Messages in this thread | | | Subject | Re: [PATCH v2 09/10] coresight: perf: Remove set_buffer call back | From | Suzuki K Poulose <> | Date | Wed, 25 Jul 2018 10:51:20 +0100 |
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On 07/24/2018 09:08 PM, Mathieu Poirier wrote: > On Mon, 23 Jul 2018 at 16:27, Suzuki K Poulose <suzuki.poulose@arm.com> wrote: >> >> Mathieu, >> >> On 07/23/2018 07:22 PM, Mathieu Poirier wrote: >>> On Fri, 20 Jul 2018 at 03:04, Suzuki K Poulose <Suzuki.Poulose@arm.com> wrote: >>>> >>>> Mathieu, >>>> >>>> On 19/07/18 21:36, Mathieu Poirier wrote: >>>>> On Tue, Jul 17, 2018 at 06:11:40PM +0100, Suzuki K Poulose wrote: >>>>>> In coresight perf mode, we need to prepare the sink before >>>>>> starting a session, which is done via set_buffer call back. >>>>>> We then proceed to enable the tracing. If we fail to start >>>>>> the session successfully, we leave the sink configuration >>>>>> unchanged. This was fine for the existing backends as they >>>>>> don't have any state associated with the buffers. But with >>>>>> ETR, we need to keep track of the buffer details and need >>>>>> to be cleaned up if we fail. In order to make the operation >>>>>> atomic and to avoid yet another call back, we get rid of >>>>>> the "set_buffer" call back and pass the buffer details >>>>>> via enable() call back to the sink. >>>>> >>>>> Suzuki, >>>>> >>>>> I'm not sure I understand the problem you're trying to fix there. From the >>>>> implementation of tmc_enable_etr_sink_perf() in the next patch, wouldn't the >>>>> same result been achievable using a callback? >>>> >>>> We can definitely achieve the results using "set_buffer". But for ETR, >>>> we track the "perf_buf" in drvdata->perf_data when we do "set_buffer". >>>> But if we failed to enable_path(), we leave the drvdata->perf_data >>>> and doesn't clean it up. Now when another session is about to set_buf, >>>> we check if perf_data is empty and WARNs otherwise. >>>> Because we can't be sure if it belongs to an abandoned session or >>>> another active session and we completely messed somewhere in the driver. >>>> So, we need a clear_buffer call back if the enable fails, something >>>> not really worth. Anyways, there is no point in separating set_buffer >>>> and enabling the sink, as the error handling becomes cumbersome as explained >>>> above. >>>> >>>>> >>>>> I'm fine with this patch and supportive of getting rid of callbacks if we can, I >>>>> just need to understand the exact problem you're after. From looking a your >>>>> code (and the current implementation), if we succeed in setting the memory for >>>>> the sink but fail in any of the subsequent steps i.e, enabling the rest of the >>>>> compoment on the path or the source, the sink is left unchanged. >>>> >>>> Yes, thats right. And we should WARN (which I missed in this version) if >>>> there is a perf_data already for a disabled ETR. Please see my response to the >>>> next patch. >>> >>> The changelog for this patch states the following: "But with ETR, we >>> need to keep track of the buffer details and need to be cleaned up if >>> we fail." >>> >>> I did a deep dive in the code and in the current implementation if the >>> source fails to be enabled in etm_event_start() the path and the sink >>> remains unchanged. With your patchset this get fixed because a goto >>> was added to disable the path when such condition occur. As such each >>> component in the path will see its ->disable() callback invoked. In >>> tmc_disable_etr_sink(), drvdata->perf_data is set to NULL in >>> tmc_etr_disable_hw(), so the cleanup on error condition is done >>> properly. As such we wouldn't need a clean_buffer() callback. >> >> All of this is right. But we still have a case. e.g, if the ETR is >> enabled in sysfs mode, coresight_enable_path() will fail after we >> have set the buffer. And since we don't try to disable the path >> when we fail at SINK (which is the right thing to do, as we could >> be potentially disabling the ETR operated in sysfs mode), we leave >> the perf_data around. And the next session finds a non-empty data. > > We are in total agreement :o) > > All I hoping for is to see the sentence "But with ETR, we need to keep > track of the buffer details and need to be cleaned up if we fail." > removed from the changelog. To me that sounds like you're adding > cleanup work in enable() which isn't the case. On the flip side you > are making the sink configuration atomic and that is the important > part.
:-), I will update the change log accordingly.
> >> >>> >>> As I said I'm in favour of removing the set_buffer() callback but I >>> wouldn't associated it with ETR state cleanup. If the code can be >>> rearranged in a way that code can be removed then that alone is enough >>> to ju
>>>>>> + if (coresight_enable_path(path, CS_MODE_PERF, handle)) >>>>> >>>>> Here we already have a handle on "event_data". As such I think this is what we >>>>> should feed to coresight_enable_path() rather than the handle. That way we >>>>> don't need to call etm_perf_sink_config(), we just use the data. >>>> >>>> The advantage of passing on the handle is, we could get all the way upto the >>>> "perf_event" for the given session. Passing the event_data will loose that >>>> information. >>>> >>>> i.e, perf_event-> |perf_ouput_handle | -> |event_data | -> sink_config >>>> | <-event | | | >>>> >>>> The purpose of the wrapper "etm_perf_sink_config()" is to abstract the way we >>>> handle the information under the event_data. i.e, if we decide to make some >>>> changes in the way we store event_data, we need to spill the changes every >>>> where. But the perf_ouput_handle has much more stable ABI than event_data, >>>> hence the choice of passing handle. >>> >>> I agree that etm_perf_sink_config() has value but it should take a >>> void * as parameter (i.e what gets returned from perf_get_aux()) >>> rather than a perf_output_handle *. >> >> Ok, did you mean : >> >> sink_config = etm_perf_sink_config(perf_get_aux(handle)); ? >> >> Or do you want to change the parameter passed to the enable_sink() as >> well ? > > I've been thinking further on this and keeping things the way you've > implemented them is probably best at this time. I'm currently adding > support for PMU HW configuration and in the end we may need the > information conveyed by event->hw.drv_config [1]. When it's all said > and done we can clean things up if need be.
Ok.
Thanks for the review.
Cheers Suzuki
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