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

Jeff Layton
2024-10-29T10:28:34Z

On Mon, 2024-10-28 at 15:05 -0400, Chuck Lever wrote:
> On Mon, Oct 28, 2024 at 10:26:27AM -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;
> > +}
> > +
>
> Isn't this the same as ffz() or "ffs(~(x))" ?
>

No. I need the _last_ cleared bit in the word. It looks though like
there is a fls() function that might be usable, so I could maybe do
fls(~(x)). I'll experiment with that.

>
> > /*
> > * 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 */
>
> This encoder doesn't hold the client's cl_lock, but does reference
> fields that are updated while that lock is held.
>
> Also, should a per-session spin lock be used instead of cl_lock?
>
> Is locking even necessary? All of these calls are prepared in a
> single-threaded workqueue. Reply processing can only free slots, so
> it doesn't need to exclude session slot selection.
>

NFSv4 callbacks use async RPCs. The encoding and transmit is done in
the context of the (single-threaded) workqueue, but the reply is
handled by rpciod. Since we release the slot after the reply in rpciod
context, I think we do require locking here. I also worry a bit that
relying on the single-threaded workqueue may end up being a bottleneck
at some point in the future, so I see having clear locking here as a
benefit.

I'm open to switching to a per-session lock of some sort, but I don't
see a real need here. Only one session will be used as the backchannel
at a time, so there shouldn't be competing access between different
sessions for the cl_lock. We are competing with the other uses of the
cl_lock, but this one should be pretty quick. My preference would be to
add extra locking only once it becomes clear that it's necessary.

>
> > *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)
>
> Nit: Can you use the name "target" instead of "highest" ?
>

Will do.

>
> > +{
> > + /* 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) {
> > + /* beyond the end of the array? */
> > + if (i >= NFSD_BC_SLOT_TABLE_MAX)
> > + break;
>
> Nit: why not cap "highest" at NFSD_BC_SLOT_TABLE_MAX before starting
> this for loop?
>

Duh -- of course. Will fix.

>
> > + 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]) {
>
> Nit: Let's rename "dummy" as "seqid", and add a "highest" variable
> for the next XDR field (not shown here).
>

Ok.

>
> > 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++);
>
> Nit: I prefer a name for this argument variable like "target".
>
> The compiler should be able to combine the usage of these variables
> into a single memory location.
>

Ok.

>
> > + 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)
>
> Nit: This patch should update the documenting comment before
> nfsd4_cb_prepare() -- the patch implements "multiple slots".
>

+1

>
> > 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
> > +
>
> The new comment is unclear about whether this is an implementation
> limit or a protocol limit. I suggest:
>
> /* Maximum number of slots that NFSD implements for NFSv4.1+ backchannel */
>
> And make this "sizeof(u32) * 8" or something similar that documents
> where the value of this limit comes from.
>

Ok.

> > /*
> > * 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",
> >
> > --
> > 2.47.0
> >
>

--
Jeff Layton <jlayton@kernel.org>