Re: [PATCH 3/3] 9pfs: reduce latency of Twalk

[Christian Schoenebeck]

On Donnerstag, 27. Mai 2021 19:05:43 CEST Christian Schoenebeck wrote:
> As on the previous performance optimization on Treaddir handling;
> reduce the overall latency, i.e. overall time spent on processing
> a Twalk request by reducing the amount of thread hops between the
> 9p server's main thread and fs worker thread(s).
> 
> In fact this patch even reduces the thread hops for Twalk handling
> to its theoritical minimum of exactly 2 thread hops:
> 
> main thread -> fs worker thread -> main thread
> 
> This is achieved by doing all the required fs driver tasks altogether
> in a single v9fs_co_run_in_worker({ ... }); code block.
> 
> This patches also changes the way how an arbitrary path is
> identified to whether it equals the 9p export root. Previously
> QIDs were compared for this, which forces to be done on main thread
> for resolving individual path element QIDs. For that reason POSIX
> stat device number and inode number pairs are compared instead now.
> Accordingly, as 9p server's root_qid member variable is no longer
> used, nor are functions fid_to_qid() and not_same_qid(), hence drop
> them.
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  hw/9pfs/9p.c | 118 +++++++++++++++++++++++++++++++++------------------
>  hw/9pfs/9p.h |   1 -
>  2 files changed, 76 insertions(+), 43 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 825de1561d..cc1b176eb5 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -971,23 +971,6 @@ static int stat_to_qid(V9fsPDU *pdu, const struct stat
> *stbuf, V9fsQID *qidp) return 0;
>  }
> 
> -static int coroutine_fn fid_to_qid(V9fsPDU *pdu, V9fsFidState *fidp,
> -                                   V9fsQID *qidp)
> -{
> -    struct stat stbuf;
> -    int err;
> -
> -    err = v9fs_co_lstat(pdu, &fidp->path, &stbuf);
> -    if (err < 0) {
> -        return err;
> -    }
> -    err = stat_to_qid(pdu, &stbuf, qidp);
> -    if (err < 0) {
> -        return err;
> -    }
> -    return 0;
> -}
> -
>  V9fsPDU *pdu_alloc(V9fsState *s)
>  {
>      V9fsPDU *pdu = NULL;
> @@ -1461,7 +1444,6 @@ static void coroutine_fn v9fs_attach(void *opaque)
>      }
>      err += offset;
> 
> -    memcpy(&s->root_qid, &qid, sizeof(qid));
>      memcpy(&s->root_st, &stbuf, sizeof(struct stat));
>      trace_v9fs_attach_return(pdu->tag, pdu->id,
>                               qid.type, qid.version, qid.path);
> @@ -1713,12 +1695,9 @@ static bool name_is_illegal(const char *name)
>      return !*name || strchr(name, '/') != NULL;
>  }
> 
> -static bool not_same_qid(const V9fsQID *qid1, const V9fsQID *qid2)
> +static bool same_stat_id(const struct stat *a, const struct stat *b)
>  {
> -    return
> -        qid1->type != qid2->type ||
> -        qid1->version != qid2->version ||
> -        qid1->path != qid2->path;
> +    return a->st_dev == b->st_dev && a->st_ino == b->st_ino;
>  }
> 
>  static void coroutine_fn v9fs_walk(void *opaque)
> @@ -1726,9 +1705,9 @@ static void coroutine_fn v9fs_walk(void *opaque)
>      int name_idx;
>      V9fsQID *qids = NULL;
>      int i, err = 0;
> -    V9fsPath dpath, path;
> +    V9fsPath dpath, path, *pathes = NULL;
>      uint16_t nwnames;
> -    struct stat stbuf;
> +    struct stat stbuf, fidst, *stbufs = NULL;
>      size_t offset = 7;
>      int32_t fid, newfid;
>      V9fsString *wnames = NULL;
> @@ -1754,6 +1733,8 @@ static void coroutine_fn v9fs_walk(void *opaque)
>      if (nwnames) {
>          wnames = g_new0(V9fsString, nwnames);
>          qids   = g_new0(V9fsQID, nwnames);
> +        stbufs = g_new0(struct stat, nwnames);
> +        pathes = g_new0(V9fsPath, nwnames);
>          for (i = 0; i < nwnames; i++) {
>              err = pdu_unmarshal(pdu, offset, "s", &wnames[i]);
>              if (err < 0) {
> @@ -1774,35 +1755,85 @@ static void coroutine_fn v9fs_walk(void *opaque)
> 
>      v9fs_path_init(&dpath);
>      v9fs_path_init(&path);
> +    /*
> +     * Both dpath and path initially point to fidp.
> +     * Needed to handle request with nwnames == 0
> +     */
> +    v9fs_path_copy(&dpath, &fidp->path);
> +    v9fs_path_copy(&path, &fidp->path);
> 
> -    err = fid_to_qid(pdu, fidp, &qid);
> +    /*
> +     * To keep latency (i.e. overall execution time for processing this
> +     * Twalk client request) as small as possible, run all the required fs
> +     * driver code altogether inside the following block.
> +     */
> +    v9fs_co_run_in_worker({
> +        if (v9fs_request_cancelled(pdu)) {
> +            err = -EINTR;
> +            break;
> +        }
> +        err = s->ops->lstat(&s->ctx, &dpath, &fidst);
> +        if (err < 0) {
> +            err = -errno;
> +            break;
> +        }
> +        stbuf = fidst;
> +        for (name_idx = 0; name_idx < nwnames; name_idx++) {
> +            if (v9fs_request_cancelled(pdu)) {
> +                err = -EINTR;
> +                break;
> +            }
> +            if (!same_stat_id(&pdu->s->root_st, &stbuf) ||
> +                strcmp("..", wnames[name_idx].data))
> +            {
> +                err = s->ops->name_to_path(&s->ctx, &dpath,
> +                                        wnames[name_idx].data, &path);
> +                if (err < 0) {
> +                    err = -errno;
> +                    break;
> +                }
> +                if (v9fs_request_cancelled(pdu)) {
> +                    err = -EINTR;
> +                    break;
> +                }
> +                err = s->ops->lstat(&s->ctx, &path, &stbuf);
> +                if (err < 0) {
> +                    err = -errno;
> +                    break;
> +                }
> +                stbufs[name_idx] = stbuf;
> +                v9fs_path_copy(&dpath, &path);
> +                v9fs_path_copy(&pathes[name_idx], &path);
> +            }
> +        }
> +    });
> +    /*
> +     * Handle all the rest of this Twalk request on main thread ...
> +     */
>      if (err < 0) {
>          goto out;
>      }

Depending on my last question below, this check might be wrong, i.e. according 
to the specs this should only error out if the first element failed.

> 
> -    /*
> -     * Both dpath and path initially poin to fidp.
> -     * Needed to handle request with nwnames == 0
> -     */
> +    err = stat_to_qid(pdu, &fidst, &qid);
> +    if (err < 0) {
> +        goto out;
> +    }
> +    stbuf = fidst;
> +
> +    /* reset dpath and path */
>      v9fs_path_copy(&dpath, &fidp->path);
>      v9fs_path_copy(&path, &fidp->path);

I am not sure what the expectation of a potential mutation of fid is here. 
Right now this code (as the previous one) assumes that fid does not mutate 
during entire Twalk request handling. But the same issue actually applies to 
any 9p request handling, not just Twalk.

> -    for (name_idx = 0; name_idx < nwnames; name_idx++) {
> -        if (not_same_qid(&pdu->s->root_qid, &qid) ||
> -            strcmp("..", wnames[name_idx].data)) {
> -            err = v9fs_co_name_to_path(pdu, &dpath, wnames[name_idx].data,
> -                                       &path);
> -            if (err < 0) {
> -                goto out;
> -            }
> 
> -            err = v9fs_co_lstat(pdu, &path, &stbuf);
> -            if (err < 0) {
> -                goto out;
> -            }
> +    for (name_idx = 0; name_idx < nwnames; name_idx++) {
> +        if (!same_stat_id(&pdu->s->root_st, &stbuf) ||
> +            strcmp("..", wnames[name_idx].data))
> +        {
> +            stbuf = stbufs[name_idx];
>              err = stat_to_qid(pdu, &stbuf, &qid);
>              if (err < 0) {
>                  goto out;
>              }
> +            v9fs_path_copy(&path, &pathes[name_idx]);
>              v9fs_path_copy(&dpath, &path);
>          }
>          memcpy(&qids[name_idx], &qid, sizeof(qid));

So far, replicating just the previous Twalk behaviour here (yet). Right now 
the request returns Rerror if any of the transmitted path elements could not 
be walked. Looking at the specs though, this seems to be wrong:

"If the first element cannot be walked for any reason, Rerror is returned. 
Otherwise, the walk will return an Rwalk mes- sage containing nwqid qids 
corresponding, in order, to the files that are visited by the nwqid successful 
elementwise walks; nwqid is therefore either nwname or the index of the first 
elementwise walk that failed. The value of nwqid can- not be zero unless 
nwname is zero. Also, nwqid will always be less than or equal to nwname. Only 
if it is equal, how- ever, will newfid be affected, in which case newfid will 
represent the file reached by the final elementwise walk requested in the 
message."

http://ericvh.github.io/9p-rfc/rfc9p2000.html#anchor33

> @@ -1838,9 +1869,12 @@ out_nofid:
>      if (nwnames && nwnames <= P9_MAXWELEM) {
>          for (name_idx = 0; name_idx < nwnames; name_idx++) {
>              v9fs_string_free(&wnames[name_idx]);
> +            v9fs_path_free(&pathes[name_idx]);
>          }
>          g_free(wnames);
>          g_free(qids);
> +        g_free(stbufs);
> +        g_free(pathes);
>      }
>  }
> 
> diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
> index 6f0b4c78c0..1567b67841 100644
> --- a/hw/9pfs/9p.h
> +++ b/hw/9pfs/9p.h
> @@ -355,7 +355,6 @@ struct V9fsState {
>      int32_t root_fid;
>      Error *migration_blocker;
>      V9fsConf fsconf;
> -    V9fsQID root_qid;
>      struct stat root_st;
>      dev_t dev_id;
>      struct qht qpd_table;

Best regards,
Christian Schoenebeck