Messages in this thread | | | Date | Wed, 22 Mar 2023 15:17:00 -0400 | Subject | Re: [PATCH] perf/core: Fix the same task check in perf_event_set_output | From | "Liang, Kan" <> |
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On 2023-03-22 10:15 a.m., Adrian Hunter wrote: > On 22/03/23 15:42, Peter Zijlstra wrote: >> On Wed, Mar 22, 2023 at 12:59:28PM +0200, Adrian Hunter wrote: >>> On 11/07/22 21:07, kan.liang@linux.intel.com wrote: >>>> From: Kan Liang <kan.liang@linux.intel.com> >>>> >>>> With the --per-thread option, perf record errors out when sampling with >>>> a hardware event and a software event as below. >>>> >>>> $ perf record -e cycles,dummy --per-thread ls >>>> failed to mmap with 22 (Invalid argument) >>>> >>>> The same task is sampled with the two events. The IOC_OUTPUT is invoked >>>> to share the mmap memory of the task between the events. In the >>>> perf_event_set_output(), the event->ctx is used to check whether the >>>> two events are attached to the same task. However, a hardware event and >>>> a software event are from different task context. The check always >>>> fails. >>>> >>>> The task struct is stored in the event->hw.target for each per-thread >>>> event. It can be used to determine whether two events are attached to >>>> the same task. >>>> >>>> The patch can also fix another issue reported months ago. >>>> https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/ >>>> The event->ctx is not ready when the perf_event_set_output() is invoked >>>> in the perf_event_open(), while the event->hw.target has been assigned >>>> at the moment. >>>> >>>> The problem should be a long time issue since commit c3f00c70276d >>>> ("perf: Separate find_get_context() from event initialization"). The >>>> event->hw.target doesn't exist at that time. Here, the patch which >>>> introduces the event->hw.target is used by the Fixes tag. >>>> >>>> The problem should still exists between the broken patch and the >>>> event->hw.target patch. This patch does not intend to fix that case. >>>> >>>> Fixes: 50f16a8bf9d7 ("perf: Remove type specific target pointers") >>>> Reviewed-by: Zhengjun Xing <zhengjun.xing@linux.intel.com> >>>> Signed-off-by: Kan Liang <kan.liang@linux.intel.com> >>> >>> Did this slip through the cracks, or is there more complexity >>> to this case than just sharing the rb? >> >> Both; I very much missed it, but looking at it now, I'm not at all sure >> it is correct prior to the whole context rewrite we did recently. >> >> So after the rewrite every cpu/task only has a single >> perf_event_context, and your change below is actually an equivalence. >>
Right, they are equivalent after the rewrite. I cannot reproduce the "failed to mmap with 22 (Invalid argument)" issue with 6.2 and later anymore.
But the issue reported by Nadav* can still be reproduced with the latest 6.3-rc.* https://lore.kernel.org/all/92645262-D319-4068-9C44-2409EF44888E@gmail.com/ Because the event->ctx is still not set when the perf_event_set_output() is invoked in the perf_event_open(). The rewrite doesn't change with it.
>> But prior to that a task could have multiple contexts. Now they got >> co-scheduled most of the times and it will probably work, but I'm not >> entirely sure. >> >> So how about we change the Fixes tag to something like: >> >> Fixes: c3f00c70276d ("perf: Separate find_get_context() from event initialization") # >= v6.2 >>
Not sure if we want the # >= v6.2.
For v6.2 and later, the patch can fixes Nadav's issue.
For v6.1 and earlier, the patch fixes both Nadav's issue and the issue described in the description.
I think I can send a V2 patch to make the history clear in the description.
Thanks, Kan
>> And anybody that wants to back-port this further gets to either do the >> full audit and/or keep the pieces. >> >> Hmm? > > Seems reasonable to me. Kan? > >> >>>> --- >>>> kernel/events/core.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/kernel/events/core.c b/kernel/events/core.c >>>> index b4d62210c3e5..22df79d3f19d 100644 >>>> --- a/kernel/events/core.c >>>> +++ b/kernel/events/core.c >>>> @@ -12080,7 +12080,7 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event) >>>> /* >>>> * If its not a per-cpu rb, it must be the same task. >>>> */ >>>> - if (output_event->cpu == -1 && output_event->ctx != event->ctx) >>>> + if (output_event->cpu == -1 && output_event->hw.target != event->hw.target) >>>> goto out; >>>> >>>> /* >>> >
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