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

[Alexander Shishkin]

Andi Kleen <ak@xxxxxxxxxxxxxxx> writes:

> On Tue, Aug 11, 2020 at 12:47:24PM +0300, Alexander Shishkin wrote:
>> 
>> 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.

Well, *all* of them will be tedious to collect, seeing as there is
ctx->task_ctx_data, there is ring_buffer, scheduling trees, there is
stuff that pmus allocate under the hood, like AUX SG tables.

>> 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.

Ok, but if we have to increase a limit anyway, we might as well increase
the NOFILE.

> 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.

We'd need to settle on charging a fixed set of structures per event,
then. And, without increasing the file limit, this would still total at
1052 events.

We could also involve perf_event_mlock_kb *and* increase it too, but I
suspect distros don't just leave it at kernel's default either.

Regards,
--
Alex