From: Tang Junhui <tang.junhui@xxxxxxxxxx>
When I saw the code "int64_t error = dirty - cache_dirty_target", I was
totally lost, I will try my best to get out.
And this patch dependent on your earlier patch(PI controller), did that
patch have applied to upstreams branch? I searched in 4.14 branch,
and did not find it.
> drivers/md/bcache/writeback.c | 38 ++++++++++++++++++++++++++++++--------
> 1 file changed, 30 insertions(+), 8 deletions(-)
>
>diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
>index 56a37884ca8b..be2840ad02d4 100644
>--- a/drivers/md/bcache/writeback.c
>+++ b/drivers/md/bcache/writeback.c
>@@ -26,16 +26,28 @@ static void __update_writeback_rate(struct cached_dev *dc)
> bcache_flash_devs_sectors_dirty(c);
> uint64_t cache_dirty_target =
> div_u64(cache_sectors * dc->writeback_percent, 100);
>- int64_t target = div64_u64(cache_dirty_target * bdev_sectors(dc->bdev),
>- c->cached_dev_sectors);
>+
>+ /*
>+ * 16384 is chosen here as something that each backing device should
>+ * be a reasonable fraction of the share, and to not blow up until
>+ * individual backing devices are a petabyte.
>+ */
>+ uint32_t bdev_share_per16k =
>+ div64_u64(16384 * bdev_sectors(dc->bdev),
>+ c->cached_dev_sectors);
>
> /*
> * PI controller:
> * Figures out the amount that should be written per second.
> *
> * First, the error (number of sectors that are dirty beyond our
>- * target) is calculated. The error is accumulated (numerically
>- * integrated).
>+ * target) is calculated.
>+ *
>+ * Unfortunately we don't know the exact share of dirty data for
>+ * each backing device; therefore the error is adjusted for this
>+ * backing disk's share of the overall error based on size.
>+ *
>+ * The error is accumulated (numerically integrated).
> *
> * Then, the proportional value and integral value are scaled
> * based on configured values. These are stored as inverses to
>
>@@ -50,12 +62,22 @@ static void __update_writeback_rate(struct cached_dev *dc)
> * variations in usage like the p term.
> */
> int64_t dirty = bcache_dev_sectors_dirty(&dc->disk);
>- int64_t error = dirty - target;
>- int64_t proportional_scaled =
>- div_s64(error, dc->writeback_rate_p_term_inverse);
>- int64_t integral_scaled;
>+ int64_t error = dirty - cache_dirty_target;
>+ int64_t proportional_scaled, integral_scaled;
> uint32_t new_rate;
>
>+ /*
>+ * Ensure even with large device size disparities that we still
>+ * writeback at least some.
>+ */
>+ if (bdev_share_per16k < 1)
>+ bdev_share_per16k = 1;
>+
>+ target = div_s64(error * bdev_share_per16k, 16384)
>+
>+ proportional_scaled = div_s64(error,
>+ dc->writeback_rate_p_term_inverse);
>+
> if ((error < 0 && dc->writeback_rate_integral > 0) ||
> (error > 0 && time_before64(local_clock(),
> dc->writeback_rate.next + NSEC_PER_MSEC))) {
>--
>2.14.1
Thanks,
Tang Junhui
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