Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <CACT4Y+YPrXGw+AtESxAgPyZ84TYkNZdP0xpocX2jwVAbZD=-XQ@mail.gmail.com>
 <20201112103125.GV2628@hirez.programming.kicks-ass.net> <CACT4Y+ayRHua-6UyRwSM3=_oi+NkXbaO3-zZ1mpDmWonbybkeA@mail.gmail.com>
 <CACT4Y+bW1gpv8bz0vswaVUt-OB07oJ3NBeTi+vchAe8TTWK+mg@mail.gmail.com>
 <CACT4Y+ZsKXfAxrzJGQc5mJ+QiP5sAw7zKWtciS+07qZzSf33mw@mail.gmail.com>
 <CACT4Y+YeRtOTsMQ8xxZg-=nbv+yuJvYYhBErT46M8jtSHmiw6g@mail.gmail.com>
 <YBqXPmbpXf4hnlj3@hirez.programming.kicks-ass.net> <CACT4Y+a-9kqX0ZkNz-ygib+ERn41HVo_8Wx6oYMQmPjTC06j7g@mail.gmail.com>
 <YBqnAYVdNM4uyGny@hirez.programming.kicks-ass.net>
In-Reply-To: <YBqnAYVdNM4uyGny@hirez.programming.kicks-ass.net>
From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 4 Feb 2021 09:10:11 +0100
Message-ID: <CACT4Y+btOt5QFKH9Q=81EnpDHoidJUHE2s0oZ8v65t-b__awuw@mail.gmail.com>
Subject: Re: Process-wide watchpoints
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Will Deacon <will@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Matt Morehouse <mascasa@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Feb 3, 2021 at 2:37 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Feb 03, 2021 at 01:49:56PM +0100, Dmitry Vyukov wrote:
> > On Wed, Feb 3, 2021 at 1:29 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, Feb 01, 2021 at 09:50:20AM +0100, Dmitry Vyukov wrote:
> > > > Or, alternatively would it be reasonable for perf to generate SIGTRAP
> > > > directly on watchpoint hit (like ptrace does)? That's what I am
> > > > ultimately trying to do by attaching a bpf program.
> > >
> > > Perf should be able to generate signals, The perf_event_open manpage
> > > lists two ways of trigering signals. The second way doesn't work for
> > > you, due to it not working on inherited counters, but would the first
> > > work?
> > >
> > > That is, set attr::wakeup_events and fcntl(F_SETSIG).
> >
> > The problem is that this sends a signal to the fd owner rather than
> > the thread that hit the breakpoint. At least that's what happened in
> > our tests. We would like to send a signal to the thread that hit the
> > breakpoint.
>
> Ah indeed.. all of this was aimed at self-monitoring.
>
> Letting perf send a signal to the monitored task is intrusive.. let me
> think on that.

I was thinking of something very similar to that bpf_send_signal that
delays sending to exit from irq:
https://elixir.bootlin.com/linux/latest/source/kernel/trace/bpf_trace.c#L1091