linux-kernel - Re: [PATCH RFC v2 8/8] selftests/perf: Add kselftest for remove_on

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Date:   Tue, 23 Mar 2021 16:58:37 +0100
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     alexander.shishkin@...ux.intel.com, acme@...nel.org,
        mingo@...hat.com, jolsa@...hat.com, mark.rutland@....com,
        namhyung@...nel.org, tglx@...utronix.de, glider@...gle.com,
        viro@...iv.linux.org.uk, arnd@...db.de, christian@...uner.io,
        dvyukov@...gle.com, jannh@...gle.com, axboe@...nel.dk,
        mascasa@...gle.com, pcc@...gle.com, irogers@...gle.com,
        kasan-dev@...glegroups.com, linux-arch@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC v2 8/8] selftests/perf: Add kselftest for
 remove_on_exec

On Tue, Mar 23, 2021 at 03:45PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 23, 2021 at 11:32:03AM +0100, Peter Zijlstra wrote:
> > And at that point there's very little value in still using
> > perf_event_exit_event()... let me see if there's something to be done
> > about that.
> 
> I ended up with something like the below. Which then simplifies
> remove_on_exec() to:
> 
[...]
> 
> Very lightly tested with that {1..1000} thing.
> 
> ---
> 
> Subject: perf: Rework perf_event_exit_event()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Tue Mar 23 15:16:06 CET 2021
> 
> Make perf_event_exit_event() more robust, such that we can use it from
> other contexts. Specifically the up and coming remove_on_exec.
> 
> For this to work we need to address a few issues. Remove_on_exec will
> not destroy the entire context, so we cannot rely on TASK_TOMBSTONE to
> disable event_function_call() and we thus have to use
> perf_remove_from_context().
> 
> When using perf_remove_from_context(), there's two races to consider.
> The first is against close(), where we can have concurrent tear-down
> of the event. The second is against child_list iteration, which should
> not find a half baked event.
> 
> To address this, teach perf_remove_from_context() to special case
> !ctx->is_active and about DETACH_CHILD.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Very nice, thanks! It seems to all hold up to testing as well.

Unless you already have this on some branch somewhere, I'll prepend it
to the series for now. I'll test some more and try to get v3 out
tomorrow.

Thanks,
-- Marco