Re: [PATCH v4 1/4] sunrpc: route to a populated pool in svc_pool_for_cpu()

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2026-07-02 at 08:13 +1000, NeilBrown wrote:
> On Thu, 02 Jul 2026, Jeff Layton wrote:
> > svc_set_num_threads() spreads the requested threads evenly across the
> > service's pools (base = nrservs / sv_nrpools).  When a service runs
> > fewer threads than it has pools -- e.g. an nfsd configured with fewer
> > threads than the host has NUMA nodes while running in "pernode" or
> > "percpu" mode -- the trailing pools are left with no threads at all.
> > 
> > svc_xprt_enqueue() selects a pool from the CPU servicing the transport,
> > queues the transport on that pool's sp_xprts, and only wakes a thread
> > from the same pool.  Each thread services exclusively its own pool, so a
> > transport that lands on a threadless pool is enqueued on sp_xprts and
> > never picked up: the connection hangs indefinitely.
> > 
> > Have svc_pool_for_cpu() skip pools that currently have no threads,
> > falling back to the next populated pool.  This trades NUMA locality for
> > a guarantee that the work is actually serviced.  sp_nrthreads is only
> > updated under the service mutex; the lockless read here is a best-effort
> > routing hint, so annotate it with data_race().
> > 
> > Fixes: 0f0257eaa5d2 ("svc: Move the xprt independent code to the svc_xprt.c file")
> 
> Why that commit?  Did this ever work correctly?
> It seems more likely that 
> Fixes: 3262c816a3d7 ("[PATCH] knfsd: split svc_serv into pools")
> is appropriate.
> 

Indeed. Good catch.

> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  net/sunrpc/svc.c | 26 +++++++++++++++++++++++++-
> >  1 file changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index dd80a2eaaa74..82fb7faf563f 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -402,6 +402,7 @@ struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
> >  	struct svc_pool_map *m = &svc_pool_map;
> >  	int cpu = raw_smp_processor_id();
> >  	unsigned int pidx = 0;
> > +	unsigned int i;
> >  
> >  	if (serv->sv_nrpools <= 1)
> >  		return serv->sv_pools;
> > @@ -414,8 +415,31 @@ struct svc_pool *svc_pool_for_cpu(struct svc_serv *serv)
> >  		pidx = m->to_pool[cpu_to_node(cpu)];
> >  		break;
> >  	}
> > +	pidx %= serv->sv_nrpools;
> > +
> > +	/*
> > +	 * Threads are spread evenly across the pools, but when there are
> > +	 * fewer threads than pools some pools can end up with none. A
> > +	 * transport enqueued on a threadless pool would never be picked
> > +	 * up, since each thread only services its own pool. Fall back to
> > +	 * the next populated pool, trading NUMA locality for a guarantee
> > +	 * that the transport is serviced.
> > +	 */
> > +	for (i = 0; i < serv->sv_nrpools; i++) {
> > +		struct svc_pool *pool = &serv->sv_pools[pidx];
> > +
> > +		/* This is set under the sp_mutex and rarely ever changes. A
> > +		 * data race here is harmless.
> > +		 */
> > +		if (data_race(pool->sp_nrthreads))
> > +			return pool;
> > +
> > +		if (++pidx >= serv->sv_nrpools)
> > +			pidx = 0;
> > +	}
> >  
> > -	return &serv->sv_pools[pidx % serv->sv_nrpools];
> > +	/* No pool has any threads; nothing can service the transport. */
> 
> Would a WARN_ON_ONCE() be appropriate here?
> 

Maybe a pr_notice_once()? A stack trace isn't particularly helpful
here, but it would be good to let someone know that this isn't
optimally configured. I'll add one for v5.


> I think this is a sensible defensive-programming approach.
> 
> Reviewed-by: NeilBrown <neil@brown.name>
> 

Thanks!

> > +	return &serv->sv_pools[pidx];
> >  }
> >  
> >  static int svc_rpcb_setup(struct svc_serv *serv, struct net *net)
> > 
> > -- 
> > 2.54.0
> > 
> > 

-- 
Jeff Layton <jlayton@kernel.org>