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

NeilBrown
2024-11-19T22:28:01Z

On Wed, 20 Nov 2024, Chuck Lever wrote:
> On Tue, Nov 19, 2024 at 11:41:31AM +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.
>
> Which begs the question "why have a 2048 slot maximum session slot
> table size?" 1025 might work too. But is there a need for any
> maximum at all, or is this just a sanity check?

Linux NFS presumably isn't the only client, and it might change in the
future. Maybe there is no need for a maximum. It was mostly as a
sanity check.

It wouldn't take much to convince me to remove the limit.

>
>
> > 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.
>
> Maybe NFSD_DEFAULT_INITIAL_SLOTS is more descriptive?

I don't think "DEFAULT" is the right word. The client requests a number
of slots. That is the "Default". The server can impose a limit - a
maximum.
Maybe we don't need a limit here either?

Thanks,
NeilBrown

>
>
> > 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++) {
> > 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.
>
> IIUC, GFP_ATOMIC allocations come from a special pool. I don't think
> we want that here. I'd rather stick with NORETRY or KERNEL.
>
>
> > + */
> > + 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
>
> The first couple of patches did so nicely at ruthlessly discarding a
> lot of arbitrary logic. I'm not convinced by the patch description
> that the INITIAL_SLOTS complexity is needed...
>
>
> > /* Maximum session per slot cache size */
> > #define NFSD_SLOT_CACHE_SIZE 2048
> > /* Maximum number of NFSD_SLOT_CACHE_SIZE slots per session */
> > --
> > 2.47.0
> >
>
> --
> Chuck Lever
>