@@ -72,6 +72,21 @@ struct btrfs_bio *btrfs_bio_alloc(unsigned int nr_vecs, blk_opf_t opf,
return bbio;
}
+void btrfs_bio_set_csum_search_commit_root(struct btrfs_bio *bbio)
+{
+ bbio->bio.bi_flags |= BTRFS_BIO_FLAG_CSUM_SEARCH_COMMIT_ROOT;
+}
+
+void btrfs_bio_clear_csum_search_commit_root(struct btrfs_bio *bbio)
+{
+ bbio->bio.bi_flags &= ~BTRFS_BIO_FLAG_CSUM_SEARCH_COMMIT_ROOT;
+}
+
+bool btrfs_bio_csum_search_commit_root(struct btrfs_bio *bbio)
+{
+ return bbio->bio.bi_flags & BTRFS_BIO_FLAG_CSUM_SEARCH_COMMIT_ROOT;
+}
+
static struct btrfs_bio *btrfs_split_bio(struct btrfs_fs_info *fs_info,
struct btrfs_bio *orig_bbio,
u64 map_length)
@@ -482,6 +497,9 @@ static void btrfs_submit_mirrored_bio(struct btrfs_io_context *bioc, int dev_nr)
static void btrfs_submit_bio(struct bio *bio, struct btrfs_io_context *bioc,
struct btrfs_io_stripe *smap, int mirror_num)
{
+ /* Clear the flag we stored in bio->bi_flags before submission. */
+ btrfs_bio_clear_csum_search_commit_root(btrfs_bio(bio));
+
if (!bioc) {
/* Single mirror read/write fast path. */
btrfs_bio(bio)->mirror_num = mirror_num;
@@ -104,6 +104,11 @@ struct btrfs_bio *btrfs_bio_alloc(unsigned int nr_vecs, blk_opf_t opf,
btrfs_bio_end_io_t end_io, void *private);
void btrfs_bio_end_io(struct btrfs_bio *bbio, blk_status_t status);
+#define BTRFS_BIO_FLAG_CSUM_SEARCH_COMMIT_ROOT (1U << (BIO_FLAG_LAST + 1))
+void btrfs_bio_set_csum_search_commit_root(struct btrfs_bio *bbio);
+void btrfs_bio_clear_csum_search_commit_root(struct btrfs_bio *bbio);
+bool btrfs_bio_csum_search_commit_root(struct btrfs_bio *bbio);
+
/* Submit using blkcg_punt_bio_submit. */
#define REQ_BTRFS_CGROUP_PUNT REQ_FS_PRIVATE
@@ -99,6 +99,21 @@ struct btrfs_bio_ctrl {
enum btrfs_compression_type compress_type;
u32 len_to_oe_boundary;
blk_opf_t opf;
+ /*
+ * If this is a data read bio, we set this to true if it is safe to
+ * search for csums in the commit root. Otherwise, it is set to false.
+ *
+ * This is an optimization to reduce the contention on the csum tree
+ * root rwsem. Due to how rwsem is implemented, there is a possible
+ * priority inversion where the readers holding the lock don't get
+ * scheduled (say they're in a cgroup stuck in heavy reclaim) which
+ * then blocks btrfs transactions. The only real help is to try to
+ * reduce the contention on the lock as much as we can.
+ *
+ * Do this by detecting when a data read is reading data from an old
+ * transaction so it's safe to look in the commit root.
+ */
+ bool csum_search_commit_root;
btrfs_bio_end_io_t end_io_func;
struct writeback_control *wbc;
@@ -770,6 +785,9 @@ static void submit_extent_folio(struct btrfs_bio_ctrl *bio_ctrl,
alloc_new_bio(inode, bio_ctrl, disk_bytenr,
folio_pos(folio) + pg_offset);
}
+ if (btrfs_op(&bio_ctrl->bbio->bio) == BTRFS_MAP_READ &&
+ is_data_inode(inode) && bio_ctrl->csum_search_commit_root)
+ btrfs_bio_set_csum_search_commit_root(bio_ctrl->bbio);
/* Cap to the current ordered extent boundary if there is one. */
if (len > bio_ctrl->len_to_oe_boundary) {
@@ -1048,6 +1066,9 @@ static int btrfs_do_readpage(struct folio *folio, struct extent_map **em_cached,
if (prev_em_start)
*prev_em_start = em->start;
+ if (em->generation < btrfs_get_fs_generation(fs_info))
+ bio_ctrl->csum_search_commit_root = true;
+
free_extent_map(em);
em = NULL;
@@ -401,6 +401,13 @@ blk_status_t btrfs_lookup_bio_sums(struct btrfs_bio *bbio)
path->skip_locking = 1;
}
+ /* See the comment on btrfs_bio_ctrl->commit_root_csum. */
+ if (btrfs_bio_csum_search_commit_root(bbio)) {
+ path->search_commit_root = 1;
+ path->skip_locking = 1;
+ down_read(&fs_info->commit_root_sem);
+ }
+
while (bio_offset < orig_len) {
int count;
u64 cur_disk_bytenr = orig_disk_bytenr + bio_offset;
@@ -446,6 +453,9 @@ blk_status_t btrfs_lookup_bio_sums(struct btrfs_bio *bbio)
bio_offset += count * sectorsize;
}
+ if (btrfs_bio_csum_search_commit_root(bbio))
+ up_read(&fs_info->commit_root_sem);
+
btrfs_free_path(path);
return ret;
}
If you run a workload like: - a cgroup that does tons of data reading, with a harsh memory limit - a second cgroup that tries to write new files e.g.: https://github.com/boryas/scripts/blob/main/sh/noisy-neighbor/run.sh then what quickly occurs is: - a high degree of contention on the csum root node eb rwsem - memory starved cgroup doing tons of reclaim on CPU. - many reader threads in the memory starved cgroup "holding" the sem as readers, but not scheduling promptly. i.e., task __state == 0, but not running on a cpu. - btrfs_commit_transaction stuck trying to acquire the sem as a writer. This results in VERY long transactions. On my test system, that script produces 20-30s long transaction commits. This then results in seriously degraded performance for any cgroup using the filesystem (the victim cgroup in the script). This reproducer is a bit silly, as the villanous cgroup is using almost all of its memory.max for kernel memory (specifically pagetables) but it sort of doesn't matter, as I am most interested in the btrfs locking behavior. It isn't an academic problem, as we see this exact problem in production at Meta with one cgroup over memory.max ruining btrfs performance for the whole system. The underlying scheduling "problem" with global rwsems is sort of thorny and apparently well known. e.g. https://lpc.events/event/18/contributions/1883/ As a result, our main lever in the short term is just trying to reduce contention on our various rwsems. In the case of the csum tree, we can either redesign btree locking (hard...) or try to use the commit root when we can. Luckily, it seems likely that many reads are for old extents written many transactions ago, and that for those we *can* in fact search the commit root! This change detects when we are trying to read an old extent (according to extent map generation) and then wires that through bio_ctrl to the btrfs_bio, which unfortunately isn't allocated yet when we have this information. Luckily, we don't need this flag in the bio after submitting, so we can save space by setting it on bbio->bio.bi_flags and clear before submitting, so the block layer is unaffected. When we go to lookup the csums in lookup_bio_sums we can check this condition on the btrfs_bio and do the commit root lookup accordingly. With the fix, on that same test case, commit latencies no longer exceed ~400ms on my system. Signed-off-by: Boris Burkov <boris@bur.io> --- Changelog: v4: - replace generic private flag machinery with specific function for the one flag - move the bio_ctrl field to take advantage of alignment v3: - add some simple machinery for setting/getting/clearing btrfs private flags in bi_flags - clear those flags before bio_submit (ensure no-op wrt block layer) - store the csum commit root flag there to save space v2: - hold the commit_root_sem for the duration of the entire lookup, not just per btrfs_search_slot. Note that we can't naively do the thing where we release the lock every loop as that is exactly what we are trying to avoid. Theoretically, we could re-grab the lock and fully start over if the lock is write contended or something. I suspect the rwsem fairness features will let the commit writer get it fast enough anyway. --- fs/btrfs/bio.c | 18 ++++++++++++++++++ fs/btrfs/bio.h | 5 +++++ fs/btrfs/extent_io.c | 21 +++++++++++++++++++++ fs/btrfs/file-item.c | 10 ++++++++++ 4 files changed, 54 insertions(+)