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
