From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DB0412E8DEA; Thu, 2 Jul 2026 12:17:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782994645; cv=none; b=cHCSrT0ibiFjold5QBTvUSDpU7KpG0SSje6eMHFbXXdifxk/ECvs9J1XOCES9t8ERh/pZmsvlIaEhpiIBM0Mo4vdZELRtGxUzajgqgYROrCaEOri3MusTBAL5s1w3myinjJVQWlZtVF8Sh9iZdGv14IQhhr+Ysm83TF2CkpORMA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782994645; c=relaxed/simple; bh=koY24Y3/FwYDTsTJxxt33spfGQnd6sCH39qugoJHN8Y=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=fZuQFv/EBQ8A5BLZBeQFDQAgJKt2q3TRKNE8Jhdq3Sxcco0iUSfcdUxYhpeUzJo5gfl7e2/lEv8c0IZO1zW5ZcEsrduN7b6thuj6apkFQeDh1DqqoSdLkblOs1XUxNbZVhmf7jER8wi0Xe0HHQs/doa2FyyW7mOOKqGkqpTdHAI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T9lXcpdi; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="T9lXcpdi" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AF4531F000E9; Thu, 2 Jul 2026 12:17:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782994643; bh=Uu+FSw9b4SMJe20t+rn2/WBPGlKa/waIUnc8Lfp/Rag=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=T9lXcpdiKPXwszQ4yCYOmZhE6oKWUtN4qQuANE58d1R6KtC4D/jmrIJ7CHKedCygW xjp8VvhJJrouMBpN7iDGaT2+VhyovE4FEJBo7DyDwD1Yg90d2ahIz4EoJuoRagt+iK +XTAx2pmJjmoc0JCYjUuiY9I8G9OhgI0y2PU9q863ouzw1aWXISE6CoQ54FPiX+Nlr AGduiqCdKwEFntVW1yRx2nOC97mXQSXt+IBC/G5mRrDbPZT9LaOiH7/bmkqKIs07He NNvVBg4WcvSz258b6mOem/zvonpKSLQ7Yp9M3kFDgE5hrbp1FSNjpp+8Y/lR70JTSt JMLMRMmE7f89Q== Message-ID: Subject: Re: [PATCH v4 1/4] sunrpc: route to a populated pool in svc_pool_for_cpu() From: Jeff Layton To: NeilBrown Cc: Trond Myklebust , Anna Schumaker , Olga Kornievskaia , Dai Ngo , Tom Talpey , Chuck Lever , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Thu, 02 Jul 2026 08:17:20 -0400 In-Reply-To: <178294402742.27465.8893159356805540635@noble.neil.brown.name> References: <20260701-sunrpc-pool-mode-v4-0-b3d867e4c8f9@kernel.org> <20260701-sunrpc-pool-mode-v4-1-b3d867e4c8f9@kernel.org> <178294402742.27465.8893159356805540635@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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 =3D 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. > >=20 > > 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. > >=20 > > 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-effor= t > > routing hint, so annotate it with data_race(). > >=20 > > Fixes: 0f0257eaa5d2 ("svc: Move the xprt independent code to the svc_xp= rt.c file") >=20 > Why that commit? Did this ever work correctly? > It seems more likely that=20 > Fixes: 3262c816a3d7 ("[PATCH] knfsd: split svc_serv into pools") > is appropriate. >=20 Indeed. Good catch. > > Signed-off-by: Jeff Layton > > --- > > net/sunrpc/svc.c | 26 +++++++++++++++++++++++++- > > 1 file changed, 25 insertions(+), 1 deletion(-) > >=20 > > 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 =3D &svc_pool_map; > > int cpu =3D raw_smp_processor_id(); > > unsigned int pidx =3D 0; > > + unsigned int i; > > =20 > > if (serv->sv_nrpools <=3D 1) > > return serv->sv_pools; > > @@ -414,8 +415,31 @@ struct svc_pool *svc_pool_for_cpu(struct svc_serv = *serv) > > pidx =3D m->to_pool[cpu_to_node(cpu)]; > > break; > > } > > + pidx %=3D 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 =3D 0; i < serv->sv_nrpools; i++) { > > + struct svc_pool *pool =3D &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 >=3D serv->sv_nrpools) > > + pidx =3D 0; > > + } > > =20 > > - return &serv->sv_pools[pidx % serv->sv_nrpools]; > > + /* No pool has any threads; nothing can service the transport. */ >=20 > Would a WARN_ON_ONCE() be appropriate here? >=20 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. >=20 > Reviewed-by: NeilBrown >=20 Thanks! > > + return &serv->sv_pools[pidx]; > > } > > =20 > > static int svc_rpcb_setup(struct svc_serv *serv, struct net *net) > >=20 > > --=20 > > 2.54.0 > >=20 > >=20 --=20 Jeff Layton