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Date:   Wed, 9 Jan 2019 11:18:08 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     lkml <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "acme@kernel.org" <acme@kernel.org>,
        "ast@kernel.org" <ast@kernel.org>,
        "daniel@iogearbox.net" <daniel@iogearbox.net>,
        Kernel Team <Kernel-team@fb.com>,
        Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH v5 perf, bpf-next 3/7] perf, bpf: introduce
 PERF_RECORD_BPF_EVENT
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On Tue, Jan 08, 2019 at 11:54:04PM +0000, Song Liu wrote:

> I think Intel PT case is at instruction granularity (instead of ksymbol
> granularity)? 

Yes.

> If this is true, modules, BPF, and PT could still share
> the ksymbol record for basic profiling. And advanced use cases like 
> annotation will depend on user space to record BPF_EVENT (and equivalent
> for other cases) timely. But at least, the ksymbol is already there. 
> 
> Does this make sense?  

I'm not sure I follow; the idea was that on ksym events we copy out the
instructions using kcore. The ksym event already has addr+len.

All we need is some means of ensuring the symbol is still there by the
time we see the event and do the copy.

I think we can do this with a new ioctl() on /proc/kcore itself:

 - when we have kcore open, we queue all text-free operations on list-1.

 - when we close kcore, we drain all (text-free) list-* and perform the
   pending frees immediately.

 - on ioctl(KCORE_QC) we perform the pending free of list-3 and advance
   list-2 to list-3 and list-1 to list-2.

Perf would then open kcore at the start of the record, make a complete
copy and keep the FD open. At the end of every buffer process, we issue
KCORE_QC IFF we observed a ksym unreg in that buffer.

We use 3 lists instead of 2 to guard against races, if there was a
reg+unreg en-route but not yet visible in the buffer, then we don't want
that free to be processed. The next buffer (read) will have the event(s)
and all should be well.