Messages in this thread | | | Subject | Re: [PATCH v6 1/4] ftrace: Implement fs notification for tracing_max_latency | From | Viktor Rosendahl <> | Date | Sun, 8 Sep 2019 19:05:04 +0200 |
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On 9/8/19 1:38 AM, Joel Fernandes wrote:> On Sat, Sep 07, 2019 at 11:12:59PM +0200, Viktor Rosendahl wrote: >> On 9/6/19 4:17 PM, Joel Fernandes wrote: >>> On Thu, Sep 05, 2019 at 03:25:45PM +0200, Viktor Rosendahl wrote: >> <clip> >>>> + >>>> +__init static int latency_fsnotify_init(void) >>>> +{ >>>> + fsnotify_wq = alloc_workqueue("tr_max_lat_wq", >>>> + WQ_UNBOUND | WQ_HIGHPRI, 0); >>>> + if (!fsnotify_wq) { >>>> + pr_err("Unable to allocate tr_max_lat_wq\n"); >>>> + return -ENOMEM; >>>> + } >>> >>> Why not just use the system workqueue instead of adding another workqueue? >>> >> >> For the the latency-collector to work properly in the worst case, when a >> new latency occurs immediately, the fsnotify must be received in less >> time than what the threshold is set to. If we always are slower we will >> always lose certain latencies. >> >> My intention was to minimize latency in some important cases, so that >> user space receives the notification sooner rather than later. >> >> There doesn't seem to be any system workqueue with WQ_UNBOUND and >> WQ_HIGHPRI. My thinking was that WQ_UNBOUND might help with the latency >> in some important cases. >> >> If we use: >> >> queue_work(system_highpri_wq, &tr->fsnotify_work); >> >> then the work will (almost) always execute on the same CPU but if we are >> unlucky that CPU could be too busy while there could be another CPU in >> the system that would be able to process the work soon enough. >> >> queue_work_on() could be used to queue the work on another CPU but it >> seems difficult to select the right CPU. > > Ok, a separate WQ is fine with me as such since the preempt/irq events are on > a debug kernel anyway.
Keep in mind that this feature is also enabled by the wakeup tracers and by hwlat. These are often enabled by production kernels.
I guess it would be possible to add some ifdefs so that we create a new workqueue only if preempt/irqsoff tracing is enabled in the kernel config and use system_highpri_wq if we only have the wakeup and hwlat tracers in the config.
However, I don't really like adding yet some more ifdefs to the code.
Since a new workqueue will not necessariy create a new worker thread nowadays, I thought that it would be OK with a new unbound workqueue, which should not add much to the tendency to create more worker threads.
> > I'll keep reviewing your patches next few days, I am at the LPC conference so > might be a bit slow. Overall I think the series look like its maturing and > getting close. >
Ok, thanks. Could you let me know when you have looked through it all so that I know when it makes sense to make another reroll of the series?
best regards,
Viktor
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