Quoting Lionel Landwerlin (2018-02-12 11:00:10)
> On 12/02/18 10:41, Chris Wilson wrote:
> > Quoting Lionel Landwerlin (2018-02-12 10:37:52)
> >> On 09/02/18 20:53, Chris Wilson wrote:
> >>> Quoting Lionel Landwerlin (2018-02-09 17:47:44)
> >>>> Hey Chris,
> >>>>
> >>>>    From the i915/perf point of view, I'm fine with this change.
> >>>> The pinning of the hw_id when monitoring a single context (with OA)
> >>>> doesn't break the existing userspace (I can only think of Mesa).
> >>>>
> >>>> I'm also trying to build up a system wide monitoring feature in GPUTop
> >>>> with a timeline display. This change makes it a bit more challenging.
> >>>> But this isn't really an expected feature, it's just nice to have.
> >>>>
> >>>> What I'm thinking of would be to keep a circular buffer of requests in
> >>>> the order they're submitted to an engine.
> >>>> Then the i915 perf driver could correlate between the context-switch
> >>>> tagged reports coming from OA and the requests submitted.
> >>>> Much like the OA buffer, this circular buffer could overflow at which
> >>>> point we signal the application using the i915 perf driver and it'll
> >>>> most likely close the driver and try again.
> >>>>
> >>>> I would need that have the hw_id added to the requests. Does that sounds
> >>>> possible?
> >>> You already add hw_id to the tracepoint. For the requests, it is just
> >>> req->ctx->hw_id, valid from submission to retirement. Hmm, the
> >>> tracepoint is broken (use-after-free in ctx->hw_id). What value do you
> >>> want for HSW? (This patch will assign all legacy submission to HW ID 0.)
> >>> But aiui, for HSW oa you want lrca not HW ID. So both the use-after-free
> >>> and alternative ids suggest storing it on the request directly.
> >> HSW doesn't have a hw_id field in the OA reports.
> >> I'll have to come up with something slightly different.
> >>
> >>> How do you get the HW ID out, via the tracepoint right?
> >> It's in the OA reports.
> > I mean how do you correlate the HW ID with userspace?
> > -Chris
> >
> Right now, with tracepoints. But as I wrote above, I don't think it's 
> reliable (because of the timestamp correlation).
> My idea was to let the i915 perf driver do it for userspace by attaching 
> the pid/tid to the drm_i915_perf_record.
> On submission i915 would put the request into a circular buffer and i915 
> perf would read back the tail of that buffer to match what is coming out 
> of the OA unit.

And I read that as doing the tracing entirely in userspace. Inside the
kernel we should be able to do a much better job of knowing what
requests/contexts are active, it may just be we take an extra ref and/or
context pin for perf tracking.
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