linux-kernel - Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY

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Date:   Wed, 21 Aug 2019 17:04:43 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Peter Zijlstra <peterz@...radead.org>, Daniel Xu <dxu@...uu.xyz>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Alexei Starovoitov <ast@...com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl

Em Wed, Aug 21, 2019 at 06:43:49PM +0000, Yonghong Song escreveu:
> On 8/21/19 11:31 AM, Peter Zijlstra wrote:
> > On Wed, Aug 21, 2019 at 04:54:47PM +0000, Yonghong Song wrote:
> >> A lot of bpf-based tracing programs uses maps to communicate and
> >> do not allocate ring buffer at all.
> > 
> > So extending PERF_RECORD_LOST doesn't work. But PERF_FORMAT_LOST might
> > still work fine; but you get to implement it for all software events.
> 
> Could you give more specifics about PERF_FORMAT_LOST? Googling 
> "PERF_FORMAT_LOST" only yields two emails which we are discussing here :-(

Perhaps he's talking about using read(perf_event_fd, ...) after having set it
up with perf_event_attr.read_format with the-to-be-implemented
PERF_FORMAT_LOST bit?

Look at perf_read() and perf_read_one() in kernel/events/core.c.
 
- Arnaldo