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

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Message-ID: <D501C4AD-3778-431B-A710-3399BFE6EE56@fb.com>
Date:   Fri, 3 Sep 2021 16:50:34 +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 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. 

Thanks,
Song