linux-kernel - Re: [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle, exception, offline

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Date:   Fri, 13 Mar 2020 08:42:43 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     mutt@...lmck-ThinkPad-P72, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle,
 exception, offline

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.

Second, for non-CPU-bound real-time programs with mostly-idle CPUs,
I should be able to decrease the likelihood of sending IPIs pretty much
to zero.

Or am I missing something here?

							Thanx, Paul