On Thu, May 20, 2021 at 09:43:10AM +0200, Geert Uytterhoeven wrote:

quoted

- values written to the file accept suffixes like K, M
- file is in the per-device directory /sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max
- 0 means use default priority of IO

The scheduler is a simple deadline one and the accuracy is up to nearest
128K.

Signed-off-by: David Sterba <dsterba@suse.com>

Thanks for your patch, which is now commit b4a9f4bee31449bc ("btrfs:
scrub: per-device bandwidth control") in linux-next.

noreply@ellerman.id.au reported the following failures for e.g.
m68k/defconfig:

ERROR: modpost: "__udivdi3" [fs/btrfs/btrfs.ko] undefined!
ERROR: modpost: "__divdi3" [fs/btrfs/btrfs.ko] undefined!

I'll fix it, thanks for the report.

quoted

+static void scrub_throttle(struct scrub_ctx *sctx)
+{
+	const int time_slice = 1000;
+	struct scrub_bio *sbio;
+	struct btrfs_device *device;
+	s64 delta;
+	ktime_t now;
+	u32 div;
+	u64 bwlimit;
+
+	sbio = sctx->bios[sctx->curr];
+	device = sbio->dev;
+	bwlimit = READ_ONCE(device->scrub_speed_max);
+	if (bwlimit == 0)
+		return;
+
+	/*
+	 * Slice is divided into intervals when the IO is submitted, adjust by
+	 * bwlimit and maximum of 64 intervals.
+	 */
+	div = max_t(u32, 1, (u32)(bwlimit / (16 * 1024 * 1024)));
+	div = min_t(u32, 64, div);
+
+	/* Start new epoch, set deadline */
+	now = ktime_get();
+	if (sctx->throttle_deadline == 0) {
+		sctx->throttle_deadline = ktime_add_ms(now, time_slice / div);

ERROR: modpost: "__udivdi3" [fs/btrfs/btrfs.ko] undefined!

div_u64(bwlimit, div)

quoted

+		sctx->throttle_sent = 0;
+	}
+
+	/* Still in the time to send? */
+	if (ktime_before(now, sctx->throttle_deadline)) {
+		/* If current bio is within the limit, send it */
+		sctx->throttle_sent += sbio->bio->bi_iter.bi_size;
+		if (sctx->throttle_sent <= bwlimit / div)
+			return;
+
+		/* We're over the limit, sleep until the rest of the slice */
+		delta = ktime_ms_delta(sctx->throttle_deadline, now);
+	} else {
+		/* New request after deadline, start new epoch */
+		delta = 0;
+	}
+
+	if (delta)
+		schedule_timeout_interruptible(delta * HZ / 1000);

ERROR: modpost: "__divdi3" [fs/btrfs/btrfs.ko] undefined!

I'm a bit surprised gcc doesn't emit code for the division by the
constant 1000, but emits a call to __divdi3().  So this has to become
div_u64(), too.

quoted

+	/* Next call will start the deadline period */
+	sctx->throttle_deadline = 0;
+}

BTW, any chance you can start adding lore Link: tags to your commits, to
make it easier to find the email thread to reply to when reporting a
regression?

Well, no I'm not going to do that, sorry. It should be easy enough to
paste the patch subject to the search field on lore.k.org and click the
link leading to the mail, I do that all the time. Making sure that
patches have all the tags and information takes time already so I'm not
too keen to spend time on adding links.