Re: [PATCH RFC 1/6] sunrpc: split svc_set_num_threads() into two functions

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2025-12-13 at 14:29 -0500, Chuck Lever wrote:
> 
> On Fri, Dec 12, 2025, at 5:39 PM, Jeff Layton wrote:
> > svc_set_num_threads() will set the number of running threads for a given
> > pool. If the pool argument is set to NULL however, it will distribute
> > the threads among all of the pools evenly.
> > 
> > These divergent codepaths complicate the move to dynamic threading.
> > Simplify the API by splitting these two cases into different helpers:
> > 
> > Add a new svc_set_pool_threads() function that sets the number of
> > threads in a single, given pool. Modify svc_set_num_threads() to
> > distribute the threads evenly between all of the pools and then call
> > svc_set_pool_threads() for each.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > ---
> >  fs/lockd/svc.c             |  4 ++--
> >  fs/nfs/callback.c          |  8 +++----
> >  fs/nfsd/nfssvc.c           | 21 ++++++++----------
> >  include/linux/sunrpc/svc.h |  3 ++-
> >  net/sunrpc/svc.c           | 54 +++++++++++++++++++++++++++++++++++++---------
> >  5 files changed, 61 insertions(+), 29 deletions(-)
> > 
> > diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
> > index 
> > d68afa196535a8785bab2931c2b14f03a1174ef9..fbf132b4e08d11a91784c21ee0209fd7c149fd9d 
> > 100644
> > --- a/fs/lockd/svc.c
> > +++ b/fs/lockd/svc.c
> > @@ -340,7 +340,7 @@ static int lockd_get(void)
> >  		return -ENOMEM;
> >  	}
> > 
> > -	error = svc_set_num_threads(serv, NULL, 1);
> > +	error = svc_set_num_threads(serv, 1);
> >  	if (error < 0) {
> >  		svc_destroy(&serv);
> >  		return error;
> > @@ -368,7 +368,7 @@ static void lockd_put(void)
> >  	unregister_inet6addr_notifier(&lockd_inet6addr_notifier);
> >  #endif
> > 
> > -	svc_set_num_threads(nlmsvc_serv, NULL, 0);
> > +	svc_set_num_threads(nlmsvc_serv, 0);
> >  	timer_delete_sync(&nlmsvc_retry);
> >  	svc_destroy(&nlmsvc_serv);
> >  	dprintk("lockd_down: service destroyed\n");
> > diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> > index 
> > c8b837006bb27277ab34fe516f1b63992fee9b7f..44b35b7f8dc022f1d8c069eaf2f7d334c93f77fc 
> > 100644
> > --- a/fs/nfs/callback.c
> > +++ b/fs/nfs/callback.c
> > @@ -119,9 +119,9 @@ static int nfs_callback_start_svc(int minorversion, 
> > struct rpc_xprt *xprt,
> >  	if (serv->sv_nrthreads == nrservs)
> >  		return 0;
> > 
> > -	ret = svc_set_num_threads(serv, NULL, nrservs);
> > +	ret = svc_set_num_threads(serv, nrservs);
> >  	if (ret) {
> > -		svc_set_num_threads(serv, NULL, 0);
> > +		svc_set_num_threads(serv, 0);
> >  		return ret;
> >  	}
> >  	dprintk("nfs_callback_up: service started\n");
> > @@ -242,7 +242,7 @@ int nfs_callback_up(u32 minorversion, struct 
> > rpc_xprt *xprt)
> >  	cb_info->users++;
> >  err_net:
> >  	if (!cb_info->users) {
> > -		svc_set_num_threads(cb_info->serv, NULL, 0);
> > +		svc_set_num_threads(cb_info->serv, 0);
> >  		svc_destroy(&cb_info->serv);
> >  	}
> >  err_create:
> > @@ -268,7 +268,7 @@ void nfs_callback_down(int minorversion, struct net 
> > *net)
> >  	nfs_callback_down_net(minorversion, serv, net);
> >  	cb_info->users--;
> >  	if (cb_info->users == 0) {
> > -		svc_set_num_threads(serv, NULL, 0);
> > +		svc_set_num_threads(serv, 0);
> >  		dprintk("nfs_callback_down: service destroyed\n");
> >  		svc_destroy(&cb_info->serv);
> >  	}
> > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > index 
> > 93f7435cafd2362d9ddb28815277c824067cb370..aafec8ff588b85b0e26d40b76ef00953dc6472b4 
> > 100644
> > --- a/fs/nfsd/nfssvc.c
> > +++ b/fs/nfsd/nfssvc.c
> > @@ -594,7 +594,7 @@ void nfsd_shutdown_threads(struct net *net)
> >  	}
> > 
> >  	/* Kill outstanding nfsd threads */
> > -	svc_set_num_threads(serv, NULL, 0);
> > +	svc_set_num_threads(serv, 0);
> >  	nfsd_destroy_serv(net);
> >  	mutex_unlock(&nfsd_mutex);
> >  }
> > @@ -702,12 +702,9 @@ int nfsd_set_nrthreads(int n, int *nthreads, 
> > struct net *net)
> >  	if (nn->nfsd_serv == NULL || n <= 0)
> >  		return 0;
> > 
> > -	/*
> > -	 * Special case: When n == 1, pass in NULL for the pool, so that the
> > -	 * change is distributed equally among them.
> > -	 */
> > +	/* Special case: When n == 1, distribute threads equally among pools. */
> >  	if (n == 1)
> > -		return svc_set_num_threads(nn->nfsd_serv, NULL, nthreads[0]);
> > +		return svc_set_num_threads(nn->nfsd_serv, nthreads[0]);
> > 
> >  	if (n > nn->nfsd_serv->sv_nrpools)
> >  		n = nn->nfsd_serv->sv_nrpools;
> > @@ -733,18 +730,18 @@ int nfsd_set_nrthreads(int n, int *nthreads, 
> > struct net *net)
> > 
> >  	/* apply the new numbers */
> >  	for (i = 0; i < n; i++) {
> > -		err = svc_set_num_threads(nn->nfsd_serv,
> > -					  &nn->nfsd_serv->sv_pools[i],
> > -					  nthreads[i]);
> > +		err = svc_set_pool_threads(nn->nfsd_serv,
> > +					   &nn->nfsd_serv->sv_pools[i],
> > +					   nthreads[i]);
> >  		if (err)
> >  			goto out;
> >  	}
> > 
> >  	/* Anything undefined in array is considered to be 0 */
> >  	for (i = n; i < nn->nfsd_serv->sv_nrpools; ++i) {
> > -		err = svc_set_num_threads(nn->nfsd_serv,
> > -					  &nn->nfsd_serv->sv_pools[i],
> > -					  0);
> > +		err = svc_set_pool_threads(nn->nfsd_serv,
> > +					   &nn->nfsd_serv->sv_pools[i],
> > +					   0);
> >  		if (err)
> >  			goto out;
> >  	}
> > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> > index 
> > 5506d20857c318774cd223272d4b0022cc19ffb8..dd5fbbf8b3d39df6c17a7624edf344557fffd32c 
> > 100644
> > --- a/include/linux/sunrpc/svc.h
> > +++ b/include/linux/sunrpc/svc.h
> > @@ -446,7 +446,8 @@ struct svc_serv *  svc_create_pooled(struct 
> > svc_program *prog,
> >  				     struct svc_stat *stats,
> >  				     unsigned int bufsize,
> >  				     int (*threadfn)(void *data));
> > -int		   svc_set_num_threads(struct svc_serv *, struct svc_pool *, int);
> > +int		   svc_set_pool_threads(struct svc_serv *, struct svc_pool *, 
> > int);
> > +int		   svc_set_num_threads(struct svc_serv *, int);
> >  int		   svc_pool_stats_open(struct svc_info *si, struct file *file);
> >  void		   svc_process(struct svc_rqst *rqstp);
> >  void		   svc_process_bc(struct rpc_rqst *req, struct svc_rqst *rqstp);
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index 
> > 4704dce7284eccc9e2bc64cf22947666facfa86a..3fe5a7f8e57e3fa3837265ec06884b357d5373ff 
> > 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -856,15 +856,12 @@ svc_stop_kthreads(struct svc_serv *serv, struct 
> > svc_pool *pool, int nrservs)
> >  }
> > 
> >  /**
> > - * svc_set_num_threads - adjust number of threads per RPC service
> > + * svc_set_pool_threads - adjust number of threads per pool
> >   * @serv: RPC service to adjust
> > - * @pool: Specific pool from which to choose threads, or NULL
> > + * @pool: Specific pool from which to choose threads
> >   * @nrservs: New number of threads for @serv (0 or less means kill all 
> > threads)
> >   *
> > - * Create or destroy threads to make the number of threads for @serv 
> > the
> > - * given number. If @pool is non-NULL, change only threads in that 
> > pool;
> > - * otherwise, round-robin between all pools for @serv. @serv's
> > - * sv_nrthreads is adjusted for each thread created or destroyed.
> > + * Create or destroy threads in @pool to bring it to @nrservs.
> >   *
> >   * Caller must ensure mutual exclusion between this and server startup 
> > or
> >   * shutdown.
> > @@ -873,12 +870,12 @@ svc_stop_kthreads(struct svc_serv *serv, struct 
> > svc_pool *pool, int nrservs)
> >   * starting a thread.
> >   */
> >  int
> > -svc_set_num_threads(struct svc_serv *serv, struct svc_pool *pool, int 
> > nrservs)
> > +svc_set_pool_threads(struct svc_serv *serv, struct svc_pool *pool, int 
> > nrservs)
> >  {
> >  	if (!pool)
> > -		nrservs -= serv->sv_nrthreads;
> > -	else
> > -		nrservs -= pool->sp_nrthreads;
> > +		return -EINVAL;
> > +
> > +	nrservs -= pool->sp_nrthreads;
> > 
> >  	if (nrservs > 0)
> >  		return svc_start_kthreads(serv, pool, nrservs);
> > @@ -886,6 +883,43 @@ svc_set_num_threads(struct svc_serv *serv, struct 
> > svc_pool *pool, int nrservs)
> >  		return svc_stop_kthreads(serv, pool, nrservs);
> >  	return 0;
> >  }
> > +EXPORT_SYMBOL_GPL(svc_set_pool_threads);
> > +
> > +/**
> > + * svc_set_num_threads - adjust number of threads in serv
> > + * @serv: RPC service to adjust
> > + * @nrservs: New number of threads for @serv (0 or less means kill all 
> > threads)
> > + *
> > + * Create or destroy threads in @serv to bring it to @nrservs. If there
> > + * are multiple pools then the new threads or victims will be 
> > distributed
> > + * evenly among them.
> > + *
> > + * Caller must ensure mutual exclusion between this and server startup 
> > or
> > + * shutdown.
> > + *
> > + * Returns zero on success or a negative errno if an error occurred 
> > while
> > + * starting a thread.
> > + */
> > +int
> > +svc_set_num_threads(struct svc_serv *serv, int nrservs)
> > +{
> > +	int base = nrservs / serv->sv_nrpools;
> > +	int remain = nrservs % serv->sv_nrpools;
> > +	int i, err;
> > +
> > +	for (i = 0; i < serv->sv_nrpools; ++i) {
> 
> If sv_nrpools happens to be zero, then the loop doesn't
> execute at all, and err is left containing stack garbage.
> Is sv_nrpools guaranteed to be non-zero? If not then err
> needs to be initialized before the loop runs. I see that
> nfsd_set_nrthreads() in fs/nfsd/nfssvc.c has "int err = 0"
> for a similar loop pattern.
> 

sv_nrpools should always be non-zero. There are many places in the rpc
layer that depend on having at least one pool. From __svc_create:

        serv->sv_nrpools = npools;
        serv->sv_pools =
                kcalloc(serv->sv_nrpools, sizeof(struct svc_pool),
                        GFP_KERNEL);
        if (!serv->sv_pools) {
                kfree(serv);
                return NULL;
        }

I think kcalloc returns NULL if you pass it 0 for "n", so creation
should fail if that happens. None of the in-tree callers ever pass in 0
for the npools, but maybe that's worth an explicit check in
__svc_create(). I can add one.

> 
> > +		int threads = base;
> > +
> > +		if (remain) {
> > +			++threads;
> > +			--remain;
> > +		}
> > +		err = svc_set_pool_threads(serv, &serv->sv_pools[i], threads);
> > +		if (err)
> > +			break;
> > +	}
> > +	return err;
> > +}
> >  EXPORT_SYMBOL_GPL(svc_set_num_threads);
> > 
> >  /**
> > 
> > -- 
> > 2.52.0

-- 
Jeff Layton <jlayton@kernel.org>