On Thu, 2019-09-05 at 13:24 -0700, Daniel Colascione wrote: > On Thu, Sep 5, 2019 at 12:56 PM Tom Zanussi <zanussi@xxxxxxxxxx> > wrote: > > On Thu, 2019-09-05 at 13:51 -0400, Joel Fernandes wrote: > > > On Thu, Sep 05, 2019 at 01:47:05PM -0400, Joel Fernandes wrote: > > > > On Thu, Sep 05, 2019 at 01:35:07PM -0400, Steven Rostedt wrote: > > > > > > > > > > > > > > > [ Added Tom ] > > > > > > > > > > On Thu, 5 Sep 2019 09:03:01 -0700 > > > > > Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > > > > On Thu, Sep 5, 2019 at 7:43 AM Michal Hocko <mhocko@kernel. > > > > > > org> > > > > > > wrote: > > > > > > > > > > > > > > [Add Steven] > > > > > > > > > > > > > > On Wed 04-09-19 12:28:08, Joel Fernandes wrote: > > > > > > > > On Wed, Sep 4, 2019 at 11:38 AM Michal Hocko <mhocko@ke > > > > > > > > rnel > > > > > > > > .org> wrote: > > > > > > > > > > > > > > > > > > On Wed 04-09-19 11:32:58, Joel Fernandes wrote: > > > > > > > > > > > > > > [...] > > > > > > > > > > but also for reducing > > > > > > > > > > tracing noise. Flooding the traces makes it less > > > > > > > > > > useful > > > > > > > > > > for long traces and > > > > > > > > > > post-processing of traces. IOW, the overhead > > > > > > > > > > reduction > > > > > > > > > > is a bonus. > > > > > > > > > > > > > > > > > > This is not really anything special for this > > > > > > > > > tracepoint > > > > > > > > > though. > > > > > > > > > Basically any tracepoint in a hot path is in the same > > > > > > > > > situation and I do > > > > > > > > > not see a point why each of them should really invent > > > > > > > > > its > > > > > > > > > own way to > > > > > > > > > throttle. Maybe there is some way to do that in the > > > > > > > > > tracing subsystem > > > > > > > > > directly. > > > > > > > > > > > > > > > > I am not sure if there is a way to do this easily. Add > > > > > > > > to > > > > > > > > that, the fact that > > > > > > > > you still have to call into trace events. Why call into > > > > > > > > it > > > > > > > > at all, if you can > > > > > > > > filter in advance and have a sane filtering default? > > > > > > > > > > > > > > > > The bigger improvement with the threshold is the number > > > > > > > > of > > > > > > > > trace records are > > > > > > > > almost halved by using a threshold. The number of > > > > > > > > records > > > > > > > > went from 4.6K to > > > > > > > > 2.6K. > > > > > > > > > > > > > > Steven, would it be feasible to add a generic tracepoint > > > > > > > throttling? > > > > > > > > > > > > I might misunderstand this but is the issue here actually > > > > > > throttling > > > > > > of the sheer number of trace records or tracing large > > > > > > enough > > > > > > changes > > > > > > to RSS that user might care about? Small changes happen all > > > > > > the > > > > > > time > > > > > > but we are likely not interested in those. Surely we could > > > > > > postprocess > > > > > > the traces to extract changes large enough to be > > > > > > interesting > > > > > > but why > > > > > > capture uninteresting information in the first place? IOW > > > > > > the > > > > > > throttling here should be based not on the time between > > > > > > traces > > > > > > but on > > > > > > the amount of change of the traced signal. Maybe a generic > > > > > > facility > > > > > > like that would be a good idea? > > > > > > > > > > You mean like add a trigger (or filter) that only traces if a > > > > > field has > > > > > changed since the last time the trace was hit? Hmm, I think > > > > > we > > > > > could > > > > > possibly do that. Perhaps even now with histogram triggers? > > > > > > > > > > > > Hey Steve, > > > > > > > > Something like an analog to digitial coversion function where > > > > you > > > > lose the > > > > granularity of the signal depending on how much trace data: > > > > https://www.globalspec.com/ImageRepository/LearnMore/20142/9ee3 > > > > 8d1a > > > > 85d37fa23f86a14d3a9776ff67b0ec0f3b.gif > > > > > > s/how much trace data/what the resolution is/ > > > > > > > so like, if you had a counter incrementing with values after > > > > the > > > > increments > > > > as: 1,3,4,8,12,14,30 and say 5 is the threshold at which to > > > > emit a > > > > trace, > > > > then you would get 1,8,12,30. > > > > > > > > So I guess what is need is a way to reduce the quantiy of trace > > > > data this > > > > way. For this usecase, the user mostly cares about spikes in > > > > the > > > > counter > > > > changing that accurate values of the different points. > > > > > > s/that accurate/than accurate/ > > > > > > I think Tim, Suren, Dan and Michal are all saying the same thing > > > as > > > well. > > > > > > > There's not a way to do this using existing triggers (histogram > > triggers have an onchange() that fires on any change, but that > > doesn't > > help here), and I wouldn't expect there to be - these sound like > > very > > specific cases that would never have support in the simple trigger > > 'language'. > > I don't see the filtering under discussion as some "very specific" > esoteric need. You need this general kind of mechanism any time you > want to monitor at low frequency a thing that changes at high > frequency. The general pattern isn't specific to RSS or even memory > in > general. One might imagine, say, wanting to trace large changes in > TCP > window sizes. Any time something in the kernel has a "level" and that > level changes at high frequency and we want to learn about big swings > in that level, the mechanism we're talking about becomes useful. I > don't think it should be out of bounds for the histogram mechanism, > which is *almost* there right now. We already have the ability to > accumulate values derived from ftrace events into tables keyed on > various fields in these events and things like onmax(). > OK, so with the histograms we already have onchange(), which triggers on any change. Would it be sufficient to just add a 'threshold' param to that i.e. onchange(x) means trigger whenever the difference between the new value and the previous value is >= x? Tom
