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 39F4F41F7FE; Mon, 6 Jul 2026 11:49:27 +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=1783338568; cv=none; b=JZ7T9dIco728rDjui8QfFmU05hDA7HUVh/BhHOD9zLRY+sttrGtTl9Lybk4loG3kgn0XaS/orstBMLZ+XxGmkrSFIWQ5YHR08zUXOksc8Pm86COKkyhNr04sSrbUrryb8t1Zn0rqjpkt4rdnvsRx3DTd8Ybl01lYm/Tvuar96nU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783338568; c=relaxed/simple; bh=x4J/BO+PpTChLIv4VoxdCb5VDWzEjHz+pgXA1FZttCg=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=fgCOqQqwsuAGgV2KW8oAXbnjFVuD92KELmz+OSwLdzEB7pMLp4hCx2Qh7xhoqSf+HgsO5pTFxxRG2O4NwHHBQ5uPMNiUgsCcWYUACDz6BcDNpobls5Jkq9FaEO+wOg+chlFTCdjT/TlVhg380ascqPeaYI3imwCVjWm7boLJPZE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=if2F6WJJ; 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="if2F6WJJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2E7C41F000E9; Mon, 6 Jul 2026 11:49:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783338566; bh=GEfw8e1qGy4AHHQLByxRukT4MBVn0dcuxeTsQ+jL/SQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=if2F6WJJ4dxRgypNK2O6metkiS1+mhArItIj+wksR/Mfk+8xZHREpWbTRtTWlmLZ7 UlJL9O/J7qb19Wv1WMzKpKB8xiFzKZzIQ80Hz+S39aff+Js+QUPQ2fmMUG1gCkSXTs tOilCop8lmdfWfWIufKCD6NMcpq3+sz5BNdZttFxkG/0FZXyF6ib9NL/U534vf2yv1 U6wXFLdDA3glmJkq1PvcBMH8M55Zd2Twv4uDd7X76FGyUhv30yobmbumujtuMRgTtd SfpMtkgKG9Sq4jDx4C7Q4kfMsiLMJVeHS3LRCVGJNdN9hZl97x2CzbYjpQIOmG4syO 4sXNVL3Iz9y4g== Message-ID: <7c27731fdb20928f8a8bedc5c286cde20057aef2.camel@kernel.org> 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: Mon, 06 Jul 2026 07:49:24 -0400 In-Reply-To: <178305016681.27465.3325737287555850173@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> <5f5ef0bb961dc0bdd34383d368261aac50509d82.camel@kernel.org> <178305016681.27465.3325737287555850173@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; 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text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-07-03 at 13:42 +1000, NeilBrown wrote: > On Thu, 02 Jul 2026, Jeff Layton wrote: > > On Thu, 2026-07-02 at 08:17 -0400, Jeff Layton wrote: > > > 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 f= ewer > > > > > threads than the host has NUMA nodes while running in "pernode" o= r > > > > > "percpu" mode -- the trailing pools are left with no threads at a= ll. > > > > >=20 > > > > > svc_xprt_enqueue() selects a pool from the CPU servicing the tran= sport, > > > > > queues the transport on that pool's sp_xprts, and only wakes a th= read > > > > > from the same pool. Each thread services exclusively its own poo= l, 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 locali= ty 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(). > > > > >=20 > > > > > Fixes: 0f0257eaa5d2 ("svc: Move the xprt independent code to the = svc_xprt.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 > > >=20 > > > Indeed. Good catch. > > >=20 > >=20 > > I had the LLM run this down. We're both wrong. > >=20 > > It briefly worked properly after 3262c816a3d7 ("split svc_serv into > > pools"), but then was broken in the same series in commit bfd241600a3b > > ("knfsd: make rpc threads pools numa aware"). So I think we want: > >=20 > > Fixes: bfd241600a3b ("knfsd: make rpc threads pools numa aware") >=20 > That is certainly credible - thanks. >=20 > >=20 > >=20 > > > > > 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 a= re > > > > > + * fewer threads than pools some pools can end up with none. A > > > > > + * transport enqueued on a threadless pool would never be picke= d > > > > > + * up, since each thread only services its own pool. Fall back = to > > > > > + * the next populated pool, trading NUMA locality for a guarant= ee > > > > > + * 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 > > >=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. > > >=20 > > >=20 > >=20 > > This is probably not feasible, as there are cases where we legitimately > > queue the call to a pool with no threads. > >=20 > > At startup, we create listeners and then threads only get spun up > > later. If we get a RPC on the listener port during that window, the > > message would fire. There's a similar window on shutdown. > >=20 > > It might not hurt to add a tracepoint there though. >=20 > and a comment explaining when this might happen? >=20 Sure, I'll add one. > The start-up window makes sense - as long as we start a thread on each > pool it should be safe. But if we don't (if the admin request zero on > some pools) we can still get stuck. Is there something simple we can do > about that, or does the admin get to keep both halves? >=20 Well, no. This patch makes sure that RPCs are always routed to a pool with CPUs. They might end up doing cross-NUMA calls and performance might suck, but it shouldn't get stuck. > And on shutdown the threads stop before the sockets are cleaned up - but > they do get cleaned up, so all good. >=20 >=20 Correct. >=20 > >=20 > > > > I think this is a sensible defensive-programming approach. > > > >=20 > > > > Reviewed-by: NeilBrown > > > >=20 > > >=20 > > > Thanks! > > >=20 > > > > > + 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 > > --=20 > > Jeff Layton > >=20 Thanks again for the review. --=20 Jeff Layton