netdev - Re: [PATCH v6 3/4] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter

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Date:	Wed, 5 Aug 2015 12:04:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kaixu Xia <xiakaixu@...wei.com>
Cc:	ast@...mgrid.com, davem@...emloft.net, acme@...nel.org,
	mingo@...hat.com, masami.hiramatsu.pt@...achi.com,
	jolsa@...nel.org, daniel@...earbox.net, wangnan0@...wei.com,
	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH v6 3/4] bpf: Implement function bpf_perf_event_read()
 that get the selected hardware PMU conuter

On Tue, Aug 04, 2015 at 08:58:15AM +0000, Kaixu Xia wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 6251b53..726ca1b 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8599,6 +8599,25 @@ struct perf_event_attr *perf_event_attrs(struct perf_event *event)
>  	return &event->attr;
>  }
>  
> +u64 perf_event_read_internal(struct perf_event *event)

Maybe: perf_event_read_local(), as this is this function only works for
events active on the current CPU.

> +{
> +	if (!event)
> +		return -EINVAL;
> +
> +	if (unlikely(event->state != PERF_EVENT_STATE_ACTIVE))
> +		return -EINVAL;

You can return perf_event_count() in that case.

> +
> +	if (event->oncpu != raw_smp_processor_id() &&

That _must_ be smp_processor_id(). If that gives a warning (ie.
preemption is not disabled or we're not affine to this one cpu) then the
warning is valid.

> +	    event->ctx->task != current)

Write it like:

	if (event->ctx->task != current &&
	    event->oncpu != smp_processor_id())

That way you'll not evaluate smp_processor_id() for current task events.

> +		return -EINVAL;
> +
> +	if (unlikely(event->attr.inherit))
> +		return -EINVAL;

This should be in your accept function, inherited events should never
get this far.

You need IRQs disabled while calling __perf_event_read(), did you test
with lockdep enabled?

> +	__perf_event_read(event);
> +	return perf_event_count(event);
> +}

Also, you probably want a WARN_ON(in_nmi()) there, this function is
_NOT_ NMI safe.


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