From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id CD3C92E65B; Mon, 18 Mar 2024 10:59:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710759547; cv=none; b=bac1KPB3QVs8hN352MKiYEesJVE34X7/aro8S+ApGUOp3gfl64jNbyFJk/WQWnI2VXvxX/JCTvhcADYKewBTDd3M2slS8fmnkagzYeCFOggIqD3sDR5ZcUM+o7sgN6kE+l6/XWCNGoPwLIOXSw4vc4J3YR0Rbhkyn6BV7ZF4qUE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710759547; c=relaxed/simple; bh=m4C7IJM/lBOjxxuMNRxJEzICHUkxmZPvIn8SOjuKfPs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=j2CGz57JgDer6HWjbHrOezesVwithqzh+mVhTrPzX+TYEr4PjxP+RTCyK1Ru3UsCTl+tut1w2BABAZidacPNBtlsdRqlYZGdZUDeKjfw1PEwFFpZbZLHcNIR5VGdtiaE9AhX+zcFR17A3ak1Y1li2dUdFyfJI65Y42fjRSTpwHU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0E971106F; Mon, 18 Mar 2024 03:59:39 -0700 (PDT) Received: from FVFF77S0Q05N (unknown [10.57.71.172]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 42E1B3F762; Mon, 18 Mar 2024 03:59:00 -0700 (PDT) Date: Mon, 18 Mar 2024 10:58:57 +0000 From: Mark Rutland To: "Paul E. McKenney" Cc: Yan Zhai , netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , 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, Joel Fernandes , Toke Hoiland-Jorgensen , Alexei Starovoitov , Steven Rostedt , Jesper Dangaard Brouer Subject: Re: [PATCH v4 net 1/3] rcu: add a helper to report consolidated flavor QS Message-ID: References: <491d3af6c7d66dfb3b60b2f210f38e843dfe6ed2.1710525524.git.yan@cloudflare.com> <790ce7e7-a8fd-4d28-aaf3-1b991a898be2@paulmck-laptop> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <790ce7e7-a8fd-4d28-aaf3-1b991a898be2@paulmck-laptop> On Fri, Mar 15, 2024 at 10:40:56PM -0700, Paul E. McKenney wrote: > On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote: > > There are several scenario in network processing that can run > > extensively under heavy traffic. In such situation, RCU synchronization > > might not observe desired quiescent states for indefinitely long period. > > Create a helper to safely raise the desired RCU quiescent states for > > such scenario. > > > > Currently the frequency is locked at HZ/10, i.e. 100ms, which is > > sufficient to address existing problems around RCU tasks. It's unclear > > yet if there is any future scenario for it to be further tuned down. > > I suggest something like the following for the commit log: > > ------------------------------------------------------------------------ > > When under heavy load, network processing can run CPU-bound for many tens > of seconds. Even in preemptible kernels, this can block RCU Tasks grace > periods, which can cause trace-event removal to take more than a minute, > which is unacceptably long. > > This commit therefore creates a new helper function that passes > through both RCU and RCU-Tasks quiescent states every 100 milliseconds. > This hard-coded value suffices for current workloads. FWIW, this sounds good to me. > > ------------------------------------------------------------------------ > > > Suggested-by: Paul E. McKenney > > Reviewed-by: Jesper Dangaard Brouer > > Signed-off-by: Yan Zhai > > --- > > v3->v4: comment fixup > > > > --- > > include/linux/rcupdate.h | 24 ++++++++++++++++++++++++ > > 1 file changed, 24 insertions(+) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 0746b1b0b663..da224706323e 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -247,6 +247,30 @@ do { \ > > cond_resched(); \ > > } while (0) > > > > +/** > > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states > > + * @old_ts: last jiffies when QS was reported. Might be modified in the macro. > > + * > > + * This helper is for network processing in non-RT kernels, where there could > > + * be busy polling threads that block RCU synchronization indefinitely. In > > + * such context, simply calling cond_resched is insufficient, so give it a > > + * stronger push to eliminate all potential blockage of all RCU types. > > + * > > + * NOTE: unless absolutely sure, this helper should in general be called > > + * outside of bh lock section to avoid reporting a surprising QS to updaters, > > + * who could be expecting RCU read critical section to end at local_bh_enable(). > > + */ > > How about something like this for the kernel-doc comment? > > /** > * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states > * @old_ts: jiffies at start of processing. > * > * This helper is for long-running softirq handlers, such as those > * in networking. The caller should initialize the variable passed in > * as @old_ts at the beginning of the softirq handler. When invoked > * frequently, this macro will invoke rcu_softirq_qs() every 100 > * milliseconds thereafter, which will provide both RCU and RCU-Tasks > * quiescent states. Note that this macro modifies its old_ts argument. > * > * Note that although cond_resched() provides RCU quiescent states, > * it does not provide RCU-Tasks quiescent states. > * > * Because regions of code that have disabled softirq act as RCU > * read-side critical sections, this macro should be invoked with softirq > * (and preemption) enabled. > * > * This macro has no effect in CONFIG_PREEMPT_RT kernels. > */ Considering the note about cond_resched(), does does cond_resched() actually provide an RCU quiescent state for fully-preemptible kernels? IIUC for those cond_resched() expands to: __might_resched(); klp_sched_try_switch() ... and AFAICT neither reports an RCU quiescent state. So maybe it's worth dropping the note? Seperately, what's the rationale for not doing this on PREEMPT_RT? Does that avoid the problem through other means, or are people just not running effected workloads on that? Mark. > > Thanx, Paul > > > +#define rcu_softirq_qs_periodic(old_ts) \ > > +do { \ > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \ > > + time_after(jiffies, (old_ts) + HZ / 10)) { \ > > + preempt_disable(); \ > > + rcu_softirq_qs(); \ > > + preempt_enable(); \ > > + (old_ts) = jiffies; \ > > + } \ > > +} while (0) > > + > > /* > > * Infrastructure to implement the synchronize_() primitives in > > * TREE_RCU and rcu_barrier_() primitives in TINY_RCU. > > -- > > 2.30.2 > > > >