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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Sun, 31 Jan 2021 11:28:32 +0100
Message-ID: <CACT4Y+ZsKXfAxrzJGQc5mJ+QiP5sAw7zKWtciS+07qZzSf33mw@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>
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On Sun, Jan 31, 2021 at 11:04 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, Nov 12, 2020 at 11:43 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> > > > for sampling race detection),
> > > > number of threads in the process can be up to, say, ~~10K and the
> > > > watchpoint is intended to be set for a very brief period of time
> > > > (~~few ms).
> > >
> > > Performance is a consideration here, doing lots of IPIs in such a short
> > > window, on potentially large machines is a DoS risk.
> > >
> > > > This can be done today with both perf_event_open and ptrace.
> > > > However, the problem is that both APIs work on a single thread level
> > > > (? perf_event_open can be inherited by children, but not for existing
> > > > siblings). So doing this would require iterating over, say, 10K
> > >
> > > One way would be to create the event before the process starts spawning
> > > threads and keeping it disabled. Then every thread will inherit it, but
> > > it'll be inactive.
> > >
> > > > I see at least one potential problem: what do we do if some sibling
> > > > thread already has all 4 watchpoints consumed?
> > >
> > > That would be immediately avoided by this, since it will have the
> > > watchpoint reserved per inheriting the event.
> > >
> > > Then you can do ioctl(PERF_EVENT_IOC_{MODIFY_ATTRIBUTES,ENABLE,DISABLE})
> > > to update the watch location and enable/disable it. This _will_ indeed
> > > result in a shitload of IPIs if the threads are active, but it should
> > > work.
> >
> > Aha! That's the possibility I missed.
> > We will try to prototype this and get back with more questions if/when
> > we have them.
> > Thanks!
>
> Hi Peter,
>
> I've tested this approach and it works, but only in half.
> PERF_EVENT_IOC_{ENABLE,DISABLE} work as advertised.
> However, PERF_EVENT_IOC_MODIFY_ATTRIBUTES does not work for inherited
> child events.
> Does something like this make any sense to you? Are you willing to
> accept such change?
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 55d18791a72d..f6974807a32c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3174,7 +3174,7 @@ int perf_event_refresh(struct perf_event *event,
> int refresh)
>  }
>  EXPORT_SYMBOL_GPL(perf_event_refresh);
>
> -static int perf_event_modify_breakpoint(struct perf_event *bp,
> +static int _perf_event_modify_breakpoint(struct perf_event *bp,
>                                          struct perf_event_attr *attr)
>  {
>         int err;
> @@ -3189,6 +3189,28 @@ static int perf_event_modify_breakpoint(struct
> perf_event *bp,
>         return err;
>  }
>
> +static int perf_event_modify_breakpoint(struct perf_event *bp,
> +                                       struct perf_event_attr *attr)
> +{
> +       struct perf_event *child;
> +       int err;
> +
> +       WARN_ON_ONCE(bp->ctx->parent_ctx);
> +
> +       mutex_lock(&bp->child_mutex);
> +       err = _perf_event_modify_breakpoint(bp, attr);
> +       if (err)
> +               goto unlock;
> +       list_for_each_entry(child, &bp->child_list, child_list) {
> +               err = _perf_event_modify_breakpoint(child, attr);
> +               if (err)
> +                       goto unlock;
> +       }
> +unlock:
> +       mutex_unlock(&bp->child_mutex);
> +       return err;
> +}
> +
>  static int perf_event_modify_attr(struct perf_event *event,
>                                   struct perf_event_attr *attr)


Not directly related to the above question, but related to my use case.
Could we extend bpf_perf_event_data with some more data re breakpoint events?

struct bpf_perf_event_data {
    bpf_user_pt_regs_t regs;
    __u64 sample_period;
    __u64 addr;
};

Ideally, I would like to have an actual access address, size and
read/write type (may not match bp addr/size). Is that info easily
available at the point of bpf hook call?
Or, if that's not available at least breakpoint bp_type/bp_size.

Is it correct that we can materialize in bpf_perf_event_data anything
that's available in bpf_perf_event_data_kern (if it makes sense in the
public interface of course)?

struct bpf_perf_event_data_kern {
    bpf_user_pt_regs_t *regs;
    struct perf_sample_data *data;
    struct perf_event *event;
};

Unfortunately I don't see perf_event_attr.bp_type/bp_size
stored/accessible anywhere in bpf_perf_event_data_kern. What would be
the right way to expose them in bpf_perf_event_data?

Thanks