From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4000C6EF02; Wed, 28 Feb 2024 17:47:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709142469; cv=none; b=CvbMd92Vc7kcldX+VWGATon9/FxCmshAetdvTOg4xhiwcyWDjKRdympdTm8CXEFQ+QM0I/fdL2w8dFCpxujMSm0yqoV/KH7t+AZHsKuPLgisdM4DJRuEnaezC4sef1zth2ZbY4E1OSUqo9pOe2EBeC+k3GJ18ISQrrXDXLOgYKo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709142469; c=relaxed/simple; bh=C1UU1WA26ToOkkgIYK5WS3L7oXs4voCYLwz9isVT8+k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=t/QYK9jTMoZOcMCpHgSH3nJkFFn4HFPABxtvh+bl6Nx7Y+yxHC2A30z26hDCOcdg263pSgSLY7iYzLU28TdMxp4yizQVPgHu7SHfN+FXoKEPc4e76q2oo7b3Ar28bPbjase+jrOXFq53ZMe+EW6BR9ViMUjGkyIZItSxB5IMa7I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=R1KhMvtx; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="R1KhMvtx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B3E49C433F1; Wed, 28 Feb 2024 17:47:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1709142468; bh=C1UU1WA26ToOkkgIYK5WS3L7oXs4voCYLwz9isVT8+k=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=R1KhMvtxM5TlHUYIwZOn+O6bgARaol0xJeSoEJFVnwSxhpbar1jquGj5XVuPFoaPW HKMHlekSlIk02JZumDoY6MJUZbXs7byFDqEE/u80HJTYB5YJqGCVAi8jy01RykgFxy XUy/LqS3EdJWBYaq4tvIfUWKBrI+F0NPKZTB0iuYkPTlQL+/CdG0/9SuzLhzMoR3Ug TnT6zoIZPl1WWejxVB/YlR58dXt0Gh8TRoa5KRWyttsNXKoo4IQFAH+R7EIglfzjbU eKAhiZbmGFdEz/mOgTkJ4JSqSkDRqSJNDGcshvqmHdOfmrBNkBAzDBNPi3BqLXpef7 xdy4OAMIHg3Bg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5F228CE0350; Wed, 28 Feb 2024 09:47:48 -0800 (PST) Date: Wed, 28 Feb 2024 09:47:48 -0800 From: "Paul E. McKenney" To: Yan Zhai Cc: Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= , Eric Dumazet , netdev@vger.kernel.org, "David S. Miller" , Jakub Kicinski , Paolo Abeni , Jiri Pirko , Simon Horman , Daniel Borkmann , Lorenzo Bianconi , Coco Li , Wei Wang , Alexander Duyck , Hannes Frederic Sowa , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, bpf@vger.kernel.org, kernel-team@cloudflare.com Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll Message-ID: Reply-To: paulmck@kernel.org References: <87edcwerj6.fsf@toke.dk> <6b6ce007-4527-494f-8d03-079f7bf139f9@paulmck-laptop> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Feb 28, 2024 at 09:48:42AM -0600, Yan Zhai wrote: > On Wed, Feb 28, 2024 at 9:10 AM Paul E. McKenney wrote: > > > > On Wed, Feb 28, 2024 at 12:50:53PM +0100, Toke Høiland-Jørgensen wrote: > > > "Paul E. McKenney" writes: > > > > > > > On Tue, Feb 27, 2024 at 05:44:17PM +0100, Eric Dumazet wrote: > > > >> On Tue, Feb 27, 2024 at 4:44 PM Yan Zhai wrote: > > > >> > > > > >> > We noticed task RCUs being blocked when threaded NAPIs are very busy in > > > >> > production: detaching any BPF tracing programs, i.e. removing a ftrace > > > >> > trampoline, will simply block for very long in rcu_tasks_wait_gp. This > > > >> > ranges from hundreds of seconds to even an hour, severely harming any > > > >> > observability tools that rely on BPF tracing programs. It can be > > > >> > easily reproduced locally with following setup: > > > >> > > > > >> > ip netns add test1 > > > >> > ip netns add test2 > > > >> > > > > >> > ip -n test1 link add veth1 type veth peer name veth2 netns test2 > > > >> > > > > >> > ip -n test1 link set veth1 up > > > >> > ip -n test1 link set lo up > > > >> > ip -n test2 link set veth2 up > > > >> > ip -n test2 link set lo up > > > >> > > > > >> > ip -n test1 addr add 192.168.1.2/31 dev veth1 > > > >> > ip -n test1 addr add 1.1.1.1/32 dev lo > > > >> > ip -n test2 addr add 192.168.1.3/31 dev veth2 > > > >> > ip -n test2 addr add 2.2.2.2/31 dev lo > > > >> > > > > >> > ip -n test1 route add default via 192.168.1.3 > > > >> > ip -n test2 route add default via 192.168.1.2 > > > >> > > > > >> > for i in `seq 10 210`; do > > > >> > for j in `seq 10 210`; do > > > >> > ip netns exec test2 iptables -I INPUT -s 3.3.$i.$j -p udp --dport 5201 > > > >> > done > > > >> > done > > > >> > > > > >> > ip netns exec test2 ethtool -K veth2 gro on > > > >> > ip netns exec test2 bash -c 'echo 1 > /sys/class/net/veth2/threaded' > > > >> > ip netns exec test1 ethtool -K veth1 tso off > > > >> > > > > >> > Then run an iperf3 client/server and a bpftrace script can trigger it: > > > >> > > > > >> > ip netns exec test2 iperf3 -s -B 2.2.2.2 >/dev/null& > > > >> > ip netns exec test1 iperf3 -c 2.2.2.2 -B 1.1.1.1 -u -l 1500 -b 3g -t 100 >/dev/null& > > > >> > bpftrace -e 'kfunc:__napi_poll{@=count();} interval:s:1{exit();}' > > > >> > > > > >> > Above reproduce for net-next kernel with following RCU and preempt > > > >> > configuraitons: > > > >> > > > > >> > # RCU Subsystem > > > >> > CONFIG_TREE_RCU=y > > > >> > CONFIG_PREEMPT_RCU=y > > > >> > # CONFIG_RCU_EXPERT is not set > > > >> > CONFIG_SRCU=y > > > >> > CONFIG_TREE_SRCU=y > > > >> > CONFIG_TASKS_RCU_GENERIC=y > > > >> > CONFIG_TASKS_RCU=y > > > >> > CONFIG_TASKS_RUDE_RCU=y > > > >> > CONFIG_TASKS_TRACE_RCU=y > > > >> > CONFIG_RCU_STALL_COMMON=y > > > >> > CONFIG_RCU_NEED_SEGCBLIST=y > > > >> > # end of RCU Subsystem > > > >> > # RCU Debugging > > > >> > # CONFIG_RCU_SCALE_TEST is not set > > > >> > # CONFIG_RCU_TORTURE_TEST is not set > > > >> > # CONFIG_RCU_REF_SCALE_TEST is not set > > > >> > CONFIG_RCU_CPU_STALL_TIMEOUT=21 > > > >> > CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=0 > > > >> > # CONFIG_RCU_TRACE is not set > > > >> > # CONFIG_RCU_EQS_DEBUG is not set > > > >> > # end of RCU Debugging > > > >> > > > > >> > CONFIG_PREEMPT_BUILD=y > > > >> > # CONFIG_PREEMPT_NONE is not set > > > >> > CONFIG_PREEMPT_VOLUNTARY=y > > > >> > # CONFIG_PREEMPT is not set > > > >> > CONFIG_PREEMPT_COUNT=y > > > >> > CONFIG_PREEMPTION=y > > > >> > CONFIG_PREEMPT_DYNAMIC=y > > > >> > CONFIG_PREEMPT_RCU=y > > > >> > CONFIG_HAVE_PREEMPT_DYNAMIC=y > > > >> > CONFIG_HAVE_PREEMPT_DYNAMIC_CALL=y > > > >> > CONFIG_PREEMPT_NOTIFIERS=y > > > >> > # CONFIG_DEBUG_PREEMPT is not set > > > >> > # CONFIG_PREEMPT_TRACER is not set > > > >> > # CONFIG_PREEMPTIRQ_DELAY_TEST is not set > > > >> > > > > >> > An interesting observation is that, while tasks RCUs are blocked, > > > >> > related NAPI thread is still being scheduled (even across cores) > > > >> > regularly. Looking at the gp conditions, I am inclining to cond_resched > > > >> > after each __napi_poll being the problem: cond_resched enters the > > > >> > scheduler with PREEMPT bit, which does not account as a gp for tasks > > > >> > RCUs. Meanwhile, since the thread has been frequently resched, the > > > >> > normal scheduling point (no PREEMPT bit, accounted as a task RCU gp) > > > >> > seems to have very little chance to kick in. Given the nature of "busy > > > >> > polling" program, such NAPI thread won't have task->nvcsw or task->on_rq > > > >> > updated (other gp conditions), the result is that such NAPI thread is > > > >> > put on RCU holdouts list for indefinitely long time. > > > >> > > > > >> > This is simply fixed by mirroring the ksoftirqd behavior: after > > > >> > NAPI/softirq work, raise a RCU QS to help expedite the RCU period. No > > > >> > more blocking afterwards for the same setup. > > > >> > > > > >> > Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support") > > > >> > Signed-off-by: Yan Zhai > > > >> > --- > > > >> > net/core/dev.c | 4 ++++ > > > >> > 1 file changed, 4 insertions(+) > > > >> > > > > >> > diff --git a/net/core/dev.c b/net/core/dev.c > > > >> > index 275fd5259a4a..6e41263ff5d3 100644 > > > >> > --- a/net/core/dev.c > > > >> > +++ b/net/core/dev.c > > > >> > @@ -6773,6 +6773,10 @@ static int napi_threaded_poll(void *data) > > > >> > net_rps_action_and_irq_enable(sd); > > > >> > } > > > >> > skb_defer_free_flush(sd); > > > > > > > > Please put a comment here stating that RCU readers cannot cross > > > > this point. > > > > > > > > I need to add lockdep to rcu_softirq_qs() to catch placing this in an > > > > RCU read-side critical section. And a header comment noting that from > > > > an RCU perspective, it acts as a momentary enabling of preemption. > > > > > > OK, so one question here: for XDP, we're basically treating > > > local_bh_disable/enable() as the RCU critical section, cf the discussion > > > we had a few years ago that led to this being documented[0]. So why is > > > it OK to have the rcu_softirq_qs() inside the bh disable/enable pair, > > > but not inside an rcu_read_lock() section? > > > > In general, it is not OK. And it is not OK in this case if this happens > > to be one of the local_bh_disable() regions that XDP is waiting on. > > Except that that region ends right after the rcu_softirq_qs(), so that > > should not be a problem. > > > > But you are quite right, that is an accident waiting to happen, so it > > would be better if the patch did something like this: > > > > local_bh_enable(); > > if (!IS_ENABLED(CONFIG_PREEMPT_RT)) { > > preempt_disable(); > > rcu_softirq_qs(); > > preempt_enable(); > > } > > > Yeah we need preempt for this call. When I first attempt it after > local_bh_enable, I got the bug call: > [ 1166.384279] BUG: using __this_cpu_read() in preemptible [00000000] > code: napi/veth2-66/8439 > [ 1166.385337] caller is rcu_softirq_qs+0x16/0x130 > [ 1166.385900] CPU: 3 PID: 8439 Comm: napi/veth2-66 Not tainted > 6.7.0-rc8-g3fbf61207c66-dirty #75 > [ 1166.386950] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), > BIOS 1.16.2-debian-1.16.2-1 04/01/2014 > [ 1166.388110] Call Trace: > [ 1166.388417] > [ 1166.388684] dump_stack_lvl+0x36/0x50 > [ 1166.389147] check_preemption_disabled+0xd1/0xe0 > [ 1166.389725] rcu_softirq_qs+0x16/0x130 > [ 1166.390190] napi_threaded_poll+0x21e/0x260 > [ 1166.390702] ? __pfx_napi_threaded_poll+0x10/0x10 > [ 1166.391277] kthread+0xf7/0x130 > [ 1166.391643] ? __pfx_kthread+0x10/0x10 > [ 1166.392130] ret_from_fork+0x34/0x50 > [ 1166.392574] ? __pfx_kthread+0x10/0x10 > [ 1166.393048] ret_from_fork_asm+0x1b/0x30 > [ 1166.393530] > > Since this patch is trying to mirror what __do_softirq has, should the > similar notes/changes apply to that side as well? Up to now, the rcu_softirq_qs() was a special function strictly for use by __do_softirq(), hence the lack of documentation. I will let the __do_softirq() maintainers decide what they would like to do there, if anything. Thanx, Paul > > Though maybe something like this would be better: > > > > local_bh_enable(); > > if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > > rcu_softirq_qs_enable(local_bh_enable()); > > else > > local_bh_enable(); > > > > A bit ugly, but it does allow exact checking of the rules and also > > avoids extra overhead. > > > > I could imagine pulling the CONFIG_PREEMPT_RT check into the body of > > rcu_softirq_qs_enable(). > > > > But is there a better way? > > > > > Also, looking at the patch in question: > > > > > > >> > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > > > >> > + rcu_softirq_qs(); > > > >> > + > > > >> > local_bh_enable(); > > > > > > Why does that local_bh_enable() not accomplish the same thing as the qs? > > > > In this case, because it does not create the appearance of a voluntary > > context switch needed by RCU Tasks. So the wait for trampoline evacuation > > could still take a very long time. > > > > Thanx, Paul > > > > > -Toke > > > > > > [0] https://lore.kernel.org/bpf/20210624160609.292325-6-toke@redhat.com/ > > >