Re: [RFC] trace: Add option for polling ring buffers

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 19 May 2021 19:57:55 +0200
Nicolas Saenz Julienne <nsaenzju@redhat.com> 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.

If you need this for trace-cmd, we can add a --poll option that would do
this.

-- Steve