linux-kernel - Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software events

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Date:   Tue, 7 Sep 2021 18:59:43 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     bpf <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kjain@...ux.ibm.com" <kjain@...ux.ibm.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v5 bpf-next 1/3] perf: enable branch record for software
 events



> On Sep 3, 2021, at 9:50 AM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Sep 3, 2021, at 1:02 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> On Thu, Sep 02, 2021 at 09:57:04AM -0700, Song Liu wrote:
>>> +static int
>>> +intel_pmu_snapshot_branch_stack(struct perf_branch_entry *entries, unsigned int cnt)
>>> +{
>>> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>>> +
>>> +	intel_pmu_disable_all();
>>> +	intel_pmu_lbr_read();
>>> +	cnt = min_t(unsigned int, cnt, x86_pmu.lbr_nr);
>>> +
>>> +	memcpy(entries, cpuc->lbr_entries, sizeof(struct perf_branch_entry) * cnt);
>>> +	intel_pmu_enable_all(0);
>>> +	return cnt;
>>> +}
>> 
>> Would something like the below help get rid of that memcpy() ?
>> 
>> (compile tested only)
> 
> We can get rid of the memcpy. But we will need an extra "size" or "num_entries" 
> parameter for intel_pmu_lbr_read. I can add this change in the next version. 
> 

This is trickier than I thought. As current lbr_read() function works with 
perf_branch_stack, while the BPF helper side uses array of perf_branch_entry. 
And the array is passed into the helper by the BPF program. Therefore, to 
really get rid of the memcpy, we need to refactor the lbr driver code more. 
How about we keep the memcpy for now, and add the optimization later (if we 
think it is necessary)?

Thanks,
Song