linux-kernel - [RFC PATCH 0/2] percpu_counter: Enable switching to global counter

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Date:	Fri,  4 Mar 2016 21:51:37 -0500
From:	Waiman Long <Waiman.Long@....com>
To:	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Dave Chinner <dchinner@...hat.com>
Cc:	xfs@....sgi.com, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Waiman Long <Waiman.Long@....com>
Subject: [RFC PATCH 0/2] percpu_counter: Enable switching to global counter

This patchset allows the degeneration of per-cpu counters back to
global counters when:

 1) The number of CPUs in the system is large, hence a high cost for
    calling percpu_counter_sum().
 2) The initial count value is small so that it has a high chance of
    excessive percpu_counter_sum() calls.

When the above 2 conditions are true, this patchset allows the user of
per-cpu counters to selectively degenerate them into global counters
with lock. This is done by calling the new percpu_counter_set_limit()
API after percpu_counter_set(). Without this call, there is no change
in the behavior of the per-cpu counters.

Patch 1 implements the new percpu_counter_set_limit() API.

Patch 2 modifies XFS to call the new API for the m_ifree and m_fdblocks
per-cpu counters.

Waiman Long (2):
  percpu_counter: Allow falling back to global counter on large system
  xfs: Allow degeneration of m_fdblocks/m_ifree to global counters

 fs/xfs/xfs_mount.c             |    1 -
 fs/xfs/xfs_mount.h             |    5 +++
 fs/xfs/xfs_super.c             |    6 +++
 include/linux/percpu_counter.h |   10 +++++
 lib/percpu_counter.c           |   72 +++++++++++++++++++++++++++++++++++++++-
 5 files changed, 92 insertions(+), 2 deletions(-)