linux-kernel - Re: [RFC PATCH 1/6] perf: Move mlock accounting to ring buffer allocation

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Date:   Mon, 26 Sep 2016 15:39:49 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        tglx@...utronix.de
Subject: Re: [RFC PATCH 1/6] perf: Move mlock accounting to ring buffer allocation

Peter Zijlstra <peterz@...radead.org> writes:

> On Mon, Sep 26, 2016 at 11:27:08AM +0300, Alexander Shishkin wrote:
>> Peter Zijlstra <peterz@...radead.org> writes:
>> > At which point we _should_ start failing fork(), which is a somewhat
>> > unexpected, and undesirable side-effect.
>> 
>> I'm not sure I see why we should fail fork() when we run out of pinned
>> memory.
>
> Well, we cannot fully honour the inherit, what other option do we have?
> Silently malfunctioning? That's far worse.

We can still put a note there saying that we tried. The user will know
to adjust their buffer size requirement or the RLIMIT_MEMLOCK.

>> > Ideally we'd unpin the old buffers and repin the new buffers on context
>> > switch, but that's impossible since faulting needs scheduling,
>> > recursion, we loose.
>> 
>> Or we can have per-cpu buffers for all user's tasks, record where each
>> task starts and ends in each buffer and cut out only bits relevant to
>> the task(s) that dump core.
>
> Which gets you the problem that when a task dumps core there might not
> be any state in the buffer, because the previous task flushed it all out
> :/

Well, there's going to be at list something that leads up to the core
dump if this task is the last one to schedule in for this buffer. It's a
bit more gambling, though.

Regards,
--
Alex