Re: [PATCH 2/2] nfsd: allow for more callback session slots

Jeff Layton
2024-10-28T14:46:14Z

On Mon, 2024-10-28 at 10:26 -0400, Jeff Layton wrote:
> nfsd currently only uses a single slot in the callback channel, which is
> proving to be a bottleneck in some cases. Widen the callback channel to
> a max of 32 slots (subject to the client's target_maxreqs value).
>
> Change the cb_holds_slot boolean to an integer that tracks the current
> slot number (with -1 meaning "unassigned"). Move the callback slot
> tracking info into the session. Add a new u32 that acts as a bitmap to
> track which slots are in use, and a u32 to track the latest callback
> target_slotid that the client reports. While they are part of the
> session, the fields are protected by the cl_lock.
>
> Fix nfsd41_cb_get_slot to always search for the lowest slotid (using
> ffs()), and change it to continually retry until there is a slot
> available.
>
> Finally, convert the session->se_cb_seq_nr field into an array of
> counters and add the necessary handling to ensure that the seqids get
> reset at the appropriate times.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> fs/nfsd/nfs4callback.c | 107 +++++++++++++++++++++++++++++++++++--------------
> fs/nfsd/nfs4state.c | 7 +++-
> fs/nfsd/state.h | 12 +++---
> fs/nfsd/trace.h | 2 +-
> 4 files changed, 89 insertions(+), 39 deletions(-)
>
> diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
> index e38fa834b3d91333acf1425eb14c644e5d5f2601..64b85b164125b244494f9805840a0d8a1ccb4c1b 100644
> --- a/fs/nfsd/nfs4callback.c
> +++ b/fs/nfsd/nfs4callback.c
> @@ -406,6 +406,16 @@ encode_cb_getattr4args(struct xdr_stream *xdr, struct nfs4_cb_compound_hdr *hdr,
> hdr->nops++;
> }
>
> +static u32 highest_unset_index(u32 word)
> +{
> + int i;
> +
> + for (i = sizeof(word) * 8 - 1; i > 0; --i)
> + if (!(word & BIT(i)))
> + return i;
> + return 0;
> +}
> +
> /*
> * CB_SEQUENCE4args
> *
> @@ -432,15 +442,38 @@ static void encode_cb_sequence4args(struct xdr_stream *xdr,
> encode_sessionid4(xdr, session);
>
> p = xdr_reserve_space(xdr, 4 + 4 + 4 + 4 + 4);
> - *p++ = cpu_to_be32(session->se_cb_seq_nr); /* csa_sequenceid */
> - *p++ = xdr_zero; /* csa_slotid */
> - *p++ = xdr_zero; /* csa_highest_slotid */
> + *p++ = cpu_to_be32(session->se_cb_seq_nr[cb->cb_held_slot]); /* csa_sequenceid */
> + *p++ = cpu_to_be32(cb->cb_held_slot); /* csa_slotid */
> + *p++ = cpu_to_be32(max(highest_unset_index(session->se_cb_slot_avail),
> + session->se_cb_highest_slot)); /* csa_highest_slotid */
> *p++ = xdr_zero; /* csa_cachethis */
> xdr_encode_empty_array(p); /* csa_referring_call_lists */
>
> hdr->nops++;
> }
>
> +static void update_cb_slot_table(struct nfsd4_session *ses, u32 highest)
> +{
> + /* No need to do anything if nothing changed */
> + if (likely(highest == READ_ONCE(ses->se_cb_highest_slot)))
> + return;
> +
> + spin_lock(&ses->se_client->cl_lock);
> + /* If growing the slot table, reset any new sequences to 1 */
> + if (highest > ses->se_cb_highest_slot) {
> + int i;
> +
> + for (i = ses->se_cb_highest_slot; i <= highest; ++i) {

That should be:

for (i = ses->se_cb_highest_slot + 1; i <= highest; ++i) {

IOW, we only want to reset the new slots, not any that could be in use.
Fixed in my tree.

> + /* beyond the end of the array? */
> + if (i >= NFSD_BC_SLOT_TABLE_MAX)
> + break;
> + ses->se_cb_seq_nr[i] = 1;
> + }
> + }
> + ses->se_cb_highest_slot = highest;
> + spin_unlock(&ses->se_client->cl_lock);
> +}
> +
> /*
> * CB_SEQUENCE4resok
> *
> @@ -485,7 +518,7 @@ static int decode_cb_sequence4resok(struct xdr_stream *xdr,
> p += XDR_QUADLEN(NFS4_MAX_SESSIONID_LEN);
>
> dummy = be32_to_cpup(p++);
> - if (dummy != session->se_cb_seq_nr) {
> + if (dummy != session->se_cb_seq_nr[cb->cb_held_slot]) {
> dprintk("NFS: %s Invalid sequence number\n", __func__);
> goto out;
> }
> @@ -496,9 +529,15 @@ static int decode_cb_sequence4resok(struct xdr_stream *xdr,
> goto out;
> }
>
> - /*
> - * FIXME: process highest slotid and target highest slotid
> - */
> + p++; // ignore current highest slot value
> +
> + dummy = be32_to_cpup(p++);
> + if (dummy == 0) {
> + dprintk("NFS: %s Invalid target highest slotid\n", __func__);
> + goto out;
> + }
> +
> + update_cb_slot_table(session, dummy);
> status = 0;
> out:
> cb->cb_seq_status = status;
> @@ -1208,31 +1247,38 @@ void nfsd4_change_callback(struct nfs4_client *clp, struct nfs4_cb_conn *conn)
> * If the slot is available, then mark it busy. Otherwise, set the
> * thread for sleeping on the callback RPC wait queue.
> */
> -static bool nfsd41_cb_get_slot(struct nfsd4_callback *cb, struct rpc_task *task)
> +static void nfsd41_cb_get_slot(struct nfsd4_callback *cb, struct rpc_task *task)
> {
> struct nfs4_client *clp = cb->cb_clp;
> + struct nfsd4_session *ses = clp->cl_cb_session;
> + int idx;
>
> - if (!cb->cb_holds_slot &&
> - test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
> + if (cb->cb_held_slot >= 0)
> + return;
> +retry:
> + spin_lock(&clp->cl_lock);
> + idx = ffs(ses->se_cb_slot_avail) - 1;
> + if (idx < 0 || idx > ses->se_cb_highest_slot) {
> + spin_unlock(&clp->cl_lock);
> rpc_sleep_on(&clp->cl_cb_waitq, task, NULL);
> - /* Race breaker */
> - if (test_and_set_bit(0, &clp->cl_cb_slot_busy) != 0) {
> - dprintk("%s slot is busy\n", __func__);
> - return false;
> - }
> - rpc_wake_up_queued_task(&clp->cl_cb_waitq, task);
> + goto retry;
> }
> - cb->cb_holds_slot = true;
> - return true;
> + /* clear the bit for the slot */
> + ses->se_cb_slot_avail &= ~BIT(idx);
> + spin_unlock(&clp->cl_lock);
> + cb->cb_held_slot = idx;
> }
>
> static void nfsd41_cb_release_slot(struct nfsd4_callback *cb)
> {
> struct nfs4_client *clp = cb->cb_clp;
> + struct nfsd4_session *ses = clp->cl_cb_session;
>
> - if (cb->cb_holds_slot) {
> - cb->cb_holds_slot = false;
> - clear_bit(0, &clp->cl_cb_slot_busy);
> + if (cb->cb_held_slot >= 0) {
> + spin_lock(&clp->cl_lock);
> + ses->se_cb_slot_avail |= BIT(cb->cb_held_slot);
> + spin_unlock(&clp->cl_lock);
> + cb->cb_held_slot = -1;
> rpc_wake_up_next(&clp->cl_cb_waitq);
> }
> }
> @@ -1265,8 +1311,8 @@ static void nfsd4_cb_prepare(struct rpc_task *task, void *calldata)
> trace_nfsd_cb_rpc_prepare(clp);
> cb->cb_seq_status = 1;
> cb->cb_status = 0;
> - if (minorversion && !nfsd41_cb_get_slot(cb, task))
> - return;
> + if (minorversion)
> + nfsd41_cb_get_slot(cb, task);
> rpc_call_start(task);
> }
>
> @@ -1292,7 +1338,7 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> return true;
> }
>
> - if (!cb->cb_holds_slot)
> + if (cb->cb_held_slot < 0)
> goto need_restart;
>
> /* This is the operation status code for CB_SEQUENCE */
> @@ -1306,10 +1352,10 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> * If CB_SEQUENCE returns an error, then the state of the slot
> * (sequence ID, cached reply) MUST NOT change.
> */
> - ++session->se_cb_seq_nr;
> + ++session->se_cb_seq_nr[cb->cb_held_slot];
> break;
> case -ESERVERFAULT:
> - ++session->se_cb_seq_nr;
> + ++session->se_cb_seq_nr[cb->cb_held_slot];
> nfsd4_mark_cb_fault(cb->cb_clp);
> ret = false;
> break;
> @@ -1335,17 +1381,16 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
> case -NFS4ERR_BADSLOT:
> goto retry_nowait;
> case -NFS4ERR_SEQ_MISORDERED:
> - if (session->se_cb_seq_nr != 1) {
> - session->se_cb_seq_nr = 1;
> + if (session->se_cb_seq_nr[cb->cb_held_slot] != 1) {
> + session->se_cb_seq_nr[cb->cb_held_slot] = 1;
> goto retry_nowait;
> }
> break;
> default:
> nfsd4_mark_cb_fault(cb->cb_clp);
> }
> - nfsd41_cb_release_slot(cb);
> -
> trace_nfsd_cb_free_slot(task, cb);
> + nfsd41_cb_release_slot(cb);
>
> if (RPC_SIGNALLED(task))
> goto need_restart;
> @@ -1565,7 +1610,7 @@ void nfsd4_init_cb(struct nfsd4_callback *cb, struct nfs4_client *clp,
> INIT_WORK(&cb->cb_work, nfsd4_run_cb_work);
> cb->cb_status = 0;
> cb->cb_need_restart = false;
> - cb->cb_holds_slot = false;
> + cb->cb_held_slot = -1;
> }
>
> /**
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 5b718b349396f1aecd0ad4c63b2f43342841bbd4..20a0d40202e40eed1c84d5d6c0a85b908804a6ba 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -2002,6 +2002,8 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
> }
>
> memcpy(&new->se_fchannel, fattrs, sizeof(struct nfsd4_channel_attrs));
> + new->se_cb_slot_avail = ~0U;
> + new->se_cb_highest_slot = battrs->maxreqs - 1;
> return new;
> out_free:
> while (i--)
> @@ -2132,7 +2134,9 @@ static void init_session(struct svc_rqst *rqstp, struct nfsd4_session *new, stru
>
> INIT_LIST_HEAD(&new->se_conns);
>
> - new->se_cb_seq_nr = 1;
> + for (idx = 0; idx < NFSD_BC_SLOT_TABLE_MAX; ++idx)
> + new->se_cb_seq_nr[idx] = 1;
> +
> new->se_flags = cses->flags;
> new->se_cb_prog = cses->callback_prog;
> new->se_cb_sec = cses->cb_sec;
> @@ -3159,7 +3163,6 @@ static struct nfs4_client *create_client(struct xdr_netobj name,
> kref_init(&clp->cl_nfsdfs.cl_ref);
> nfsd4_init_cb(&clp->cl_cb_null, clp, NULL, NFSPROC4_CLNT_CB_NULL);
> clp->cl_time = ktime_get_boottime_seconds();
> - clear_bit(0, &clp->cl_cb_slot_busy);
> copy_verf(clp, verf);
> memcpy(&clp->cl_addr, sa, sizeof(struct sockaddr_storage));
> clp->cl_cb_session = NULL;
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 41cda86fea1f6166a0fd0215d3d458c93ced3e6a..2987c362bdd56251e736879dc89302ada2259be8 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -71,8 +71,8 @@ struct nfsd4_callback {
> struct work_struct cb_work;
> int cb_seq_status;
> int cb_status;
> + int cb_held_slot;
> bool cb_need_restart;
> - bool cb_holds_slot;
> };
>
> struct nfsd4_callback_ops {
> @@ -307,6 +307,9 @@ struct nfsd4_conn {
> unsigned char cn_flags;
> };
>
> +/* Max number of slots that the server will use in the backchannel */
> +#define NFSD_BC_SLOT_TABLE_MAX 32
> +
> /*
> * Representation of a v4.1+ session. These are refcounted in a similar fashion
> * to the nfs4_client. References are only taken when the server is actively
> @@ -325,7 +328,9 @@ struct nfsd4_session {
> struct nfsd4_cb_sec se_cb_sec;
> struct list_head se_conns;
> u32 se_cb_prog;
> - u32 se_cb_seq_nr;
> + u32 se_cb_slot_avail; /* bitmap of available slots */
> + u32 se_cb_highest_slot; /* highest slot client wants */
> + u32 se_cb_seq_nr[NFSD_BC_SLOT_TABLE_MAX];
> struct nfsd4_slot *se_slots[]; /* forward channel slots */
> };
>
> @@ -459,9 +464,6 @@ struct nfs4_client {
> */
> struct dentry *cl_nfsd_info_dentry;
>
> - /* for nfs41 callbacks */
> - /* We currently support a single back channel with a single slot */
> - unsigned long cl_cb_slot_busy;
> struct rpc_wait_queue cl_cb_waitq; /* backchannel callers may */
> /* wait here for slots */
> struct net *net;
> diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
> index f318898cfc31614b5a84a4867e18c2b3a07122c9..a9c17186b6892f1df8d7f7b90e250c2913ab23fe 100644
> --- a/fs/nfsd/trace.h
> +++ b/fs/nfsd/trace.h
> @@ -1697,7 +1697,7 @@ TRACE_EVENT(nfsd_cb_free_slot,
> __entry->cl_id = sid->clientid.cl_id;
> __entry->seqno = sid->sequence;
> __entry->reserved = sid->reserved;
> - __entry->slot_seqno = session->se_cb_seq_nr;
> + __entry->slot_seqno = session->se_cb_seq_nr[cb->cb_held_slot];
> ),
> TP_printk(SUNRPC_TRACE_TASK_SPECIFIER
> " sessionid=%08x:%08x:%08x:%08x new slot seqno=%u",
>

--
Jeff Layton <jlayton@kernel.org>