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

Jeff Layton
2024-11-19T19:34:12Z

On Tue, 2024-11-19 at 11:41 +1100, NeilBrown wrote:
> If a client ever uses the highest available slot for a given session,
> attempt to allocate another slot so there is room for the client to use
> more slots 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.
>
> We would expect to stablise with one more slot available than the client
> actually uses.
>
> Now that we grow the slot table on demand we can start with a smaller
> allocation. Define NFSD_MAX_INITIAL_SLOTS and allocate at most that
> many when session is created.
>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> fs/nfsd/nfs4state.c | 32 ++++++++++++++++++++++++++------
> fs/nfsd/state.h | 2 ++
> 2 files changed, 28 insertions(+), 6 deletions(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 31ff9f92a895..fb522165b376 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -1956,7 +1956,7 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
> if (!slot || xa_is_err(xa_store(&new->se_slots, 0, slot, GFP_KERNEL)))
> goto out_free;
>
> - for (i = 1; i < numslots; i++) {
> + for (i = 1; i < numslots && i < NFSD_MAX_INITIAL_SLOTS; i++) {

nit: maybe just clamp numslots at NFSD_MAX_INITIAL_SLOTS?

> slot = kzalloc(slotsize, GFP_KERNEL | __GFP_NORETRY);
> if (!slot)
> break;
> @@ -4248,11 +4248,6 @@ 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);
> @@ -4302,6 +4297,31 @@ 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 one.
> + */
> + if (seq->slotid == session->se_fchannel.maxreqs - 1 &&
> + session->se_fchannel.maxreqs < NFSD_MAX_SLOTS_PER_SESSION) {
> + int s = session->se_fchannel.maxreqs;
> +
> + /*
> + * 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.
> + */
> + slot = kzalloc(slot_bytes(&session->se_fchannel), GFP_NOWAIT);
> + if (slot && !xa_is_err(xa_store(&session->se_slots, s, slot,
> + GFP_ATOMIC)))
> + session->se_fchannel.maxreqs += 1;
> + else
> + kfree(slot);
> + }
> + seq->maxslots = session->se_fchannel.maxreqs;
> +
> out:
> switch (clp->cl_cb_state) {
> case NFSD4_CB_DOWN:
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index e97626916a68..a14a823670e9 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -249,6 +249,8 @@ static inline struct nfs4_delegation *delegstateid(struct nfs4_stid *s)
> * get good throughput on high-latency servers.
> */
> #define NFSD_MAX_SLOTS_PER_SESSION 2048
> +/* Maximum number of slots per session to allocate for CREATE_SESSION */
> +#define NFSD_MAX_INITIAL_SLOTS 32
> /* Maximum session per slot cache size */
> #define NFSD_SLOT_CACHE_SIZE 2048
> /* Maximum number of NFSD_SLOT_CACHE_SIZE slots per session */

--
Jeff Layton <jlayton@kernel.org>