Messages in this thread | | | Date | Fri, 30 Oct 2020 22:56:09 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf() |
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Hi Mathieu,
On 2020-10-30 22:18, Mathieu Poirier wrote: > On Fri, Oct 30, 2020 at 01:29:56PM +0530, Sai Prakash Ranjan wrote: >> Hello guys, >> >> On 2020-10-24 02:07, Mathieu Poirier wrote: >> > On Fri, Oct 23, 2020 at 03:44:16PM +0200, Peter Zijlstra wrote: >> > > On Fri, Oct 23, 2020 at 02:29:54PM +0100, Suzuki Poulose wrote: >> > > > On 10/23/20 2:16 PM, Peter Zijlstra wrote: >> > > > > On Fri, Oct 23, 2020 at 01:56:47PM +0100, Suzuki Poulose wrote: >> > > >> > > > > > That way another session could use the same sink if it is free. i.e >> > > > > > >> > > > > > perf record -e cs_etm/@sink0/u --per-thread app1 >> > > > > > >> > > > > > and >> > > > > > >> > > > > > perf record -e cs_etm/@sink0/u --per-thread app2 >> > > > > > >> > > > > > both can work as long as the sink is not used by the other session. >> > > > > >> > > > > Like said above, if sink is shared between CPUs, that's going to be a >> > > > > trainwreck :/ Why do you want that? >> > > > >> > > > That ship has sailed. That is how the current generation of systems are, >> > > > unfortunately. But as I said, this is changing and there are guidelines >> > > > in place to avoid these kind of topologies. With the future >> > > > technologies, this will be completely gone. >> > > >> > > I understand that the hardware is like that, but why do you want to >> > > support this insanity in software? >> > > >> > > If you only allow a single sink user (group) at the same time, your >> > > problem goes away. Simply disallow the above scenario, do not allow >> > > concurrent sink users if sinks are shared like this. >> > > >> > > Have the perf-record of app2 above fail because the sink is in-user >> > > already. >> > >> > I agree with you that --per-thread scenarios are easy to deal with, but >> > to >> > support cpu-wide scenarios events must share a sink (because there is >> > one event >> > per CPU). CPU-wide support can't be removed because it has been around >> > for close to a couple of years and heavily used. I also think using the >> > pid of >> > the process that created the events, i.e perf, is a good idea. We just >> > need to >> > agree on how to gain access to it. >> > >> > In Sai's patch you objected to the following: >> > >> > > + struct task_struct *task = READ_ONCE(event->owner); >> > > + >> > > + if (!task || is_kernel_event(event)) >> > >> > Would it be better to use task_nr_pid(current) instead of event->owner? >> > The end >> > result will be exactly the same. There is also no need to check the >> > validity of >> > @current since it is a user process. >> > >> >> We have devices deployed where these crashes are seen consistently, >> so for some immediate relief, could we atleast get some fix in this >> cycle without major design overhaul which would likely take more time. >> Perhaps my first patch [1] without any check for owner or >> I can post a new version as Suzuki suggested [2] dropping the export >> of is_kernel_event(). Then we can always work on top of it based on >> the >> conclusion of this discussion, we will atleast not have the systems >> crash in the meantime, thoughts? > > For the time being I think [1], exactly the way it is, is a reasonable > way > forward. >
Sure, I just checked now and [1] still applies neatly on top of coresight next branch.
[1] https://lore.kernel.org/patchwork/patch/1318098/
Thanks, Sai
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