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

[Peter Zijlstra]

On Mon, Sep 26, 2016 at 11:27:08AM +0300, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > Well, we could 'force' inject a VMA into the process's address space, we
> > do that for a few other things as well. It also makes for less
> > exceptions with the actual core dumping.
> 
> Threads then will end up with the same buffer (through sharing the mm),
> but they can't really share trace buffers.
> 
> Also, system core dump is still a problem.

Hurm, true on both counts.

> > But the worry I have is the total amount of pinned memory. If you want
> > to inherit this on fork(), as is a reasonable expectation, then its
> > possible to quickly exceed the total amount of pinnable memory.
> >
> > 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.

> > 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
:/