linux-kernel - Re: [PATCH v12] perf: Sharing PMU counters across compatible events

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Date:   Wed, 29 Apr 2020 20:44:54 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>, Tejun Heo <tj@...nel.org>,
        "kernel test robot" <rong.a.chen@...el.com>
Subject: Re: [PATCH v12] perf: Sharing PMU counters across compatible events



> On Apr 20, 2020, at 4:04 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> static inline u64 perf_event_count(struct perf_event *event)
>> {
>> -	return local64_read(&event->count) + atomic64_read(&event->child_count);
>> +	u64 count;
>> +
>> +	if (likely(event->dup_master != event))
>> +		count = local64_read(&event->count);
>> +	else
>> +		count = local64_read(&event->master_count);
>> +
>> +	return count + atomic64_read(&event->child_count);
>> }
> 
> So lsat time I said something about SMP ordering here. Where did that
> go?

I am not quite sure about this one. I guess the following is sufficient. 
Could you please help me double check? 

Thanks,
Song


static inline u64 perf_event_count(struct perf_event *event)
{
        struct perf_event *master;
        u64 count;

again:
        master = READ_ONCE(event->dup_master);
        if (master == EVENT_TOMBSTONE)
                goto again;

        if (likely(master != event))
                count = local64_read(&event->count);
        else
                count = local64_read(&event->master_count);

        return count + atomic64_read(&event->child_count);
}