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Date: Tue, 30 May 2017 21:03:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <ast@fb.com>
Cc: "David S . Miller" <davem@davemloft.net>,
        Brendan Gregg <bgregg@netflix.com>,
        Daniel Borkmann <daniel@iogearbox.net>, Teng Qin <qinteng@fb.com>,
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Subject: Re: [PATCH v2 net-next 1/3] perf, bpf: Add BPF support to all
 perf_event types
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On Tue, May 30, 2017 at 10:37:46AM -0700, Alexei Starovoitov wrote:
> On 5/30/17 9:51 AM, Peter Zijlstra wrote:

> > I'm not entirely sure I see how that is required. Should per task not
> > already work? The WARN that's there will only trigger if you call them
> > on the wrong task, which is something you shouldn't do anyway.
> 
> The kernel WARN is considered to be a bug of bpf infra. That's the
> reason we do all these checks at map update time and at run-time.
> The bpf program authors should be able to do all possible experiments
> until their scripts work. Dealing with kernel warns and reboots is not
> something user space folks like to do.
> Today bpf_perf_event_read() for per-task events isn't really
> working due to event->oncpu != cpu runtime check in there.
> If we convert warns to returns the existing scripts will continue
> to work as-is and per-task will be possible.

Ah yes.. I always forget that side of things (not ever having seen a
bpf thing working doesn't help either of course).

Let me consider that for a bit..