netdev - Re: [PATCH net-next v5 1/4] bpf: add helper bpf_perf_event_read

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Date:   Wed, 20 Sep 2017 19:26:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yonghong Song <yhs@...com>
Cc:     rostedt@...dmis.org, ast@...com, daniel@...earbox.net,
        netdev@...r.kernel.org, kernel-team@...com,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next v5 1/4] bpf: add helper
 bpf_perf_event_read_value for perf event array map

On Tue, Sep 19, 2017 at 11:09:32PM -0700, Yonghong Song wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 3e691b7..2d5bbe5 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3684,10 +3684,12 @@ static inline u64 perf_event_count(struct perf_event *event)
>   *     will not be local and we cannot read them atomically
>   *   - must not have a pmu::count method
>   */
> -int perf_event_read_local(struct perf_event *event, u64 *value)
> +int perf_event_read_local(struct perf_event *event, u64 *value,
> +			  u64 *enabled, u64 *running)
>  {
>  	unsigned long flags;
>  	int ret = 0;
> +	u64 now;
>  
>  	/*
>  	 * Disabling interrupts avoids all counter scheduling (context
> @@ -3718,14 +3720,21 @@ int perf_event_read_local(struct perf_event *event, u64 *value)
>  		goto out;
>  	}
>  
> +	now = event->shadow_ctx_time + perf_clock();
> +	if (enabled)
> +		*enabled = now - event->tstamp_enabled;
>  	/*
>  	 * If the event is currently on this CPU, its either a per-task event,
>  	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
>  	 * oncpu == -1).
>  	 */
> -	if (event->oncpu == smp_processor_id())
> +	if (event->oncpu == smp_processor_id()) {
>  		event->pmu->read(event);
> -
> +		if (running)
> +			*running = now - event->tstamp_running;
> +	} else if (running) {
> +		*running = event->total_time_running;
> +	}
>  	*value = local64_read(&event->count);
>  out:
>  	local_irq_restore(flags);

Yeah, this looks about right.

Dave, could we have this in a topic tree of sorts, because I have a
pending series to rework all the timekeeping and it might be nice to not
have sfr run into all sorts of conflicts.