Message-ID: <Zx/gbQmNS1vXw2Jq@tissot.1015granger.net>
Date: Mon, 28 Oct 2024 15:05:17 -0400
From: Chuck Lever <chuck.lever@...cle.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Neil Brown <neilb@...e.de>, Dai Ngo <Dai.Ngo@...cle.com>,
        Tom Talpey <tom@...pey.com>, Olga Kornievskaia <okorniev@...hat.com>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] nfsd: allow for more callback session slots

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@...nel.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))" ?


>  /*
>   * 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.


>  	*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" ?


> +{
> +	/* 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?


> +			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).


>  		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.


> +	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".


>  	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.

>  /*
>   * 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
> 

-- 
Chuck Lever

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