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

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-12-06 at 12:43 +1100, NeilBrown wrote:
> On Fri, 06 Dec 2024, Jeff Layton wrote:
> > 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.
> 
> I think that if we need a tunable, then it is a failure of design.
> If?when someone complains we may well need to redesign.  I hope we could
> avoid a tunable in that design!
> 

I hope so too.

> >  
> > > +	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.
> 
> Maybe.  I'm open simple using GFP_NOWAIT both places.
> Most often xa_store won't need to allocate anything - it adds slots to
> the array in batches (at least I assume it does - anything else would be
> inefficient).  So it mostly won't matter.
> So if seems at all inelegant - let's drop it.
> 
> 

I'd prefer we drop that part. It probably won't matter much in the long
run anyway.

> 
> > 
> > > +			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>
> > 
> > 
> 

-- 
Jeff Layton <jlayton@kernel.org>