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

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Message-Id: <BWENHQJIN885.216UOYEIWNGFU@dlxu-fedora-R90QNFJV>
Date:   Tue, 20 Aug 2019 10:58:47 -0700
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Peter Zijlstra" <peterz@...radead.org>
Cc:     <bpf@...r.kernel.org>, <songliubraving@...com>, <yhs@...com>,
        <andriin@...com>, <mingo@...hat.com>, <acme@...nel.org>,
        <ast@...com>, <alexander.shishkin@...ux.intel.com>,
        <jolsa@...hat.com>, <namhyung@...nel.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl

Hi Peter,

On Tue Aug 20, 2019 at 4:45 PM Peter Zijlstra wrote:
> On Fri, Aug 16, 2019 at 03:31:46PM -0700, Daniel Xu wrote:
> > It's useful to know [uk]probe's nmissed and nhit stats. For example with
> > tracing tools, it's important to know when events may have been lost.
> > debugfs currently exposes a control file to get this information, but
> > it is not compatible with probes registered with the perf API.
> 
> What is this nmissed and nhit stuff?

nmissed is the number of times the probe's handler should have been run
but didn't. nhit is the number of times the probes handler has run. I've
documented this information in the uapi header. If you'd like, I can put
it in the commit message too.

Daniel