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

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Date:   Tue, 3 Oct 2017 16:43:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        acme@...hat.com, kirill.shutemov@...ux.intel.com,
        Borislav Petkov <bp@...en8.de>, rric@...nel.org
Subject: Re: [RFC PATCH 09/17] perf: Use shmemfs pages for userspace-only
 per-thread detached events

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?