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Date:   Mon, 16 Mar 2020 08:39:42 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     mutt@paulmck-ThinkPad-P72, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
        jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
        rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
        fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle,
 exception, offline
Message-ID: <20200316153942.GW3199@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <20200312181618.GA21271@paulmck-ThinkPad-P72>
 <20200313144145.GA31604@lenoir>
 <20200313154243.GU3199@paulmck-ThinkPad-P72>
 <20200316144535.GA501@lenoir>
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On Mon, Mar 16, 2020 at 03:45:36PM +0100, Frederic Weisbecker wrote:
> On Fri, Mar 13, 2020 at 08:42:43AM -0700, Paul E. McKenney wrote:
> > On Fri, Mar 13, 2020 at 03:41:46PM +0100, Frederic Weisbecker wrote:
> > > On Thu, Mar 12, 2020 at 11:16:18AM -0700, Paul E. McKenney wrote:
> > > > Hello!
> > > > 
> > > > This series provides two variants of Tasks RCU, a rude variant inspired
> > > > by Steven Rostedt's use of schedule_on_each_cpu(), and a tracing variant
> > > > requested by the BPF folks and perhaps also of use for other tracing
> > > > use cases.
> > > > 
> > > > The tracing variant has explicit read-side markers to permit finite grace
> > > > periods even given in-kernel loops in PREEMPT=n builds It also protects
> > > > code in the idle loop, on exception entry/exit paths, and on the various
> > > > CPU-hotplug online/offline code paths, thus having protection properties
> > > > similar to SRCU.  However, unlike SRCU, this variant avoids expensive
> > > > instructions in the read-side primitives, thus having read-side overhead
> > > > similar to that of preemptible RCU.
> > > > 
> > > > There are of course downsides.  The grace-period code can send IPIs to
> > > > CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.
> > > > It is necessary to scan the full tasklist, much as for Tasks RCU.  There
> > > > is a single callback queue guarded by a single lock, again, much as for
> > > > Tasks RCU.  If needed, these downsides can be at least partially remedied
> > > 
> > > So what we trade to fix the issues we are having with tracing against extended
> > > grace periods, we lose in CPU isolation. That worries me a bit as tracing can
> > > be thoroughly used with nohz_full and CPU isolation.
> > 
> > First, disturbing nohz_full CPUs can be avoided by the sysadm simply
> > refusing to remove tracepoints while sensitive applications are running
> > on nohz_full CPUs.
> 
> So, in that case we'll need to modify the tools such as perf tools to avoid
> releasing the related buffers until we are ready to do so.
> 
> That's possible but it's kindof an ABI breakage. Also what if there is a
> long running service on that nohz full CPU polling on the networking card...

In the near term, I do admit that Mathieu's point about using smp_mb()
in readers but only on nohz_full CPUs is attractive.

I have some other ideas, but simplicity has its advantages, and if no
one complains, perhaps those advantages are also good for the long term.

							Thanx, Paul