From: Dmitry Vyukov <dvyukov@google.com>
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>
Subject: Re: Process-wide watchpoints
Date: Thu, 12 Nov 2020 11:43:53 +0100 [thread overview]
Message-ID: <CACT4Y+ayRHua-6UyRwSM3=_oi+NkXbaO3-zZ1mpDmWonbybkeA@mail.gmail.com> (raw)
In-Reply-To: <20201112103125.GV2628@hirez.programming.kicks-ass.net>
On Thu, Nov 12, 2020 at 11:31 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Nov 12, 2020 at 08:46:23AM +0100, Dmitry Vyukov 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!
next prev parent reply other threads:[~2020-11-12 10:44 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-12 7:46 Process-wide watchpoints Dmitry Vyukov
2020-11-12 10:31 ` Peter Zijlstra
2020-11-12 10:43 ` Dmitry Vyukov [this message]
[not found] ` <CACT4Y+bW1gpv8bz0vswaVUt-OB07oJ3NBeTi+vchAe8TTWK+mg@mail.gmail.com>
[not found] ` <CACT4Y+ZsKXfAxrzJGQc5mJ+QiP5sAw7zKWtciS+07qZzSf33mw@mail.gmail.com>
2021-02-01 8:50 ` Dmitry Vyukov
2021-02-03 12:29 ` Peter Zijlstra
2021-02-03 12:49 ` Dmitry Vyukov
2021-02-03 12:50 ` Dmitry Vyukov
2021-02-03 13:37 ` Peter Zijlstra
2021-02-04 8:10 ` Dmitry Vyukov
2021-02-04 9:38 ` Peter Zijlstra
2021-02-04 9:54 ` Dmitry Vyukov
2021-02-04 12:09 ` Peter Zijlstra
2021-02-04 12:53 ` Dmitry Vyukov
2021-02-04 13:10 ` Peter Zijlstra
2021-02-04 13:35 ` Dmitry Vyukov
2021-02-04 13:45 ` Peter Zijlstra
2021-02-04 14:59 ` Peter Zijlstra
2021-02-04 13:33 ` Peter Zijlstra
2021-02-04 13:37 ` Dmitry Vyukov
2021-02-04 13:06 ` Peter Zijlstra
2021-02-03 5:38 ` Namhyung Kim
2021-02-04 8:10 ` Dmitry Vyukov
2021-02-03 12:22 ` Peter Zijlstra
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