V2 fixes a crash issue of using an uninitialized hashmap.
Add a hashmap to map the string offsets from a source btf to the
string offsets from a target btf to reduce overheads.
btf__add_btf() calls btf__add_str() to add strings from a source to a
target btf. It causes many string comparisons, and it is a major
hotspot when adding a big btf. btf__add_str() uses strcmp() to check
if a hash entry is the right one. The extra hashmap here compares
offsets of strings, that are much cheaper. It remembers the results
of btf__add_str() for later uses to reduce the cost.
We are parallelizing BTF encoding for pahole by creating separated btf
instances for worker threads. These per-thread btf instances will be
added to the btf instance of the main thread by calling btf__add_str()
to deduplicate and write out. With this patch and -j4, the running
time of pahole drops to about 6.0s from 6.6s.
The following lines are the summary of 'perf stat' w/o the change.
6.668126396 seconds time elapsed
13.451054000 seconds user
0.715520000 seconds sys
The following lines are the summary w/ the change.
5.986973919 seconds time elapsed
12.939903000 seconds user
0.724152000 seconds sys
Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxx>
---
tools/lib/bpf/btf.c | 31 +++++++++++++++++++++++++++++++
1 file changed, 31 insertions(+)
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 9aa19c89f758..8b96418000cc 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -1620,20 +1620,38 @@ static int btf_commit_type(struct btf *btf, int data_sz)
struct btf_pipe {
const struct btf *src;
struct btf *dst;
+ struct hashmap *str_off_map; /* map string offsets from src to dst */
};
static int btf_rewrite_str(__u32 *str_off, void *ctx)
{
struct btf_pipe *p = ctx;
+ void *mapped_off;
int off;
+ int err;
if (!*str_off) /* nothing to do for empty strings */
return 0;
+ if (p->str_off_map &&
+ hashmap__find(p->str_off_map, (void *)(long)*str_off, &mapped_off)) {
+ *str_off = (__u32)(long)mapped_off;
+ return 0;
+ }
+
off = btf__add_str(p->dst, btf__str_by_offset(p->src, *str_off));
if (off < 0)
return off;
+ /* Remember string mapping from src to dst. It avoids
+ * performing expensive string comparisons.
+ */
+ if (p->str_off_map) {
+ err = hashmap__append(p->str_off_map, (void *)(long)*str_off, (void *)(long)off);
+ if (err)
+ return err;
+ }
+
*str_off = off;
return 0;
}
@@ -1680,6 +1698,9 @@ static int btf_rewrite_type_ids(__u32 *type_id, void *ctx)
return 0;
}
+static size_t btf_dedup_identity_hash_fn(const void *key, void *ctx);
+static bool btf_dedup_equal_fn(const void *k1, const void *k2, void *ctx);
+
int btf__add_btf(struct btf *btf, const struct btf *src_btf)
{
struct btf_pipe p = { .src = src_btf, .dst = btf };
@@ -1713,6 +1734,11 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
if (!off)
return libbpf_err(-ENOMEM);
+ /* Map the string offsets from src_btf to the offsets from btf to improve performance */
+ p.str_off_map = hashmap__new(btf_dedup_identity_hash_fn, btf_dedup_equal_fn, NULL);
+ if (p.str_off_map == NULL)
+ return libbpf_err(-ENOMEM);
+
/* bulk copy types data for all types from src_btf */
memcpy(t, src_btf->types_data, data_sz);
@@ -1754,6 +1780,8 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
btf->hdr->str_off += data_sz;
btf->nr_types += cnt;
+ hashmap__free(p.str_off_map);
+
/* return type ID of the first added BTF type */
return btf->start_id + btf->nr_types - cnt;
err_out:
@@ -1767,6 +1795,9 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
* wasn't modified, so doesn't need restoring, see big comment above */
btf->hdr->str_len = old_strs_len;
+ if (p.str_off_map)
+ hashmap__free(p.str_off_map);
+
return libbpf_err(err);
}
--
2.30.2
