Re: [RFC PATCH 09/17] perf: Use shmemfs pages for userspace-only per-thread detached events

[Peter Zijlstra]

On Tue, Sep 05, 2017 at 04:30:18PM +0300, Alexander Shishkin wrote:
> In order to work around the problem of using up mlocked memory for the
> detached events, we can pin the ring buffer pages only while they are
> in use (that is, the event is ACTIVE), and unpin them for the rest of
> the time. When not pinned in, these pages can be swapped out. This way,
> one user can have at most mlock_limit*nr_cpus kB of memory pinned at
> any given moment, however many events they actually have.
> 
> This enforces a constraint: pinning and unpinning may sleep and thus
> can't be done in the event scheduling path. Instead, we use a task
> work to do this, which this pattern us to userspace-only events.
> Also, since one userspace thread only needs one buffer (for whatever
> CPU it's running on at any given moment), we only do this for per-thread
> events.
> 
> The source for such swappable pages is shmemfs. This patch allows
> allocating perf ring buffer pages from an shmemfs file if the above
> constraints are met.

Right, so why still allow that previous icky thing? What cases do we
need that for?