linux-kernel - Re: Process-wide watchpoints

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Message-ID: <YBqnAYVdNM4uyGny@hirez.programming.kicks-ass.net>
Date:   Wed, 3 Feb 2021 14:37:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Matt Morehouse <mascasa@...gle.com>
Subject: Re: Process-wide watchpoints

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@...radead.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.