Re: [PATCH 4/6] nfsd: allocate new session-based DRC slots on demand.

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-12-06 at 11:43 +1100, NeilBrown wrote:
> If a client ever uses the highest available slot for a given session,
> attempt to allocate more slots so there is room for the client to use
> them if wanted.  GFP_NOWAIT is used so if there is not plenty of
> free memory, failure is expected - which is what we want.  It also
> allows the allocation while holding a spinlock.
> 
> Each time we increase the number of slots by 20% (rounded up).  This
> allows fairly quick growth while avoiding excessive over-shoot.
> 
> We would expect to stablise with around 10% more slots available than
> the client actually uses.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  fs/nfsd/nfs4state.c | 40 +++++++++++++++++++++++++++++++++++-----
>  1 file changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 67dfc699e411..ec4468ebbd40 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -4235,11 +4235,6 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>  	slot = xa_load(&session->se_slots, seq->slotid);
>  	dprintk("%s: slotid %d\n", __func__, seq->slotid);
>  
> -	/* We do not negotiate the number of slots yet, so set the
> -	 * maxslots to the session maxreqs which is used to encode
> -	 * sr_highest_slotid and the sr_target_slot id to maxslots */
> -	seq->maxslots = session->se_fchannel.maxreqs;
> -
>  	trace_nfsd_slot_seqid_sequence(clp, seq, slot);
>  	status = check_slot_seqid(seq->seqid, slot->sl_seqid,
>  					slot->sl_flags & NFSD4_SLOT_INUSE);
> @@ -4289,6 +4284,41 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
>  	cstate->session = session;
>  	cstate->clp = clp;
>  
> +	/*
> +	 * If the client ever uses the highest available slot,
> +	 * gently try to allocate another 20%.  This allows
> +	 * fairly quick growth without grossly over-shooting what
> +	 * the client might use.
> +	 */

20% seems like a reasonable place to start, but I do wonder if this
might need to be tunable under some workloads. Oh well, we can cross
that bridge if/when someone complains.
 
> +	if (seq->slotid == session->se_fchannel.maxreqs - 1 &&
> +	    session->se_fchannel.maxreqs < NFSD_MAX_SLOTS_PER_SESSION) {
> +		int s = session->se_fchannel.maxreqs;
> +		int cnt = DIV_ROUND_UP(s, 5);
> +
> +		do {
> +			/*
> +			 * GFP_NOWAIT is a low-priority non-blocking
> +			 * allocation which can be used under
> +			 * client_lock and only succeeds if there is
> +			 * plenty of memory.
> +			 * Use GFP_ATOMIC which is higher priority for
> +			 * xa_store() so we are less likely to waste the
> +			 * effort of the first allocation.
> +			 */

I don't know here. Why not just use GFP_NOWAIT for the xa_store too? If
we're so memory constrained that that fails, we're probably better off
releasing the slot.

> +			slot = kzalloc(slot_bytes(&session->se_fchannel),
> +				       GFP_NOWAIT);
> +			if (slot &&
> +			    !xa_is_err(xa_store(&session->se_slots, s, slot,
> +						GFP_ATOMIC | __GFP_NOWARN))) {
> +				s += 1;
> +				session->se_fchannel.maxreqs = s;
> +			} else {
> +				kfree(slot);
> +			}
> +		} while (slot && --cnt > 0);
> +	}
> +	seq->maxslots = session->se_fchannel.maxreqs;
> +
>  out:
>  	switch (clp->cl_cb_state) {
>  	case NFSD4_CB_DOWN:

-- 
Jeff Layton <jlayton@kernel.org>