linux/include
Mel Gorman cc9a6c8776 cpuset: mm: reduce large amounts of memory barrier related damage v3
Commit c0ff7453bb ("cpuset,mm: fix no node to alloc memory when
changing cpuset's mems") wins a super prize for the largest number of
memory barriers entered into fast paths for one commit.

[get|put]_mems_allowed is incredibly heavy with pairs of full memory
barriers inserted into a number of hot paths.  This was detected while
investigating at large page allocator slowdown introduced some time
after 2.6.32.  The largest portion of this overhead was shown by
oprofile to be at an mfence introduced by this commit into the page
allocator hot path.

For extra style points, the commit introduced the use of yield() in an
implementation of what looks like a spinning mutex.

This patch replaces the full memory barriers on both read and write
sides with a sequence counter with just read barriers on the fast path
side.  This is much cheaper on some architectures, including x86.  The
main bulk of the patch is the retry logic if the nodemask changes in a
manner that can cause a false failure.

While updating the nodemask, a check is made to see if a false failure
is a risk.  If it is, the sequence number gets bumped and parallel
allocators will briefly stall while the nodemask update takes place.

In a page fault test microbenchmark, oprofile samples from
__alloc_pages_nodemask went from 4.53% of all samples to 1.15%.  The
actual results were

                             3.3.0-rc3          3.3.0-rc3
                             rc3-vanilla        nobarrier-v2r1
    Clients   1 UserTime       0.07 (  0.00%)   0.08 (-14.19%)
    Clients   2 UserTime       0.07 (  0.00%)   0.07 (  2.72%)
    Clients   4 UserTime       0.08 (  0.00%)   0.07 (  3.29%)
    Clients   1 SysTime        0.70 (  0.00%)   0.65 (  6.65%)
    Clients   2 SysTime        0.85 (  0.00%)   0.82 (  3.65%)
    Clients   4 SysTime        1.41 (  0.00%)   1.41 (  0.32%)
    Clients   1 WallTime       0.77 (  0.00%)   0.74 (  4.19%)
    Clients   2 WallTime       0.47 (  0.00%)   0.45 (  3.73%)
    Clients   4 WallTime       0.38 (  0.00%)   0.37 (  1.58%)
    Clients   1 Flt/sec/cpu  497620.28 (  0.00%) 520294.53 (  4.56%)
    Clients   2 Flt/sec/cpu  414639.05 (  0.00%) 429882.01 (  3.68%)
    Clients   4 Flt/sec/cpu  257959.16 (  0.00%) 258761.48 (  0.31%)
    Clients   1 Flt/sec      495161.39 (  0.00%) 517292.87 (  4.47%)
    Clients   2 Flt/sec      820325.95 (  0.00%) 850289.77 (  3.65%)
    Clients   4 Flt/sec      1020068.93 (  0.00%) 1022674.06 (  0.26%)
    MMTests Statistics: duration
    Sys Time Running Test (seconds)             135.68    132.17
    User+Sys Time Running Test (seconds)         164.2    160.13
    Total Elapsed Time (seconds)                123.46    120.87

The overall improvement is small but the System CPU time is much
improved and roughly in correlation to what oprofile reported (these
performance figures are without profiling so skew is expected).  The
actual number of page faults is noticeably improved.

For benchmarks like kernel builds, the overall benefit is marginal but
the system CPU time is slightly reduced.

To test the actual bug the commit fixed I opened two terminals.  The
first ran within a cpuset and continually ran a small program that
faulted 100M of anonymous data.  In a second window, the nodemask of the
cpuset was continually randomised in a loop.

Without the commit, the program would fail every so often (usually
within 10 seconds) and obviously with the commit everything worked fine.
With this patch applied, it also worked fine so the fix should be
functionally equivalent.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-03-21 17:54:59 -07:00
..
acpi Merge 3.3-rc2 into the driver-core-next branch. 2012-02-02 11:24:44 -08:00
asm-generic mm: thp: fix pmd_bad() triggering in code paths holding mmap_sem read mode 2012-03-21 17:54:54 -07:00
crypto crypto: remove the second argument of k[un]map_atomic() 2012-03-20 21:48:16 +08:00
drm drm/exynos: exynos_drm.h header file fixes 2012-02-15 10:29:12 +09:00
keys keys: add a "logon" key type 2012-01-17 22:39:40 -06:00
linux cpuset: mm: reduce large amounts of memory barrier related damage v3 2012-03-21 17:54:59 -07:00
math-emu
media Merge branch 'v4l_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media 2012-01-18 12:53:54 -08:00
misc
mtd mtd: document that MEMWRITE ioctl is NAND-specific 2012-01-09 18:18:36 +00:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-03-20 21:04:47 -07:00
pcmcia
rdma Merge branches 'cma', 'cxgb3', 'cxgb4', 'ehca', 'iser', 'mad', 'nes', 'qib', 'srp' and 'srpt' into for-next 2012-03-19 09:50:33 -07:00
rxrpc
scsi SCSI & usb-storage: add flags for VPD pages and REPORT LUNS 2012-02-08 17:36:41 -08:00
sound Merge branch 'fix/asoc' into for-linus 2012-01-31 15:13:14 +01:00
target target: Change target_submit_cmd() to return void 2012-02-07 06:41:04 +00:00
trace Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial 2012-03-20 21:12:50 -07:00
video fbdev fixes for 3.3 2012-02-07 15:54:02 -08:00
xen Merge branch 'for-3.3/drivers' of git://git.kernel.dk/linux-block 2012-01-15 12:48:41 -08:00
Kbuild