Re: [PATCH] fuse: add a new "connections" file to show longest waiting reqeust

[Joanne Koong]

On Mon, Feb 3, 2025 at 8:37 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
> Add a new file to the "connections" directory that shows how long (in
> seconds) the oldest fuse_req in the processing hash or pending queue has
> been waiting.
>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> This is based on top of Joanne's work, as it requires the "create_time"
> field in fuse_req.  We have some internal detection of hung fuse server
> processes that relies on seeing elevated values in the "waiting" sysfs
> file. The problem with that method is that it can't detect when highly
> serialized workloads on a FUSE mount are hung. This adds another metric
> that we can use to detect when fuse mounts are hung.
> ---
>  fs/fuse/control.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/fuse/fuse_i.h  |  2 +-
>  2 files changed, 57 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/control.c b/fs/fuse/control.c
> index 2a730d88cc3bdb50ea1f8a3185faad5f05fc6e74..b213db11a2d7d85c4403baa61f9f7850fed150a8 100644
> --- a/fs/fuse/control.c
> +++ b/fs/fuse/control.c
> @@ -180,6 +180,55 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
>         return ret;
>  }
>
> +/* Show how long (in s) the oldest request has been waiting */
> +static ssize_t fuse_conn_oldest_read(struct file *file, char __user *buf,
> +                                     size_t len, loff_t *ppos)
> +{
> +       char tmp[32];
> +       size_t size;
> +       unsigned long oldest = jiffies;
> +
> +       if (!*ppos) {
> +               struct fuse_conn *fc = fuse_ctl_file_conn_get(file);
> +               struct fuse_iqueue *fiq = &fc->iq;
> +               struct fuse_dev *fud;
> +               struct fuse_req *req;
> +
> +               if (!fc)
> +                       return 0;
> +
> +               spin_lock(&fc->lock);
> +               list_for_each_entry(fud, &fc->devices, entry) {
> +                       struct fuse_pqueue *fpq = &fud->pq;
> +                       int i;
> +
> +                       spin_lock(&fpq->lock);
> +                       for (i = 0; i < FUSE_PQ_HASH_SIZE; i++) {
> +                               if (list_empty(&fpq->processing[i]))
> +                                       continue;
> +                               /*
> +                                * Only check the first request in the queue. The
> +                                * assumption is that the one at the head of the list
> +                                * will always be the oldest.
> +                                */
> +                               req = list_first_entry(&fpq->processing[i], struct fuse_req, list);

This probably doesn't matter in actuality, but maybe
list_first_entry_or_null() on fpq->processing[i] would be more optimal
here than "list_empty()" and "list_first_entry()" since that'll
minimize the number of READ_ONCE() calls we'd need to do.

> +                               if (req->create_time < oldest)
> +                                       oldest = req->create_time;
> +                       }
> +                       spin_unlock(&fpq->lock);
> +               }
> +               if (!list_empty(&fiq->pending)) {

I think we'll need to grab the fiq->lock here first before checking fiq->pending

> +                       req = list_first_entry(&fiq->pending, struct fuse_req, list);
> +                       if (req->create_time < oldest)
> +                               oldest = req->create_time;
> +               }
> +               spin_unlock(&fc->lock);
> +               fuse_conn_put(fc);
> +       }
> +       size = sprintf(tmp, "%ld\n", (jiffies - oldest)/HZ);

If there are no requests, I think this will still return a non-zero
value since jiffies is a bit more than what the last "oldest =
jiffies" was, which might be confusing. Maybe we should just return 0
in this case?


Thanks,
Joanne

> +       return simple_read_from_buffer(buf, len, ppos, tmp, size);
> +}
> +
>  static const struct file_operations fuse_ctl_abort_ops = {
>         .open = nonseekable_open,
>         .write = fuse_conn_abort_write,
> @@ -202,6 +251,11 @@ static const struct file_operations fuse_conn_congestion_threshold_ops = {
>         .write = fuse_conn_congestion_threshold_write,
>  };
>
> +static const struct file_operations fuse_ctl_oldest_ops = {
> +       .open = nonseekable_open,
> +       .read = fuse_conn_oldest_read,
> +};
> +
>  static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
>                                           struct fuse_conn *fc,
>                                           const char *name,
> @@ -264,6 +318,8 @@ int fuse_ctl_add_conn(struct fuse_conn *fc)
>
>         if (!fuse_ctl_add_dentry(parent, fc, "waiting", S_IFREG | 0400, 1,
>                                  NULL, &fuse_ctl_waiting_ops) ||
> +           !fuse_ctl_add_dentry(parent, fc, "oldest", S_IFREG | 0400, 1,
> +                                NULL, &fuse_ctl_oldest_ops) ||
>             !fuse_ctl_add_dentry(parent, fc, "abort", S_IFREG | 0200, 1,
>                                  NULL, &fuse_ctl_abort_ops) ||
>             !fuse_ctl_add_dentry(parent, fc, "max_background", S_IFREG | 0600,
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index dcc1c327a0574b1fd1adda4b7ca047aa353b6a0a..b46c26bc977ad2d75d10fb306d3ecc4caf2c53bd 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -42,7 +42,7 @@
>  #define FUSE_NAME_MAX 1024
>
>  /** Number of dentries for each connection in the control filesystem */
> -#define FUSE_CTL_NUM_DENTRIES 5
> +#define FUSE_CTL_NUM_DENTRIES 6
>
>  /* Frequency (in seconds) of request timeout checks, if opted into */
>  #define FUSE_TIMEOUT_TIMER_FREQ 15
>
> ---
> base-commit: 9afd7336f3acbe5678cca3b3bc5baefb51ce9564
> change-id: 20250203-fuse-sysfs-ce351d105cf0
>
> Best regards,
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
>
>