Re: [PATCH 1/2] perf: Add closing sibling events' file descriptors

[Andi Kleen <ak@linux.intel.com>]

On Tue, Aug 11, 2020 at 12:47:24PM +0300, Alexander Shishkin wrote:
> Andi Kleen <ak@linux.intel.com> writes:
> 
> >> It didn't. I can't figure out what to charge on the locked memory, as
> >> all that memory is in kernel-side objects. It also needs to make sense
> >
> > I don't see how that makes a difference for the count. It just account
> > bytes. Can you elaborate?
> 
> Right, but which bytes? One byte per event? That's
> arbitrary. sizeof(struct perf_event)? Then, probably also sizeof(struct
> perf_event_context).

Yes the sum of all the sizeofs needed for a perf_event.

> 
> >> as iirc the default MLOCK_LIMIT is quite low, you'd hit it sooner than
> >> the file descriptor limit.
> >
> > For a single process? 
> 
> The above two structs add up to 2288 bytes on my local build. Given the
> default RLIMIT_MEMLOCK of 64k, that's 28 events. As opposed to ~1k
> events if we keep using the RLIMIT_NOFILE. Unless I'm missing your
> point.

Yes that's true. We would probably need to increase the limit to a few
MB at least.

Or maybe use some combination with the old rlimit for compatibility.
The old rlimit would give an implicit extra RLIMIT_NFILE * 2288 limit
for RLIMIT_MEMLOCK. This would only give full compatibility for a single
perf process, but I suspect that's good enough for most users.

-Andi