On Fri, Oct 30, 2020 at 4:33 PM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> >
> > From: Andrii Nakryiko <andriin@xxxxxx>
> >
> > Revamp BTF dedup's string deduplication to match the approach of writable BTF
> > string management. This allows to transfer deduplicated strings index back to
> > BTF object after deduplication without expensive extra memory copying and hash
> > map re-construction. It also simplifies the code and speeds it up, because
> > hashmap-based string deduplication is faster than sort + unique approach.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
>
> LGTM, with a couple nitpick below:
>
> Acked-by: Song Liu <songliubraving@xxxxxx>
>
> > ---
> > tools/lib/bpf/btf.c | 265 +++++++++++++++++---------------------------
> > 1 file changed, 99 insertions(+), 166 deletions(-)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index 89fecfe5cb2b..db9331fea672 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -90,6 +90,14 @@ struct btf {
> > struct hashmap *strs_hash;
> > /* whether strings are already deduplicated */
> > bool strs_deduped;
> > + /* extra indirection layer to make strings hashmap work with stable
> > + * string offsets and ability to transparently choose between
> > + * btf->strs_data or btf_dedup->strs_data as a source of strings.
> > + * This is used for BTF strings dedup to transfer deduplicated strings
> > + * data back to struct btf without re-building strings index.
> > + */
> > + void **strs_data_ptr;
> > +
> > /* BTF object FD, if loaded into kernel */
> > int fd;
> >
> > @@ -1363,17 +1371,19 @@ int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
> >
> > static size_t strs_hash_fn(const void *key, void *ctx)
> > {
> > - struct btf *btf = ctx;
> > - const char *str = btf->strs_data + (long)key;
> > + const char ***strs_data_ptr = ctx;
> > + const char *strs = **strs_data_ptr;
> > + const char *str = strs + (long)key;
>
> Can we keep using btf as the ctx here? "char ***" hurts my eyes...
>
yep, changed to struct btf *
> [...]
>
> > - d->btf->hdr->str_len = end - start;
> > + /* replace BTF string data and hash with deduped ones */
> > + free(d->btf->strs_data);
> > + hashmap__free(d->btf->strs_hash);
> > + d->btf->strs_data = d->strs_data;
> > + d->btf->strs_data_cap = d->strs_cap;
> > + d->btf->hdr->str_len = d->strs_len;
> > + d->btf->strs_hash = d->strs_hash;
> > + /* now point strs_data_ptr back to btf->strs_data */
> > + d->btf->strs_data_ptr = &d->btf->strs_data;
> > +
> > + d->strs_data = d->strs_hash = NULL;
> > + d->strs_len = d->strs_cap = 0;
> > d->btf->strs_deduped = true;
> > + return 0;
> > +
> > +err_out:
> > + free(d->strs_data);
> > + hashmap__free(d->strs_hash);
> > + d->strs_data = d->strs_hash = NULL;
> > + d->strs_len = d->strs_cap = 0;
> > +
> > + /* restore strings pointer for existing d->btf->strs_hash back */
> > + d->btf->strs_data_ptr = &d->strs_data;
>
> We have quite some duplicated code in err_out vs. succeed_out cases.
> How about we add a helper function, like
nope, that won't work, free(d->strs_data) vs free(d->btf->strs_data),
same for hashmap__free(), plus there are strict requirements about the
exact sequence of assignments in success case
>
> void free_strs_data(struct btf_dedup *d)
> {
> free(d->strs_data);
> hashmap__free(d->strs_hash);
> d->strs_data = d->strs_hash = NULL;
> d->strs_len = d->strs_cap = 0;
> }
>
> ?
