bcachefs: Tweak threshold for allocator triggering discards

The allocator path has a "if we're really low on free buckets, check if
we should issue discards" - tweak this to also trigger discards if more
than 1/128th of the device is in need_discard state.

Signed-off-by: Kent Overstreet <[email protected]>

Author: Kent Overstreet

fs/bcachefs/alloc_foreground.c

@@ -511,7 +511,8 @@ static struct open_bucket *bch2_bucket_alloc_trans(struct btree_trans *trans,
 	bch2_dev_usage_read_fast(ca, &req->usage);
 	avail = dev_buckets_free(ca, req->usage, req->watermark);
 
-	if (req->usage.buckets[BCH_DATA_need_discard] > avail)
+	if (req->usage.buckets[BCH_DATA_need_discard] >
+	    min(avail, ca->mi.nbuckets >> 7))
 		bch2_dev_do_discards(ca);
 
 	if (req->usage.buckets[BCH_DATA_need_gc_gens] > avail)