linux-kernel - Re: [PATCH bpf-next 1/3] perf: enable branch record for software events

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Date:   Thu, 26 Aug 2021 13:26:08 +0530
From:   kajoljain <kjain@...ux.ibm.com>
To:     Song Liu <songliubraving@...com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "open list:BPF (Safe dynamic programs and tools)" 
        <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/3] perf: enable branch record for software
 events



On 8/25/21 8:52 PM, Song Liu wrote:
> 
> 
>> On Aug 25, 2021, at 5:09 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Mon, Aug 23, 2021 at 11:01:55PM -0700, Song Liu wrote:
>>
>>> arch/x86/events/intel/core.c |  5 ++++-
>>> arch/x86/events/intel/lbr.c  | 12 ++++++++++++
>>> arch/x86/events/perf_event.h |  2 ++
>>> include/linux/perf_event.h   | 33 +++++++++++++++++++++++++++++++++
>>> kernel/events/core.c         | 28 ++++++++++++++++++++++++++++
>>> 5 files changed, 79 insertions(+), 1 deletion(-)
>>
>> No PowerPC support :/
> 
> I don't have PowerPC system for testing at the moment. I guess we can decide
> the overall framework now, and ask PowerPC folks' help on PowerPC support
> later? 

Hi Song,
   I will look at powerpc side to enable this.

Thanks,
Kajol Jain

> 
>>
>>> +void intel_pmu_snapshot_branch_stack(void)
>>> +{
>>> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>> +
>>> +	intel_pmu_lbr_disable_all();
>>> +	intel_pmu_lbr_read();
>>> +	memcpy(this_cpu_ptr(&perf_branch_snapshot_entries), cpuc->lbr_entries,
>>> +	       sizeof(struct perf_branch_entry) * x86_pmu.lbr_nr);
>>> +	*this_cpu_ptr(&perf_branch_snapshot_size) = x86_pmu.lbr_nr;
>>> +	intel_pmu_lbr_enable_all(false);
>>> +}
>>
>> Still has the layering violation and issues vs PMI.
> 
> Yes, this is the biggest change after I test with this more. I tested with 
> perf_[disable|enable]_pmu(), and function pointer in "struct pmu". However,
> all these logic consumes LBR entries. In one of the version, 22 out of the
> 32 LBR entries are branches after the fexit event. Most of them are from
> perf_disable_pmu(). And each function pointer consumes 1 or 2 entries. 
> This would be worse for systems with fewer LBR entries. 
> 
> On the other hand, I think current version was not too bad. It may corrupt
> some samples when there is collision between this and PMI. But it should not
> cause serious issues. Did I miss anything more serious? 
> 
>>
>>> +#ifdef CONFIG_HAVE_STATIC_CALL
>>> +DECLARE_STATIC_CALL(perf_snapshot_branch_stack,
>>> +		    perf_default_snapshot_branch_stack);
>>> +#else
>>> +extern void (*perf_snapshot_branch_stack)(void);
>>> +#endif
>>
>> That's weird, static call should work unconditionally, and fall back to
>> a regular function pointer exactly like you do here. Search for:
>> "Generic Implementation" in include/linux/static_call.h
> 
> Thanks for the pointer. Let me look into it. 
>>
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index 011cc5069b7ba..b42cc20451709 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>
>>> +#ifdef CONFIG_HAVE_STATIC_CALL
>>> +DEFINE_STATIC_CALL(perf_snapshot_branch_stack,
>>> +		   perf_default_snapshot_branch_stack);
>>> +#else
>>> +void (*perf_snapshot_branch_stack)(void) = perf_default_snapshot_branch_stack;
>>> +#endif
>>
>> Idem.
>>
>> Something like:
>>
>> DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack, void (*)(void));
>>
>> with usage like: static_call_cond(perf_snapshot_branch_stack)();
>>
>> Should unconditionally work.
>>
>>> +int perf_read_branch_snapshot(void *buf, size_t len)
>>> +{
>>> +	int cnt;
>>> +
>>> +	memcpy(buf, *this_cpu_ptr(&perf_branch_snapshot_entries),
>>> +	       min_t(u32, (u32)len,
>>> +		     sizeof(struct perf_branch_entry) * MAX_BRANCH_SNAPSHOT));
>>> +	cnt =  *this_cpu_ptr(&perf_branch_snapshot_size);
>>> +
>>> +	return (cnt > 0) ? cnt : -EOPNOTSUPP;
>>> +}
>>
>> Doesn't seem used at all..
> 
> At the moment, we only use this from BPF side (see 2/3). We sure can use it
> from perf side, but that would require discussions on the user interface. 
> How about we have that discussion later? 
> 
> Thanks,
> Song
>