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Davidlohr Bueso deb9b13eb2 powerpc/qspinlock: Use generic smp_cond_load_relaxed 
49a7d46a06 (powerpc: Implement smp_cond_load_relaxed()) added
busy-waiting pausing with a preferred SMT priority pattern, lowering
the priority (reducing decode cycles) during the whole loop slowpath.

However, data shows that while this pattern works well with simple
spinlocks, queued spinlocks benefit more being kept in medium priority,
with a cpu_relax() instead, being a low+medium combo on powerpc.

Data is from three benchmarks on a Power9: 9008-22L 64 CPUs with
2 sockets and 8 threads per core.

1. locktorture.

This is data for the lowest and most artificial/pathological level,
with increasing thread counts pounding on the lock. Metrics are total
ops/minute. Despite some small hits in the 4-8 range, scenarios are
either neutral or favorable to this patch.

+=========+==========+==========+=======+
| # tasks | vanilla  | dirty    | %diff |
+=========+==========+==========+=======+
| 2       | 46718565 | 48751350 | 4.35  |
+---------+----------+----------+-------+
| 4       | 51740198 | 50369082 | -2.65 |
+---------+----------+----------+-------+
| 8       | 63756510 | 62568821 | -1.86 |
+---------+----------+----------+-------+
| 16      | 67824531 | 70966546 | 4.63  |
+---------+----------+----------+-------+
| 32      | 53843519 | 61155508 | 13.58 |
+---------+----------+----------+-------+
| 64      | 53005778 | 53104412 | 0.18  |
+---------+----------+----------+-------+
| 128     | 53331980 | 54606910 | 2.39  |
+=========+==========+==========+=======+

2. sockperf (tcp throughput)

Here a client will do one-way throughput tests to a localhost server, with
increasing message sizes, dealing with the sk_lock. This patch shows to put
the performance of the qspinlock back to par with that of the simple lock:

		     simple-spinlock           vanilla			dirty
Hmean     14        73.50 (   0.00%)       54.44 * -25.93%*       73.45 * -0.07%*
Hmean     100      654.47 (   0.00%)      385.61 * -41.08%*      771.43 * 17.87%*
Hmean     300     2719.39 (   0.00%)     2181.67 * -19.77%*     2666.50 * -1.94%*
Hmean     500     4400.59 (   0.00%)     3390.77 * -22.95%*     4322.14 * -1.78%*
Hmean     850     6726.21 (   0.00%)     5264.03 * -21.74%*     6863.12 * 2.04%*

3. dbench (tmpfs)

Configured to run with up to ncpusx8 clients, it shows both latency and
throughput metrics. For the latency, with the exception of the 64 case,
there is really nothing to go by:
				     vanilla                dirty
Amean     latency-1          1.67 (   0.00%)        1.67 *   0.09%*
Amean     latency-2          2.15 (   0.00%)        2.08 *   3.36%*
Amean     latency-4          2.50 (   0.00%)        2.56 *  -2.27%*
Amean     latency-8          2.49 (   0.00%)        2.48 *   0.31%*
Amean     latency-16         2.69 (   0.00%)        2.72 *  -1.37%*
Amean     latency-32         2.96 (   0.00%)        3.04 *  -2.60%*
Amean     latency-64         7.78 (   0.00%)        8.17 *  -5.07%*
Amean     latency-512      186.91 (   0.00%)      186.41 *   0.27%*

For the dbench4 Throughput (misleading but traditional) there's a small
but rather constant improvement:

			     vanilla                dirty
Hmean     1        849.13 (   0.00%)      851.51 *   0.28%*
Hmean     2       1664.03 (   0.00%)     1663.94 *  -0.01%*
Hmean     4       3073.70 (   0.00%)     3104.29 *   1.00%*
Hmean     8       5624.02 (   0.00%)     5694.16 *   1.25%*
Hmean     16      9169.49 (   0.00%)     9324.43 *   1.69%*
Hmean     32     11969.37 (   0.00%)    12127.09 *   1.32%*
Hmean     64     15021.12 (   0.00%)    15243.14 *   1.48%*
Hmean     512    14891.27 (   0.00%)    15162.11 *   1.82%*

Measuring the dbench4 Per-VFS Operation latency, shows some very minor
differences within the noise level, around the 0-1% ranges.

Fixes: 49a7d46a06 ("powerpc: Implement smp_cond_load_relaxed()")
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210318204702.71417-1-dave@stgolabs.net
 		2021-03-29 12:48:46 +11:00
Documentation A set of irqchip updates: 2021-03-14 13:33:33 -07:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
arch powerpc/qspinlock: Use generic smp_cond_load_relaxed 2021-03-29 12:48:46 +11:00
block block: Discard page cache of zone reset target range 2021-03-11 11:49:25 -07:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto crypto: mips/poly1305 - enable for all MIPS processors 2021-03-08 11:52:17 +01:00
drivers A set of irqchip updates: 2021-03-14 13:33:33 -07:00
fs Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
include A set of irqchip updates: 2021-03-14 13:33:33 -07:00
init Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel prctl: fix PR_SET_MM_AUXV kernel stack leak 2021-03-14 14:33:27 -07:00
lib kasan: fix KASAN_STACK dependency for HW_TAGS 2021-03-13 11:27:31 -08:00
mm Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
net NFS Client Bugfixes for Linux 5.10-rc3 2021-03-12 14:19:35 -08:00
samples Merge git://git.kernel.org:/pub/scm/linux/kernel/git/netdev/net 2021-03-09 17:15:56 -08:00
scripts kbuild: fix ld-version.sh to not be affected by locale 2021-03-13 11:12:13 +09:00
security Revert 95ebabde38 ("capabilities: Don't allow writing ambiguous v3 file capabilities") 2021-03-12 15:27:14 -06:00
sound ALSA: hda/hdmi: Cancel pending works before suspend 2021-03-10 12:52:01 +01:00
tools selftests/powerpc: Fix L1D flushing tests for Power10 2021-03-24 14:09:29 +11:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
Makefile Linux 5.12-rc3 2021-03-14 14:41:02 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.