Hi Steven, thanks for having a look at this. On Fri, 2021-05-28 at 13:32 -0400, Steven Rostedt wrote: > On Wed, 19 May 2021 19:57:55 +0200 > Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx> wrote: > > > To minimize trace's effect on isolated CPUs. That is, CPUs were only a > > handful or a single, process are allowed to run. Introduce a new trace > > option: 'poll-rb'. > > > > This option changes the heuristic used to wait for data on trace > > buffers. The default one, based on wait queues, will trigger an IPI[1] > > on the CPU responsible for new data, which will take care of waking up > > the trace gathering process (generally trace-cmd). Whereas with > > 'poll-rb' we will poll (as in busy-wait) the ring buffers from the trace > > gathering process, releasing the CPUs writing trace data from doing any > > wakeup work. > > > > This wakeup work, although negligible in the vast majority of workloads, > > may cause unwarranted latencies on systems running trace on isolated > > CPUs. This is made worse on PREEMPT_RT kernels, as they defer the IPI > > handling into a kernel thread, forcing unwarranted context switches on > > otherwise extremely busy CPUs. > > > > To illustrate this, tracing with PREEMPT_RT=y on an isolated CPU with a > > single process pinned to it (NO_HZ_FULL=y, and plenty more isolation > > options enabled). I see: > > - 50-100us latency spikes with the default trace-cmd options > > - 14-10us latency spikes with 'poll-rb' > > - 11-8us latency spikes with no tracing at all > > > > The obvious drawback of 'poll-rb' is putting more pressure on the > > housekeeping CPUs. Wasting cycles. Hence the notice in the documentation > > discouraging its use in general. > > > > [1] The IPI, in this case, an irq_work, is needed since trace might run > > in NMI context. Which is not suitable for wake-ups. > > Can't this simply be done in user-space? > > Set the reading of the trace buffers to O_NONBLOCK and it wont wait for > buffering to happen, and should prevent it from causing the IPI wake ups. Yes, I hadn't thought of O_NONBLOCK... > If you need this for trace-cmd, we can add a --poll option that would do > this. I just sent a patch with my attempt at implementing --poll. -- Nicolás Sáenz
