-hch@lst.de?part=2