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Date:   Tue, 11 Aug 2020 07:34:24 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc:     peterz@infradead.org, Arnaldo Carvalho de Melo <acme@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Jiri Olsa <jolsa@kernel.org>, alexey.budankov@linux.intel.com,
        adrian.hunter@intel.com
Subject: Re: [PATCH 1/2] perf: Add closing sibling events' file descriptors
Message-ID: <20200811143424.GD1448395@tassilo.jf.intel.com>
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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