lightnvm: pblk: add padding distribution sysfs attribute

When pblk receives a sync, all data up to that point in the write buffer
must be comitted to persistent storage, and as flash memory comes with a
minimal write size there is a significant cost involved both in terms
of time for completing the sync and in terms of write amplification
padded sectors for filling up to the minimal write size.

In order to get a better understanding of the costs involved for syncs,
Add a sysfs attribute to pblk: padded_dist, showing a normalized
distribution of sectors padded. In order to facilitate measurements of
specific workloads during the lifetime of the pblk instance, the
distribution can be reset by writing 0 to the attribute.

Do this by introducing counters for each possible padding:
{0..(minimal write size - 1)} and calculate the normalized distribution
when showing the attribute.

Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Javier González <javier@cnexlabs.com>
Rearranged total_buckets statement in pblk_sysfs_get_padding_dist
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 9 insertions, 8 deletions

diff --git a/drivers/lightnvm/pblk-rb.c b/drivers/lightnvm/pblk-rb.c
index 7044b5599cc4..8b1434060fb3 100644
--- a/drivers/lightnvm/pblk-rb.c
+++ b/drivers/lightnvm/pblk-rb.c
@@ -437,9 +437,7 @@ static int pblk_rb_may_write_flush(struct pblk_rb *rb, unsigned int nr_entries,
 	if (bio->bi_opf & REQ_PREFLUSH) {
 		struct pblk *pblk = container_of(rb, struct pblk, rwb);
 
-#ifdef CONFIG_NVM_DEBUG
-		atomic_long_inc(&pblk->nr_flush);
-#endif
+		atomic64_inc(&pblk->nr_flush);
 		if (pblk_rb_flush_point_set(&pblk->rwb, bio, mem))
 			*io_ret = NVM_IO_OK;
 	}
@@ -620,14 +618,17 @@ try:
 			pr_err("pblk: could not pad page in write bio\n");
 			return NVM_IO_ERR;
 		}
-	}
 
-	atomic64_add(pad, &((struct pblk *)
-			(container_of(rb, struct pblk, rwb)))->pad_wa);
+		if (pad < pblk->min_write_pgs)
+			atomic64_inc(&pblk->pad_dist[pad - 1]);
+		else
+			pr_warn("pblk: padding more than min. sectors\n");
+
+		atomic64_add(pad, &pblk->pad_wa);
+	}
 
 #ifdef CONFIG_NVM_DEBUG
-	atomic_long_add(pad, &((struct pblk *)
-			(container_of(rb, struct pblk, rwb)))->padded_writes);
+	atomic_long_add(pad, &pblk->padded_writes);
 #endif
 
 	return NVM_IO_OK;